HORN ANTENNA: 1.

1. Sectoral E-plane: The horn is flared out in the plane of the electric field E and hence is called
DEFINITION and INTRODUCTION: an E-plane sectoral horn.
2. H-plane sectoral horn: The horn is flared out in a plane perpendicular to electric field E (in
A horn antenna may be regarded as a flared out (or opened out) waveguide. The function plane of magnetic field H), hence is called H-plane sectoral horn.
of the horn is to produce a uniform phase front with a larger aperture than that of the waveguide 3. Pyramidal horn: A rectangular horn with flare in both planes is called pyramidal horn.
and hence greater directivity. 4. Exponentially tapered pyramidal: Similar to the pyramidal horn. To minimize reflections of
A waveguide when excited at one end and open at second end radiates in open space. The the guided wave, the transition region or horn between the waveguide at the throat and free space
radiation is much greater through waveguide than two wire transmission line. Because of at the aperture could be given a gradual exponential taper.
mismatch between the waveguide and free space, small portion of incident wave is radiated and 5. Conical horn: By flaring walls of a circular waveguide, a conical horn is formed. Excited by
larger portion is reflected back. In order to overcome this difficulty, sudden discontinuity is TE11 mode. E distribution is shown by arrows.
replaced by gradual transformation. Then the entire energy incident in forward direction will 6. Biconical (TEM): It is excited by TEM mode by a vertical radiator.
now be radiated, provided the impedance matching is proper. 7. Biconical (TE01): It is excited by TE01 mode by a small horizontal loop antenna.
8) Exponentially tapered Circular horn: Excited by circular waveguide. Horn between the
TYPES OF HORN ANTENNA: waveguide at the throat and free space could be given a gradual exponential taper.
(i) Rectangular horn antenna (energized by circular waveguide)
PRINCIPLE: (HUYGENE’S PRINCIPLE):
1. Sectoral E-plane
“Huygene’s principle says that, each point on a primary wavefront can be considered to
2. Sectoral H-plane be a new source of a secondary spherical wave and the secondary wavefront can be constructed
as the envelope of these secondary spherical waves.”
3. Pyramidal
Thus according to the principle, the fields also spread laterally and thus we can get
4. Exponentially tapered pyramidal spherical wavefront.
(ii) Circular horn antenna (energized by circular waveguide) OPERATION:
5. Conical The propagation of EM wave in waveguide and in free space are different. In waveguide,
6. Biconical (TEM) waves are bounded (waves will not spread and restricted by the conducting wall).After reaching
the mouth, waves spread laterally and wave front becomes spherical.
7. Biconical (TE01) The flaring structure provides impedance matching, more directivity and narrow
beamwidth. Horn produces an uniform plane wavefront with a larger aperture in comparison to a
8. Exponentially tapered waveguide.
Note: As the aperture is large, directivity is high.
Note: (i) Sectoral means flaring in one dimension (ii) TE10 mode is used in rectangular WG
(iii) Arrow in figure indicates the direction and its length gives magnitude
DESIGN: (Principle of equality of path length) (Fermat’s principle)
The principle of equality of path length is applicable to the horn design but with a
different emphasis. Instead of requiring a constant phase across the horn mouth, the requirement
is relaxed to one where the phase may deviate but by less than a specified amount δ, equal to the
path length difference between a ray travelling along the side and along the axis of the horn.

1 2
 Ap = physical aperture, m²
From the geometry, 𝜀𝑎𝑝 = aperture efficiency = Ae/Ap
𝜃 𝐿
cos = (1) λ = Wavelength, m.
2 𝐿+ 𝛿
𝑎 Note (i): For Rectangular horn, Ap = aE aH
𝜃 2
tan 2 = (2)
𝐿
Note (ii): For Conical horn, Ap = πr 2
𝑎
𝜃 2
sin 2 = (3) Where, r = aperture radius.
𝐿+ 𝛿

where, θ = flare angle (θE for E plane, θH for H plane) (in degree) Note (iii): For rectangular horn, aE , aH (or) r are all atleast 1λ, taking εap =0.6
4π(0.6)Ap 7.5Ap
a = aperture (aE for E plane, aH for H plane) (in meter) D=
λ2
≈
λ2
L = horn length (in meter) 7.5Ap
D = 10 log ( ) ≈ 10 log(7.5aEλ aHλ )
λ2
δ = path length difference, (in meter)
Where, aEλ =E plane aperture in λ
From the above equation
aHλ =H plane aperture in λ
𝐚 𝐋
𝛉 = 𝟐𝐭𝐚𝐧−𝟏 (𝟐𝐋)=2𝐜𝐨𝐬 −𝟏 (𝐋+𝛅) --------(A)
Note (iv): Assuming 60% antenna efficiency
This equation gives optimum flaring angle/aperture angle.
4.5Ap
From the geometry (Using right angle triangle) Power gain, G =
λ2

2 2 a 2
(L + δ) =L +( ) OBSERVATIONS:
2

a2
(1) If δ is small fraction of wavelength, the field has nearly uniform phase over the entire
L2 +δ2 +2δL = L2 + ( 4
) aperture.

δ2 is neglected as δ is very small (2) For constant L, the directivity of the horn increases (beamwidth decreases) as aperture a
a2
and flare angle θ are increased.
Therefore 2 δL=( 4 )
(3) If a & θ becomes so large, δ is equivalent to 180 electrical degree, the field at the edge of
𝐚𝟐 aperture is in phase opposite to the field on the axis. Reduces the directivity (increases
L=( ) ------------(B)
𝟖𝛅 side lobes).
Equation (A) and (B) are design equations. (4) δ = < 0.25 λ ; for E plane horn
Half power beam width: = 0.4 λ ; for H plane horn
(5) Maximum directivity occurs at the largest flare angle (θ) for which δ does not exceed the
HPBW (E plane) = (56° λ) / aE = 56°/aEλ certain value (δ₀).

HPBW (H plane) = (67°λ) / aH = 67°/aHλ

𝐿
..̇̇͘͘͘͘Optimum δ = δ₀ = 𝜃 -L (from equation 1)
Directivity: cos(
2
)

The directivity (or gain, assuming no loss) of a horn antenna can be expressed in terms of
effective aperture. 𝜃
𝛿0 cos( )
2
Optimum length, L = 𝜃
(4πAe) 4π∈𝑎𝑝 Ap 1 − cos( )
2
D= =
λ2 λ2

where, Ae = effective aperture in m²

3 4
RHODE’S EXPERIMENT: ADVANTAGES:

Directivity is highest as they have more than one flare angle

It can be operated over a wide range of high frequency as there is no resonant element in
Antenna.
High gain, low VSWR, wide bandwidth, low weight and easy to construct

DISADVANTAGES:

Still for high power gain, lens and parabolic reflectors are preferred rather than horns.

APPLICATIONS:

Widely used as antenna at UHF and microwave frequencies above 300 MHz
Microwave applications
Used as feed for reflectors
Used for experimentation in X band
Used in short range radar systems

VARIOUS OTHER HORN ANTENNA (just for information)

(a) Ridge horn:

Increasing bandwidth by lowering the resonant frequency. By continuing a double ridge
structure from a waveguide into pyramidal horn, BW can be increased many fold.

Referring to figure,
θE = Total flare angle in E-plane
θH = Total flare angle in H-plane
L = Axial length from throat to aperture
R = Radial length
(b)Septem horn:
(a) Patterns are compared as a function of R Reduces the side lobe level.
θ = 20°.
E plane patterns has minor lobe.
H plane patterns has minor lobe practically none.

(b) Patterns are compared as a function of θ

R = 8λ.
Upper row contains E plane pattern.
Lower row contains H plane pattern.
When θ = 50°, E plane pattern is split whereas H plane pattern not.
This is because a given phase shift at the aperture in E plane horn has more effect on the pattern
than the same phase shift in the H plane horn.
(E Field goes to zero at the edge of aperture, so the phase near the edges is relatively less
important).
(c) Corrugated horns:
It can provide reduced edge diffraction, improved pattern symmetry and reduced cross
polarization (less E field in H-field).

5 6
 REFLECTOR ANTENNA

Reflectors are widely used to modify the radiation pattern of a radiating element to direct electromagnetic
energy in desired direction. In fact polarization of the primary antenna and its position with respect to the
reflecting surface is used to control the pattern characteristics impedance, gain and directivity. The antenna
which is radiating is called primary antenna (or) feed, while the reflector antenna is called the secondary
antenna. The most common feeds are dipole, horn and slot.

TYPES OF REFLECTOR ANTENNA:

1. Rod reflector

2. Flat (plane) reflector

3. Corner reflector
(d) Aperture matched horn:
By attaching a smooth curved surface section, a significant improvement in the pattern, 4. Common curved reflector shapes:
impedance and bandwidth is achieved.
a) Parabolic (Microwave dish)

b) Hyperbolic

c) Circular

d) Elliptical reflector

ROD REFLECTOR:
Mainly used in Yagi-Uda antenna. It is placed behind the driven element. It is slightly larger than that of
driven element (i.e.> λ/2) offering inductive reactance and contributes in increasing the gain. The main
disadvantage is that it alters the impedance of the driven element.
FLAT (PLANE) REFLECTOR:
1) The flat reflector has a large flat sheet near a linear dipole antenna.
2) It is the simplest form of the reflector antenna.
3) The main advantage is that backward radiation are reduced and the gain in the forward direction
increases (D = 6)
4) Large Flat sheet can convert a bidirectional into unidirectional pattern.
5) The desirable properties of the sheet may be largely preserved with the reflector reduced size and even
in limiting case (thin layer reflector).
6) The flat sheet reflector is relatively insensitive to small frequency changes. Therefore, this reflector
element is highly sensitive to the frequency change.
7) To increase directivity further (double than the value of flat reflector, D=12) array of two half wave
dipoles are used.

7 8
 CORNER REFLECTOR:

DEFINITION:

A corner reflector is a reflecting object which consists of two or three mutually intersecting, conducting flat
surface with some angle.

1) A corner reflector with a driven element is called active corner reflector (or) simply corner reflector (α <
1800) producing sharper radiation pattern than flat sheet reflector (α = 1800).

2) A corner reflector without any driven element is called passive corner (or) retro reflector (or) square
corner reflector (α = 900).

Example:

ADVANTAGES:

A corner reflector is designed to improve the collimation of electromagnetic energy in the forward
direction.

DESIGN:

For passive corner reflector, using geometry

DA = √𝑙2 + 𝑙2 = √2 𝑙 = 1.414 𝑙
Let 𝑙 = 2S, DA = 1.414 (2S) =2.828 S

9 10
For good corner reflector,

(i) λ < DA < 2λ

(ii) 𝑙 = 2S
(iii) (λ/3) < DA < (2λ/3)
(iv) α = 900 (mostly preferred)

COMPARISON BETWEEN FLAT AND CORNER REFLECTOR:

FLAT REFLECTOR CORNER REFLECTOR

Non directional feed Directional feed Equation of parabola, y2 = 4fx
Has no specific feed point Has specific feed point  The open mouth (Da) of the parabola is known as the aperture.
Linear dipole antenna with reflector Two flat sheet induced at an angle.  The ratio of focal length (F) to aperture size (Da) (i.e) F/Da is known as “F over Da ratio”.
Corner angle α = 1800 Flat sheet inserted at α < 1800
 It varies between 0.25 and 0.5

OPERATION:
If a feed antenna is placed at the focus all the waves are incident on the reflector and they are
reflected back forming a plane wave front .By the time the reflected waves reach the directrix all of them
will be in phase, irrespective of the point on the parabola from which they are reflected. Since all the waves
are in phase, a very strong and concentrated beam of radiation is there along the parabolic axis.
The main purpose of parabola is to convert a spherical wave front from the focus into a plane wave front at
the mouth of the parabola.
CURVED REFLECTORS:

Elliptical reflector produces a diverging beam with all reflected waves passing through the second focus of
the ellipse. Other shapes are hyperbolic and circular reflector.

PARABOLIC REFLECTOR: SHIELDING:

PRINCIPLE AND DEFINITION: Radiation not striking the parabolic curve appears as a minor lobe (waste of power), this can be minimized
A reflector antenna which has the shape of paraboloid and employs the properties of parabola. with the help of shielding.

"A parabola may be defined as the locus of a point which moves in such way that its distance from the
fixed point (called focus) plus its distance from a straight line (called directrix) is constant.

(i.e) FP + PP’ = FQ + QQ’= FS + SS’= constant = k (say)

Let, F = focus
A = Vertex
AB = Axis of parabola
AF = Focal length

11 12
Transmitting: Ray reflected by parabola to form plane wavefront. The radiation portion (B) of source is intercepted and it is reflected as a plane. As the mouth of the
paraboloid is circular in shape, the parallel beams produced are of the circular cross section. Let distance
Receiving: Beam of parallel wave incident on parabola to focus at the focal point. between the focus (F) and vertex (V) is ‘L’.
L = n (λ/4) n = 1,3,5,…
If L is in odd number, the direct ray and the reflected ray are in the same phase and will tend to reinforce in
the central region of the reflected wave.
If L is in even number, the direct ray and reflected ray will be in opposite phase and will tend to cancel the
central region of reflected wave.

SIGNIFICANCE OF F/Da RATIO:

The ratio F/Da is an important design constraint. The paraboloid can be designed to obtain pencil shape
radiation by keeping Da fixed and changing the focal length (F).

CASE (i): Focal point inside the aperture (F < Da /4). It is very difficult to obtain uniform illumination
COMPARISON BETWEEN PARABOLIC AND CORNER REFLECTOR because it involves illumination over a wide angle.

1) A parabolic reflector has a directional feed which radiates all or most of energy into the parabola. CASE (ii): Focal point along the plane of open mouth (F = Da /4).It gives pencil shaped radiation with
2) A corner reflector does not require a directional feed, where the direct and the reflected wave are maximum gain.
properly combined using image theory and it does not has a specific focal point.
CASE (iii): Focal point beyond the open mouth (F > Da /4). It is difficult to direct all the radiations from
PARABOLOID (or) PARABOLOID REFLECTOR (or) MICROWAVE DISH: the source.
3D structure can be obtained by rotating parabola (2D plane) around its axis and it is named as
“paraboloid” (Microwave dish). It produces sharp major lobe and smaller minor lobe.

SPILL OVER AND BACK LOBE:

1) In addition to the desired radiation, some of the desired rays are not captured by the reflector and these
constitute spillover. While receiving, spillover increases noise pickup which is particularly troublesome in
satellite ground station.

2) Some radiations from the primary radiator, occurs in the forward direction (backside) in addition to the
desired parallel beam. This is known as back lobe. (Not desired one and it should be minimized)

F/Da > λ/4 => F is outside, spill over occurs.

F/Da < λ/4 => F is inside, No spillover. Reduces aperture efficiency.

𝑭 𝟏 𝜽
In general, 𝑫
= 𝟒
𝐜𝐨𝐭 ( 𝟐 ) ; aperture number.

13 14
 Gain, G = KD = 0.65 * 9.87 (Da/ λ)² = 6 (Da/ λ) ² (or) 6 (Daλ) ²

(K= 0.65 for dipole antenna) (Daλ = Diameter of aperture in λ)

(ii) For rectangular aperture: Assume that the primary antenna is isotropic.

BWFN = 115 λ / L (in degree)

where, L = length of aperture

(iii) For square aperture: Assume that the primary antenna is isotropic.
4 π L2
Directivity, D = λ2
= 12.6 (Lλ)2

Gain, G = KD = 0.65 * 12.6 (Lλ)² = 7.7 (Lλ)²

ANALYSIS (DESIGN OF PARABOLOID):
(K= 0.65 for dipole antenna)

OTHER TYPES OF PARABOLOID REFLECTOR:

(a) Parabolic cylinder: Constructed by moving the parabolic contour parallel to itself. Used to generate fan
beam (required for RADAR) with large aspect ratio.
(b) Paraboloid reflector: Generate pencil beam.
(c) Cut (or) truncated paraboloid: Generate fan beam in azimuthal (or) elevation plane.
(d) Pill box: Cylinder that is short in axial direction and provided with conducting end plates. Generate fan
beam.
(e) Cheese antenna: Combination of pill box and parabolic reflector.

ADVANTAGES:

1) Produces directional beam with sharp main lobe.

(i) For circular aperture: Assume that the primary antenna is isotropic. 2) Extremely high gain & directivity (narrow beamwidth).
BWFN = 140 λ / Da (in degree) 3) For small input power, ERP is very high.
HPBW = 58 λ / Da (in degree) DISADVANTAGES:
4 π Ae 4π (π Da2 / 4)
Directivity, D =
λ2
= = 9.87 (Da/ λ)2 (or) 9.87 (Daλ)2 1) Large size and hence not suited for low frequency.
λ2

15 16
2) Main beam is pencil shaped surrounded by number of side lobes. It produces EM interference. WAVEGUIDE HORN

3) Cannot locate the focus exactly.

APPLICATIONS:

1) Microwave and Radar applications.

2) Satellite transmission & reception.

3) Radio astronomy.

FEEDING STRUCTURE (METHODS) (SYSTEM):

A source of radiation placed at focus is called primary radiator (or) feed. The reflector is called secondary
radiator. The various feeds used in parabolic reflector:

a) Dipole antenna
b) End fire array of dipoles CASSEGRAIN FEED
c) Waveguide horn
d) Cassegrain feed
e) Offset feed

DIPOLE ANTENNA

END FIRE ARRAY OF DIPOLES

17 18
OFFSET FEED
SLOT ANTENNA:

DEFINITION AND APPLICATION:

The slot antenna is an opening cut in a conducting sheet(or) in one of the walls of waveguides
which acts as an antenna. It is excited suitably either by coaxial cable or waveguide.

Slot antennas are useful in many applications, especially where low profile or flush mounting is
required as, for example on high speed aircraft. It is best suitable radiator at frequencies above
300 MHz in specific mobile and radar systems. It is useful for TV broadcasting with
omnidirectional pattern. The slot antenna with cavity design has been utilized by variety of law
reinforcement applications.

APERTURE BLOCKAGE:

It is clear that there is a region of blocked rays in front of cassegrain reflector. Some of the radiations from
the parabolic reflector are blocked by hyperboloid creating region of blocked ray. It is known as aperture
blockage. This is not very serious problem in case of lager dimension parabolic reflector, but it is the
drawback for small dimensions.
CONSTRUCTION:
The paraboloidal reflector can be fed using HWD or horn antenna with small ground plane in order to
redirect the back lobe towards main lobe.

19 20
FEEDING METHODS AND VARIOUS SHAPE OF SLOT ANTENNA: 4. Horizontally polarized slot antenna:

a) Excited by Coaxial cable

1. Center Feed:
2. Off-center feed;
3. Vertically polarized slot antenna
4. Horizontally polarized slot antenna
b) Excited by waveguide
5. Boxed-in slot (cavity backed slot)
6. Waveguide feed
7. T-fed slot
8. Broadside array of slot in waveguide
1.Center Feed:

5. Boxed-in slot:

2. Off-center feed:

3. Vertically polarized slot antenna:

21 22
6. Waveguide Feed: 8. Broadside array of slot in waveguide:

RADIATION PATTERN:

According to Booker’s theory, the radiation pattern of slot antenna and half wave dipole are
identical, but difference lies in polarization. In order to increase the directivity and gain, array of
slots is used.

7. T-Fed slot:

23 24
COMPARISON BETWEEN DIPOLE AND SLOT:

1. Dipole is inductive and the slot is capacitive and vice-versa.

2. Widening the slot (small L/W ratio) increases the bandwidth, whereas increasing the
thickness of dipole (small L/D ratio) increases the bandwidth.
3. A dipole and a slot in a conducting sheet have similar shapes except that the metal region
and free space are interchanged.
4. According to Booker’s theory, the radiation pattern of slot antenna and half wave dipole
are identical, but difference lies in polarization. If E field of slot antenna is horizontally
polarized, then the E field of dipole is vertically polarized. In order to increase the
directivity and gain, array of slots is used.

INPUT IMPEDANCE:
Let the slot is excited by a two wire transmission line. E field is maximum at the centre and
tapers off towards the edges. The distribution is sinusoidal with the terminal impedance of 500
Ohms.

Using (2), (3) & (5)

Using (1), (4) and (6)

25 26
Case (i):

The impedance of dipole antenna (HWD) = 73 + j 42.5 Ohms

Therefore, impedance of slot, Zs = 35476 / (73 + j 42.5)

Zs = 366 – j211 Ohms

PRINCIPLE: The discussion is based largely on a generalized and extension of Babinet’s

principle by Henry Booker.

27 28
MICROSTRIP antenna (Patch antenna) (Printed antenna): VARIOUS SHAPES OF PATCH ANTENNA:
DEFINITION AND INTRODUCTION:
An antenna made from metal patch on a dielectric substrate above the ground plane is called
microstrip patch antenna.

CONSTRUCTION:

RADIATION MECHANISM (RMSA) AND DESIGN PROCEDURE:

29 30
 FEEDING METHODS:

(i) Contacting Feed

1. Microstrip line feed
2. Coaxial probe feed
(ii) Non-contacting Feed
3. Aperture coupling (Electromagnetic Coupling)
4. Proximity coupling
1) Microstrip line feed:

31 32
 3. Aperture coupled feed:

4. Proximity coupled feed (indirect feed):

2. Coaxial probe feed:

Equivalent circuits of feed methods:

33 34
 6. Military application:
High velocity aircrafts, space crafts, missiles, rocket requires low profile, light weight
and conformally mounted antenna. The ideal choice is Microstrip antenna.
7. Space Communication:
Microstrip antenna are used in space programs like Earth Limb Measurement Satellites
(ELMS), International Sun Earth Explorer (ISEE), Shuttle Imaging Radar(SIR), etc.
8. Commercial applications:
Mobile Satellite Communications, Direct Broadcast Satellites, Global Positioning
Systems, etc.

ADVANTAGES:

DISADVANTAGES:

Others: (Just for information)

1. Analysis: (a) Transmission line model (b) Cavity model (c) Full wave analysis
2. Method of tuning: (a) Using two stubs (b) using shorting post (c) adjusting thickness
of substrate (d) Using defected structure
3. Techniques to improve bandwidth: (a) Use of thick substrate (b) Use of low
dielectric constant (c) use stacked Microstrip antenna (d) use of non-contacting feed
4. Size reduction: (a) Shorting of patch (b)Change of shape (c) Partial shorting
APPLICATIONS: 5. Array of Microstrip antenna increases gain and directivity.
1. Mobile communication application:
Mobile communication requires low cost and low profile antennas. Microstrip patch
antenna meets all the requirements and various types of Microstrip antenna have been
designed for use in mobile communication, cellular communication and pagers.
2. RF identification:
RFID system generally uses frequencies between 30Hz and 5.8 GHz depending on its
applications. It is tag/reader/transponder. It is used in different areas like
manufacturing, transportations, health care, logistics, etc.
3. WiMax:
IEEE 802.16 standard covers 30 miles radius with 70Mbps.
4. RADAR application:
Radar can be used for detecting moving targets such as people and vehicles. It
demands a low profile, light weight antenna, the Microstrip antenna is an ideal choice.
5. Medical application:
Wearable Microstrip antenna is suitable for wireless body area network with gain and
FBR operated at 2.45GHz. It is useful for treatment of tumors.

35 36
37 38
39 40
41 42
43 44
 UNIT 5 The angle of rotation C depends on K neither but depends on 𝜃 𝑛𝑜𝑟 𝑜𝑛 𝜙. Physical congruence
implies that the original antenna would electrically behave the same both frequency. However the radiation
SPECIAL ANTENNAS AND MEASUREMENTS
pattern will be rotated azimuthally through the angle C. For unrestricted value of K (0 ≤ K ≤ ∞) the pattern will
1) PRINCIPLE OF FREQUENCY INDEPENDENT ANTENNAS (RUMSEY’S PRINCIPLE): rotate by angle C in𝜑 with frequency, because C depends on K, but its shape will be unaltered.

STATEMENT: It was pointed out by V.H.Rumsey that,” the performance of a lossless antenna (impedance Differentiate equation (3) w.r.t the angle C we have
and pattern properties) is independent of frequency, if its dimensions in terms of wavelength remain constant”.
Such a result could be achieved if the antenna is specified in terms of angles. It was showed by Rumsey that this
requirement would be fulfilled by any antenna whose equation in spherical coordinates is of the form.

r = 𝑒 𝑎(𝜙+𝜙0 ) f (𝜃)

where f (𝜃) is any function of 𝜃. In case of planar antenna the equation reduces to

r = 𝑒 𝑎(𝜙+𝜙0 )

GENERAL ANALYSIS: (GENERAL SOLUTION OF FREQUENCY INDEPENDENT ANTENNA):

Consider an antenna, whose surface is described by,

r = F (𝜃,𝜙) (1)

where r is the distance along the surface or the edges.

Suppose that one wishes to scale the antenna to a new frequency that K times lower than the original frequency.
The antenna must, then, be made K times bigger than to maintain the same electrical dimensions. Thus the new
surface this antenna is described by, 2) SPIRAL ANTENNA (EQUIANGULAR SPIRAL ANTENNA):

𝑟 ′ = KF (𝜃,𝜙) (2) The equiangular spiral is one geometrical configuration whose surface can be described by angles. It thus
fulfils all the requirements for shapes that can be used to design frequency independent antennas. Since a curve
where K depends neither on 𝜃 𝑛𝑜𝑟 𝑜𝑛 𝜙. along its surface extends to infinity, it is necessary to designate the length of the arm to specify a finite size
For the second antenna to achieve congruence with the first, it must be rotated by an angle C so that antenna. The lowest frequency of operation occurs when the total arm length is comparable to the wavelength.
For all frequencies above this, the pattern and impedance characteristics are frequency independent.
KF (𝜃, 𝜙) = F (𝜃, 𝜙 + 𝐶) (3)
(i) PLANAR SPIRAL:

This is the equation of an equiangular or logarithmic spiral where a is the rate of expansion and 𝜙0 is is the
orientation

As the parameter A is arbitrary, it follows that in equation (10) 𝜌0 can be considered as fixed with 𝜙0
playing the role of parameter . If 𝜙0 is given the value of 0 and π, the antenna shown in fig (b) results. If 𝜙0 is
allowed to take on the values 0, 𝜋⁄2 , π and 3𝜋⁄2 four spirals as shown in fig. (c) & (d) are formed with several
symmetrical possibilities for connecting the terminals. Numerous other combinations are also possible. If now
𝜙0 is allowed to take all values from 0 to 𝜙1 and all values from π to 𝜋+𝜙1 , an antenna of the type shown in fig.
(e) arises, where 𝜙1 is arbitrary.
Babinet principle to spiral antenna: with ground plane. The input impedance is between 100 to 150 ohms for a pitch angle 𝛼 = 17° and full angles
of 20° 𝑡𝑜 60° . The smaller cone angles (30° or less) have higher front to back ratios of radiation.
Let 𝑍1 = Input impedance of the antenna for a value 𝜙1 = 𝛼

and 𝑍2 = input impedance of the antenna for value 𝜙1 = 𝜋 − 𝛼

then the two antennas form the complementary screens and hence as usual
𝜂2
𝑍1 𝑍2 =
4

𝜋
For a special case when 𝜙1 =
2

𝑍1 = 𝑍2
𝜂2 3) HELICAL ANTENNA:
Then 𝑍1 2 = 4
=𝑍2 2
DEFINITION:
𝜂2
and 𝑍1 = 𝑍2 = Helical antenna is broadband VHF and UHF antenna to provide circular polarization characteristics and uses
2
shape of helix. The characteristics is mainly depends on the diameter, pitch, number of turns, wavelength,
and 𝑍1 = 𝑍2 = 60𝜋 = 188.4 𝑜ℎ𝑚𝑠 excitation and spacing between the loops. It is used in extra terrestrial communication where satellite relay are
involved.
The experimentally measured value of input impedance is 164 ohms and theoretical value is 188.4 ohms. The
difference is due to the finite arm length, finite thickness of the plate and non-identical feeding conditions. CONSTRUCTION:

(ii) CONICAL SPIRAL ANTENNA: Helical antenna consists of a helix of thick copper wire or tubing wound in the shape of a screw thread and used
with a flat metal called a ground plane or ground plate. The structure of the helical antenna is shown in figure.

𝜋
Larger the values of 𝜃0 in the range 0 ≤ 𝜃 ≤ lesser the tightly wound spirals. The conducting conical spiral
2
is constructed easily by forming the conical arms on the dielectric cone, using printed circuit, which also act as
support. The feed cable is bound to the metal arms which are wrapped around a cone. Symmetry is preserved The helix is fed by a co-axial cable and it is connected between helix and ground plane. One end of the helix is
by using as a dummy cable as is done in planar surface. The conical equi-angular spiral antenna is fed at the connected to the centre conductor of the cable and the outer conductor is connected to the ground plane.
The ground plane is simply made of sheet or of screen or of radial and concentric conductors. The mode of
apex by means of a balanced transmission line carried up inside and along the axis of the cone. Alternatively by
radiation depends on the diameter of the helix "D" and turn spacing "S" (turn spacing is a measure between two
coaxial cable carried along and soldered in contact with one of the arms. centres of the turns).
The difference between the planar and conical spiral is that conical spiral provides a unidirectional radiation
single lobe toward the apex of the cone with maximum along the axis. Circular polarization is obtained and
relative constant impedances are preserved over bandwidth conical spiral antenna can be used in conjunction
 1) The radiation pattern is a combination of the equivalent radiation from a short dipole positioned on the
same helix axis and a small loop which is also coaxial with the helix axis.
2) Pitch angle, α = 0° corresponds to a loop
α = 90°, the helix become a linear antenna

The following symbols are used to describe a helix

C = Circumference of helix = πD
d = Diameter of helix conductor
A = Axial length = NS
N = Number of turns
L = Length of one turn
NL = Total length of wire
l = Spacing of helix from ground plane
α = Pitch angle

3) If S → 0, D = constant, helix collapse to a loop

4) If D → 0, S = constant, helix straighten into a linear conductor (Short dipole).

For N turn of helix, the total length of antenna is equal to NS. If one turn of helix is unrolled, then
circumference (πD), spacing ‘S’, turn length ‘L’ and pitch angle ‘α’ are related by the triangle as shown in
figure.
(i) 𝐿 = √𝑆 2 + 𝐶 2 = √𝑆 2 + (𝜋𝐷)2 (1)
𝑆 𝑆
(ii) tan 𝛼 = =
𝐶 𝜋𝐷
𝑺
𝜶 = 𝒕𝒂𝒏−𝟏 ( 𝝅𝑫 ) (2)
Pitch angle (α) is the angle between a line tangent to the helix wire and the plane normal to the helix axis.

The different radiation characteristics are obtained by changing the above parameter in relation to wavelength.
After the point "A" the conductor lies in the surface of imaginary helix cylinder. The axial length of the helix 5) The radiation pattern of loop and dipole are same but the polarization is right angled (90° apart)
starts from here. Coaxial line coincident with helix axis. 6) Hence the polarization may be either circular or elliptical depending on the field strength and pitch angle
(α).
The component of the feed wire length parallel to the axis length is "l". This length is equal to S/2. The antenna 7) α = small, loop radiation pre-dominates and α = large, dipole radiation pre-dominates and in between
terminals are considered at the point "B" and all the impedances are referred to this point. The variation of feed values of α, polarization is circular or elliptical for particular values of α.
wire geometry affects the input impedances of the antenna. 8) Helix antenna may be considered to be having a number of small loops and short dipoles connected in
series in which the loop diameter is same as helix diameter and helix spacing "S" is same as dipole
MODES OF RADIATION: length.
In general, a helical antenna can radiate in many modes. But the most important modes of radiation are as Axial Ratio:
follows: Let the far field of the small loop is,
(i) Normal mode or Perpendicular (Broadside) mode. 120 𝜋 2 𝐼 sin 𝜃 𝐴
(ii) Axial or End fire or Beam mode of radiation. 𝐸𝜙 = . 𝜆2 (3)
𝑟

where, I = Retarded current

(i) NORMAL MODE OF RADIATION:
In this normal mode of radiation, the radiation field is maximum in broadside (i.e.,) in the direction normal to r = distance
the helix axis and is circularly polarized waves. Here, the dimensions of the helix is small compared with 𝜋 𝐷2
wavelength (i.e.,) NL << λ. A= Area of loop = 4
  The helix is operated in conjunction with a ground plane and it is fed by a coaxial cable. The ground
plane will be atleast half wavelength in diameter.
Let the far field of the short dipole is,
𝑗 60 𝜋 𝐼 sin 𝜃 𝑆
𝐸𝜃 = 𝑟
. (4)
𝜆

where, L = S = Length of dipole.

Equation (3) and (4) shows that there is 90° phase between them due to the presence of j operator.
60 𝜋 𝐼 sin 𝜃 𝑆
𝐸𝜃 .
𝐴𝑅 = = 120 𝜋2𝑟𝐼 sin 𝜃 𝜆𝐴
𝐸𝜙 . 2
𝑟 𝜆

𝑆𝜆
𝐴𝑅 =
2𝜋 𝐴
𝟐𝑺𝝀
𝑨𝑹 = 𝝅𝟐 𝑫𝟐
(5)
Analysis:
1) If AR = 0, then elliptical polarization becomes linear horizontal polarization
 The pitch angle "α" varies from 12° to 18° and about 14° is an optimum pitch angle. The antenna gain
2) If AR = ∞, then elliptical polarization becomes linear vertical polarization
and beamwidth depends upon the helix length (NS).
3) When AR = 1,then elliptical polarization becomes circular polarization

 The terminal impedance is 100Ω resistive, at frequency C = λ and at higher and lower frequencies the
For circular polarization: resistive value changes followed by reactive components.
AR = 1
 In general, the terminal impedance of helical antenna lies between 100 Ω to 200 Ω pure resistive. Within
|𝐸𝜃 | = |𝐸∅ | 20% approximation, the terminal impedance is given by
2 S λ = 𝜋 2 𝐷2
𝟏𝟒𝟎 𝑪
𝝅𝟐 𝑫𝟐 𝑪𝟐 𝑹= 𝜴 (8)
𝝀
S= = (6)
𝟐𝛌 𝟐𝛌
 The HPBW (Beamwidth between half power points) is given by,
Substitute (6) in (2)
52 𝜆3
𝑆
𝝅𝟐 𝑫 𝟐
𝜋𝐷 𝑪
HPBW = √ (in degrees) (9)
−1 −1 −1 −𝟏 𝐶 𝑁𝑆
𝜶 = 𝑡𝑎𝑛 ( 𝜋𝐷 ) = 𝑡𝑎𝑛 ( 𝟐𝛌
) = 𝑡𝑎𝑛 ( 𝟐 𝛌 ) = 𝒕𝒂𝒏 ( 𝟐𝛌 ) (7)
𝜋𝐷 where, λ = free space wavelength
The above expression is the condition for pitch angle to get the circular polarization. This normal mode of S = Spacing
operation is very narrow in bandwidth and its radiation efficiency is very small. Also this mode of operation is
limited and it is hardly used.  The beamwidth between first nulls is given by

115 𝜆3
BWFN = √ (in degrees) (10)
𝐶 𝑁𝑆

 The maximum directive gain (directivity) for axial mode is given by

ii) AXIAL MODE OF RADIATION:
15NSC2
D= (11)
 The radiation field is maximum in the end fire direction that is along the helical axis and the polarization 𝜆3
1
is circular or nearly circular. This mode occurs when the helix circumference (D) and spacing (S) are Axial Ratio (AR) = 1 + 2𝑁 (12)
appreciable of the order of one wavelength.
 This mode produces a broad and fairly directional beam in the axial direction with minor lobes at  The normalized far field pattern is given as,
oblique angles. Because of this features, the helical antenna is used for many practical applications.
Nψ
𝜋 sin ( )
𝐸 = sin ( ) cos θ ψ
2
(13a)
 C/ λ is of the order of one wavelength and spacing is λ/4. 2𝑁 sin ( )
2
 𝑆 1 1
𝜓 = 2𝜋 [ 𝜆 (1 − cos 𝜃) + ] (13b) log f2 –log f1 = log ( )
2𝑁 𝜏

3𝜆 4𝜆 𝑓2 1
where, α = 12° to 15°, N ≥ 3, NS ≤ 10 and C = to
4 3 =
𝑓1 𝜏
ADVANTAGES:
The advantages of helical antenna are, f1 = τf2
(i) N-turn helix is an end fire array of "n" sources.
(ii) The helix not only has a nearly uniform resistance input over a wide bandwidth, but it also operates as a In general, frequency will be repeated given by τ (n-1)
f
super gain end fire array over the same bandwidth.
(iii) It is non-critical with respect to conductor size and turn spacing; therefore it can achieve circular CONSTRUCTION:
polarization over a wide bandwidth.
(iv) It is easy to use in arrays because of almost negligible mutual impedance. All dimensions increases in proportion to the distance from the origin. It has a number of dipoles of different
(v) Because of circular polarization, the helical antenna is capable of receiving signals of arbitrary lengths and spacing. It is fed by a balanced two wire transmission line which is transposed between each
polarization. adjacent pairs of dipoles. It is fed at narrow end and the maximum beam radiation is as shown. The length of the
dipoles increases from feed point towards other end such that the included angle α remains constant.
APPLICATIONS:
The applications of helical antenna are,
(i) Wide bandwidth, simplicity, highest directivity and circular polarization of helical beam antenna have
made it indispensable for space communication application like telemetry, radio astronomy, satellite
and space communications, ballistic missiles, etc. Transmitting telemetry data from moon to earth.
(ii) A single or an array of helical antenna is used to receive or transmit the VHF signals through ionosphere
(Faraday effect).
(iii) In axial mode, the helix dimensions are not critical. Hence, the bandwidth and radiation efficiency, both
are greater. Thus, the axial mode helical antennas are used to achieve circularly polarized waves over
extremely wide bandwidth and are capable of receiving waves of arbitrary polarization.

4) LOG PERIODIC ANTENNA (LOG PERIODIC DIPOLE ARRAY) (LPDA)

A Log periodic antenna is a broadband narrow beam antenna used in VHF and UHF bands. It is a frequency
independent antenna.

FREQUENCY INDEPENDENT ANTENNA:

The antenna for which the impedance and pattern (& hence the directivity) remains constant as a function of the
frequency.

LOG PERIODIC ANTENNA – DEFINITION:

The dipole length and the spacing are related through parameter called design ratio (or) scale factor (or)
It is an array antenna which has structural geometry such that its impedance is periodic with the logarithm of the periodicity factor. (τ < 1)
frequency. In other words, input impedance Zin will go through identical cycles (each cycle exactly like
𝑅1 𝑅 𝑅3 𝑅𝑛 𝐿 𝐿 𝐿3 𝐿𝑛
preceding one) and hence the name. The geometry of this antenna involves a basic geometry that is repeated (by = 𝑅2 = = ………. = 𝑅 = τ = 𝐿 1 = 𝐿2 = = ………. = 𝐿
𝑅2 3 𝑅4 𝑛+1 2 3 𝐿4 𝑛+1
a scale factor, τ) but with a changing size of the structure, by which structure can either expanded or contracted.
𝑅𝑛 𝐿𝑛
=𝐿 =τ
𝑅𝑛+1 𝑛+1

𝑆𝑛+1 𝐿𝑛+1 1
Also, 𝑆𝑛
= 𝐿𝑛
= 𝜏 = k; K > 1

The spacing factor (𝜎) is defined as,

𝑆𝑛
𝜎=
2 𝐿𝑛
where, L = length and radiated by the active region. This region presents large reactive impedance to the line and thus, any small
R= spacing from the start position amount of the incident wave from active region is reflected back towards backward direction.
S = spacing between dipole
d = diameter of the dipole DESIGN OF LPDA:
a = gap spacing at dipole centre. The following are design parameters of LPDA,
Typical values: 1) Apex angle (α)
To achieve gain between 7.5dB and 12 dB in comparison isotropic antenna, α = 300 and τ = 0.7. 2) Design ratio (𝜏)
3) Spacing factor (σ)
WORKING PRINCIPLE:
We know that,
When the log periodic antenna is operated at a given frequency, it is observed that all the structure does not
𝑆𝑛+1 𝐿𝑛+1 𝑅𝑛+1 𝑑𝑛+1 𝑎𝑛+1 1
radiates but only a certain portion radiates known as active element (λ/2). The analysis of a log periodic dipole = = = = = =
𝑆𝑛 𝐿𝑛 𝑅𝑛 𝑑𝑛 𝑎𝑛 𝜏
array can be done by considering three region of the antenna:
𝑅𝑛+1 −𝑅𝑛 𝑆𝑛 𝑠 𝜆
𝞴
Spacing factor, σ = 2𝐿𝑛
=
2𝐿𝑛
= 𝜆 (for active element Ln =
2
)
(1) Transmission-line region (Inactive region L < 𝟐
)
𝐿𝑛+1
𝞴 𝐿1
= k n = f = Frequency ratio (or) Bandwidth.
(2) Active region (L ≅ )
𝟐

𝞴
(3) Reflective region (Inactive region L > )
𝟐

𝞴
(1) Inactive transmission line region (L < ):
𝟐

𝞴
At middle of the operating range, the antenna elements are short with the resonant length i.e. L≤ 𝟐, therefore,
the elements present a relatively high capacitance impedance. The element current is small and leads the base
voltage supplied by transmission line by 90° (approx.). The element spacing in wavelength is also small. By
transposition of transmission introduces 180° phase shift between adjacent dipoles. Hence currents in elements
of these regions are small and hence the small radiation in backward direction (towards left). From the fig,

𝞴 𝐿𝑛+1 − 𝐿𝑛
(2) Active region (L ≅ ): 𝛼 2
𝟐
tan =
2 𝑆
𝞴
In this region the dipole lengths are approximately resonant length (i.e. L ≅ ) and hence the impedance
𝟐 𝐿
offered by the dipoles of the region are resistive appreciably in nature. Hence the element currents are large and 𝛼 𝐿𝑛+1 [1 − 𝐿 𝑛 ]
𝑛+1
tan =
in phase with base voltage. The current just below resonance is slightly leading and above resonance slightly 2 2𝑆
lagging. The spacing between two elements is now sufficiently large, causing the phase in a particular element
𝜆 𝐿𝑛
to lead approximately by 90°. For example, by the time field radiated from element Ln+1 reaches Ln phase of Ln for active element Ln+1 ≈ 2 and 𝐿 =𝜏
𝑛+1
advances by 90° and its field adds to the field of Ln+1 elements, in phase producing a large resultant field
𝜆
towards left. Hence there is strong radiation towards left in backward direction and a little radiation towards 𝛼 [1 − 𝜏]
right. tan = 2
2 2𝑆
𝞴
(3) Inactive reflective region (L > ): 𝛼 [1 − 𝜏]
𝟐 tan =
2 𝑆
4( )
The element (dipoles) length are longer than the resonant length (i.e. L ≥
𝞴
) hence the impedance becomes 𝜆
𝟐
inductive, causing the currents in the elements to lag the base voltage. The base voltage supplied by 𝜶 [𝟏− 𝝉]
𝐭𝐚𝐧 𝟐 = (1)
𝟒( 𝝈 )
transmission line is now very much small as almost all the energy transmitted down the line has been attracted
 [𝟏− 𝝉] Need for transposing the line: In log periodic antenna, it is necessary to introduce a 180° phase reversal
Also, 𝝈 = 𝜶 (2)
𝟒( 𝐭𝐚𝐧 ) between elements. This is accomplished by using a twisted transmission line (transposed lines).
𝟐

[𝟏− 𝝉]
𝛂 = 𝟐 𝒕𝒂𝒏−𝟏 (3) In the inactive transmission line region (L > λ/2), the spacing between the dipoles is very small. Therefore the
𝟒( 𝝈 )
transmission provides 180° phase shift between adjacent dipoles. Hence the radiation is very small in backward
Out of the three parameters, two are specified and the third is determined. direction.

Note: When α is replaced by 2α, then the above equation are modified as, While in active region, the spacing between the dipoles is sufficiently large and the transmission provides 90°
phase shift. When the field radiated from element (n + 1) reaches nth element, the phase advances by 90° and
[1 − 𝜏] field of nth element adds to that of (n + 1)th element in phase. Thus a large field is resulted towards left
α = 𝑡𝑎𝑛−1 (backward direction). So it is necessary to feed the neighboring dipoles at opposite phase and, this is
4( 𝜎 )
accomplished by transposing the transmission line.
[1− 𝜏]
,𝜎 =
4( tan 𝛼 )
Effects of decreasing (α):
[1 − 𝜏]
tan 𝛼 = [1− 𝜏]
4( 𝜎 ) We know that, α = 2 𝑡𝑎𝑛−1
4( 𝜎 )

GENERAL CHARACTERISTICS (SALIENT FEATURES):

It is clear that α and σ is inversely proportional. When α is reduced, σ increases. Therefore spacing factor
(1) Excited from the shorter length side (high frequency side) for one active element or centre of two active increases and length decreases. This in turn increases the number of dipoles and they will become more closer
regions. to each other. Hence the radiation efficiency is increased with increased gain and directivity.

(2) There is infinite variety of the log periodic structures possible but not all structure would be frequency APPLICATIONS:
independent.
(i) Mainly used in HF communication. It has advantage that no power is wasted in terminating resistance.
(3) Used to receive a good number of TV channels without deterioration of field strength. (ii) Used in TV reception. Design will accommodate all channels even upto UHF band.
(iii) Best suited for all round monitoring and covers all the higher frequencies bands with low cost of
(4) Radiation pattern may be unidirectional (backward direction) and bidirectional (maximum radiation is in installation.
broadside direction). 5. MODERN ANTENNAS
(5) Transmission line inactive region (between active and vertex) must have proper characteristics impedance The antennas used with modern trends are called modern antennas. Such antennas are mostly widely used
with negligible radiation. in modern complex communication systems. It increases channel capacity, extended range, better link quality,
(6) In active region, currents magnitude and phasing should be proper so that strong radiation occur along signal quality, coverage and spectrum efficiency. They are namely,
backward direction and zero radiation along forward direction.  Reconfigurable antennas,
(7) In inactive reflective region, there should be rapid decay of current for a successful antenna.  Wearable antennas,
 Active antennas, and
Reason for feeding from end with shorter dipoles: When the log periodic antenna is operated at a given  Dielectric antennas.
frequency, it is observed that all the structure does not radiated but only a certain portion radiates known as
"active region". Active region is that region in which dipoles have nearly resonant length i.e. λ/2. The cut off 6. RECONFIGURABLE ANTENNA
frequencies are those frequencies at which the longest and shortest dipoles are nearly half wavelength (i.e.  A reconfigurable antenna is an antenna which is capable of modifying dynamically its frequency and
resonant length). Hence the active region of the antenna is towards apex (shorter element) for highest radiation properties in a controlled and reversible manner.
frequencies, at middle for intermediate frequencies and near longest elements for lowest frequencies. In other  Reconfigurable antennas, has the ability to radiate more than one pattern at different frequencies and
words phase centre of the antenna shifts from longest end to shortest end as the frequencies change from polarizations, which is necessary in modern telecommunication systems.
minimum to maximum. The maximum to minimum ratio of frequencies determines the bandwidth. Therefore  Reconfigurability has become an important and desired feature of modern, agile, radio-frequency (RF)
for high frequency application, it is necessary to feed the antenna from end with shorter dipole. systems for wireless and satellite communications, sensing, and imaging.
 These antennas can address complex system requirements by modifying their geometry and electrical
Radiation pattern and input impedance: The radiation pattern of a log periodic antenna may be bi-directional behaviour, thereby adapting to changes in environmental conditions or system requirements (i.e.,
and unidirectional depending upon the log periodic structures. The input impedance depends chiefly on the enhanced bandwidth, changes in operating frequency, polarization, and radiation pattern).
characteristic impedance of the transmission line that feeds the antenna.
  In order to provide a dynamical response, reconfigurable antennas integrate an inner mechanism (such (3) POLARIZATION RECONFIGUARTION
as RF switches, varactors, mechanical actuators or tunable materials) that enable the intentional  Polarization reconfigurable antennas are capable of switching between different polarizations modes.
redistribution of the RF currents over the antenna surface and produce reversible modifications over its The capability of switching between horizontal, vertical and circular polarizations can be used to reduce
properties. the polarization mismatch losses in the portable devices.
 Reconfigurable antennas differ from the smart antennas because the reconfiguration mechanism lies  Polarization reconfigurability can be provided by adjusting the balance between the different modes of a
inside the antenna rather than in an external beamforming network. multimode structure.
 The reconfiguration capability of reconfigurable antennas is used to maximize the antenna performance (4) COMPOUND RECONFIGURATION
in a changing scenario or to satisfy the changing operating requirements.  The compound reconfiguration is the capability of simultaneously tuning several antenna parameters, for
instance frequency and radiation pattern. The most common application of compound reconfiguration is
the combination of frequency agility and beam-scanning to provide improved spectral efficiencies.
 This reconfigurability is achieved by combining the different single-parameter reconfiguration
techniques in the same structure or by reshaping a pixel surface dynamically.
RECONFIGURATION TECHNIQUES

The four most important reconfiguration techniques used to implement reconfigurable antennas, are:

(a) Electrical technique:

Antennas based on radio-frequency microelectromechanical systems RF-MEMS, PIN diodes, and
varactors to redirect their surface currents are called electrically reconfigurable.
(b) Optical technique:
Antennas that rely on photoconductive switching elements are called optically reconfigurable antennas.
(c) Physical technique:
In this technique, in order to achieve reconfigurability physical alternations in the antenna structure is
made. Such antennas are called physically reconfigurable antennas.
(d) Material change:
TYPES OF ANTENNA RECONFIGURATION In this technique, some smart materials such as ferrites and liquid crystals are used.
Reconfigurable antennas can be classified according to the antenna parameter that is dynamically
adjusted, typically the frequency of operation, radiation pattern or polarization. TYPE OF SWITCHES:
The different type of switches used in the design of electrically and optically reconfigurable antennas.
(1) Frequency RF MEMS:
(2) Radiation Pattern
(3) Polarization They use mechanical movement to achieve a short circuit or an open circuit in a surface current path of an
(4) Compound antenna structure. The forces required for the mechanical movement can be obtained using electrostatic,
magnetostatic, piezoelectric, or thermal designs.
(1) FREQUENCY RECONFIGURATION PIN Diodes:
 Frequency reconfigurable antennas can adjust dynamically their frequency of operation. They are
particularly useful in situations where several communication systems converge because the requirement They operate in two modes. The “ON” state, where the diode is forward biased and the “OFF” state, where
of multiple antennas can be replaced by a single reconfigurable antenna. the diode is not biased.
 The frequency reconfiguration is generally achieved by modifying physically or electrically the antennas Varactors:
dimensions using RF-switches, impedance loading or tunable materials.
They consist of a p-n junction diode. As the bias voltage applied to the diode is varied, the varactor
(2) RADIATION PATTERN RECONFIGURATION capacitance is going to be changed. Typical values are from tens to hundreds pico Farads.
 Radiation pattern reconfigurability is based on the intentional modification of the spherical distribution Photoconductive Elements:
of radiation pattern. Beam steering is the most extended application and in steering considers the
direction of maximum radiation to maximize the antenna gain in a link with the mobile devices. The movement of electrons from the valence band to the conduction band allows the switch to go from
 Pattern reconfigurable antennas are usually designed using movable/rotatable structures or including the “OFF” state to “ON” state. This is achieved by illuminating the switch by light of appropriate wavelength from
switchable and reactively-loaded parasitic elements. a laser diode.
CHALLENGES: supports wireless communication, which include in-body communication, on-body communication and off-
body communication. A WBAN connects sensors, actuators and IoT nodes on human body, or on cloths or
Three challenges that have to address while designing reconfigurable antennas are: under the skin, establishing a wireless communication channel. Wearable antennas can be employed on
people of all ages, athletes and patients for a continuous monitoring of vital signs, oxygen level (Oximetry)
1) Reconfigurable property (e.g., frequency, radiation pattern, or polarization) needs to be modified. and stress level, among others.
2) Different radiating elements of the antennas structure reconfigured to achieve the required property.
3) Reconfiguration technique minimizes negative effects on the antenna radiation/ impedance Wearable Antenna Applications
characteristics.
ADVANTAGES:

There are several advantages in using reconfigurable antennas are

(1) Ability to support more than one wireless standard

a) Minimizes cost
b) Minimizes volume requirement
c) Simplifies integration
d) Good isolation between different wireless standards.
(2) Lower front end processing
a) No need for front end filtering
b) Good out-of-band rejection.
(3) Automated via a microcontroller or a Field Programmable Gate Array (FPGA)
a) Capability to adapt and learn.
(4) Multifunctional capabilities The advent of high efficiency miniature antennas is greatly enabling invasive/non-invasive devices
a) Change functionality as the mission changes; in consumer, healthcare and several military applications. A few examples of consumer-bound wearable
devices that use wearable antennas are smartwatches (integrated Bluetooth Antennas), smart glasses
b) Act as a single element or as array;
(integrated Wi-Fi, GPS and IR Antennas), Body worn action cameras (Wi-Fi and Bluetooth), and small
c) Provide narrow band or wideband operation.
sensor devices in sports shoes (Wi-Fi / Bluetooth) that can be paired with smartphones.
APPLICATIONS:

Reconfigurable antennas are required to cover different wireless services that are spanned over a wide
frequency range. The main applications are as follows:

(1) Frequency Reconfigurable Antennas for a Cognitive Radio System,

(2) Pattern Reconfigurable Antenna for MIMO Systems, and
(3) Mainly used for Satellite Applications.

7. WEARABLE ANTENNAS – Applications, Technologies, and their Impact on Human Body

The demands for wearable electronics and related technologies have grown tremendously in recent
years. Some of the key developments that accelerated this growth are miniaturization of wireless devices,
advent of high-speed wireless networks, availability of ultra-compact, low-power SoCs and ever-evolving
battery technologies. Wearable electronics find numerous applications these days, and most of these
applications use different types of antennas to sense, fetch, and exchange data wirelessly to and from a host A WBAN device ensures continuous health monitoring of an elderly person or a patient without
device or an IoT gateway. hindering his day-to-day activities. The implantable antenna sensors are also used for several biomedical
applications such as heart pacemakers, cochlear implants and intraocular implants among others. In
What is a Wearable Antenna? military, wearable antennas find several applications such as soldier’s live-location tracking, real-time
transmission of image and video for instant decentralized communications, etc. These antennas are also
Wearable antennas are designed to function while being worn. These antennas are commonly used
used for access / identity management, navigation, RFID applications, etc.
in wearable wireless communication and bio-medical RF systems. Wearable antennas are used within the
context of Wireless Body Area Networks (WBAN). In a WBAN, antenna is the key component that
Wearable Antenna Technologies

Compact antennas are an integrable part of wearable devices. Antennas are implemented based on
the bandwidth requirements, efficiency, electrical performance, polarization effects, size and and
application of the wearable device. Some of the commonly used antenna technologies include microstrip
antennas, printed dipole, monopole, printed loops, slot antennas, and planar inverted-Fs (PIFAs) antennas.

Microstrip Antennas

Printed Loop Antennas

Microstrip antennas are metallic strip or patch mounted on a substrate. Microstrip antennas are
simple and inexpensive to design and manufacture due to its 2-dimensional structure. They are easy to
fabricate using modern printed circuit technology. Microstrip antennas are of low profile and conformable
to planar & non-planar surfaces. These antennas allow linear & circular polarization. These antennas can
be easily mounted on rigid surfaces and are available in several forms such as rectangle, square, circle,
triangular, elliptical patterns. Most GPS devices use a Microstrip / patch Antenna.

Printed Dipole Antennas

The Printed Loop Antenna is made of single or multiple loops, in the shape of a circle, square or
any other closed geometric shape. The Loop Antenna has a dimension less than a wavelength, which
ensures the current throughout the loop remains in phase. These antennas are light in weight and have a
simple, compact structure. The Loop Antennas have relatively poor efficiency (very low value of radiation
resistance), which results in power loss in the form of heat due to the flow of high current. Two distinct
Loop Antennas are available – Large Loop Antennas and Small Loop Antennas. The Large Loop Antennas
are used for both transmission and reception whereas the Small Loop Antennas are majorly used for
Printed Dipole Antennas are popular due to its low profile, ease of fabrication, low-cost, reception. These antennas are ideal for small radio devices, and body worn communication systems
polarisation purity and wide frequency band coverage. Other major advantages of this antenna are its suitable for military applications.
structure (two arms printed on two sides of a dielectric substrate), large bandwidth and the single-ended
microstrip input. Dipole Antennas are relatively large in size, which makes it a little complex to implement Slot Antennas
in applications with space restrictions. In addition, the degradation of omnidirectional radiation patterns
and the likely need of a balun may pose challenges in small form-factor designs. Printed Dipole Antennas
are widely used in wireless communication and mmWave applications.

Monopole Antennas
Monopole Antennas are half the size of dipole antenna and are mostly mounted above a ground
plane. Due to its relatively smaller size, Monopole Antennas are ideal for applications where a smaller
antenna design is required. Monopole Antennas exhibit good radiation performance if they are placed over
High Impedance Surfaces (HIS). Monopole antennas are of low-profile, low-cost, and easy to fabricate,
which meets the basic requirements for wearable antennas. The simple, lightweight structure of Monopole
Antennas make them ideal to integrate into clothes.
 The Slot Antenna consists of a flat metal surface with fine narrow slots. The Slot Antennas are very
versatile and are used typically at frequencies between 300 MHz and 24 GHz. This antenna has
omnidirectional radiation patterns and a linear polarization. The slot size (length and width), shape and
material characteristics, determine the operating characteristics of the antenna. The simple structure and
flexible nature make it suitable for small form-factor wearable applications. Since the antenna can be easily
implemented on flexible surfaces like denim, it is ideal for medical and military applications. This antenna
provides effective wireless data transmission even when the human posture is changed.

Planar Inverted-F Antennas (PIFA)

The Federal Communication Commission (FCC) introduced Specific Absorption Rate (SAR) limits
for wireless devices to ensure acceptable radiations level in human body. The SAR limit is set to 1.6 W/kg
averaged over 1g of actual tissue, while the limit is set to 2W/kg averaged over 10g of actual tissue by the
Council of European Union. SAR is a parameter that is used to measure the rate at which RF
(radiofrequency) energy is absorbed by human tissues. SAR values ensure that any wearable device or
wireless smart gadget does not exceed the maximum permissible exposure levels.

Antennas without a ground plane exhibit higher SAR value since the SAR of on‐body antennas
relies on near‐field coupling to the body. Hence, many of the methods to reduce SAR value rely on altering
the ground plane. One of the techniques is to use Electromagnetic Bandgap (EBG) structures, or Periodic
Conductive Structures to filter electromagnetic waves within certain frequency bands. Similarly, using
High Impedance Surfaces (HIS) help block electromagnetic waves within a certain frequency band. High
T Impedance Surfaces placed behind wearable antennas increase the front-to-back radiation ratio reducing
the Specific Absorption Rate (SAR) in a human body. HIS also prevents propagating surface waves and
The Planar Inverted-F antenna (PIFA) finds majority of applications in portable smart devices. reflects electromagnetic waves with no phase reversal. Another effective method is to integrate Artificial
These antennas resemble an inverted ‘F’, as the names indicates. The low profile and omnidirectional Magnetic Conductor (AMC) ground plane, which serves as an isolator. The SAR reduction techniques such
pattern make the antenna popular among wearable product developers. PIFAs can also be printed like as integration of Ferrite Sheets and Metamaterials are also popular among antenna designers.
microstrips, a technology that allows antennas to be printed on the substrate or circuit board. The Planar
Inverted-F Antennas have compact size, dual-band functionality, and very good on-body results (good Effect of Human Body on Wearable Antenna
SAR values), making it suitable for body worn electronics devices. The human body also has some effects on wearable antenna when it is in close proximity. The
lossy, high dielectric constant characteristics of human body may result in the variation of input
Impact of Human body on Wearable Antenna and Vice-versa impedance, frequency shifts and reduced efficiency of Antenna. It disturbs the communication link
between antenna and the external host device.
In WBAN, the close proximity of human body poses significant challenges to the wearable antennas
and vice-versa.
Based on the application, various techniques can be adopted to address the effect of human body on
antenna. One of the key aspects is the placement and orientation of antenna. An ideal position/orientation
o Impact of electromagnetic radiations on human body and of the antenna, location and distance from the body significantly reduce the impact of human body on
o The reduced efficiency of the antenna due to electromagnetic immersion in body tissue, antennas. For high performance devices, automatic tunable circuits and reconfigurable antennas can also be
fragmentation of radiation pattern, impedance variations and frequency detuning. implemented. Antenna designers also implement EBG ground plane and High Impedance Surfaces to
address the impact of body on wearable antennas.

These factors call for special attention during antenna design for wearable devices. Developers should
focus on structural deformation, accuracy and precision in antenna fabrication methods and size during
wearable antenna design.

Effects of Antenna on Human Body

Unlike ionizing radiations, the non-ionizing radiations such as microwaves, visible light or sound
waves may not have sufficient energy to ionize atoms or molecules in a body, however this energy can
increase the cell temperature by moving atoms or make them vibrate. This rise in temperature due to
dielectric heating, a thermal effect due to microwave radiation when a dielectric material is heated by
rotations of polar molecules induced by the electromagnetic field, may have severe effects in human
tissues.
8. MEASUREMENT OF GAIN:

DEFINITION:

Gain of an antenna (in a given direction) is defined as “the ratio of the intensity, in a given direction, to the
radiation intensity that would be obtained if the power accepted by the antenna were radiated isotropically.

In most cases we deal with relative gain, which is defined as “the ratio of the power gain in a given
direction to the power gain of a reference antenna in its referenced direction.” 1)At first, the standard antenna is connected to the receiver (Rx) with the help of switch “S” and the
𝑅𝑎𝑑𝑖𝑎𝑡𝑖𝑜𝑛 𝑖𝑛𝑡𝑒𝑛𝑠𝑖𝑡𝑦 𝑖𝑛 𝑔𝑖𝑣𝑒𝑛 𝑑𝑖𝑟𝑒𝑐𝑡𝑖𝑜𝑛 antenna is aimed at secondary antenna in the direction of maximum signal intensity. The input to the
𝐺= transmitting antenna (secondary antenna) is adjusted to a convenient level and corresponding reading at
𝑅𝑎𝑑𝑖𝑎𝑡𝑖𝑜𝑛 𝑖𝑛𝑡𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝑎𝑛𝑡𝑒𝑛𝑛𝑎 𝑟𝑎𝑑𝑖𝑎𝑡𝑒𝑑 𝑖𝑠𝑜𝑡𝑟𝑜𝑝ℎ𝑖𝑐𝑎𝑙𝑙𝑦
the receiver (primary antenna circuit) is recorded. The attenuator dial setting and the power bridge
reading are also recorded. Say it is 𝑊1 and 𝑃1 respectively.
2)Now connect the subject antenna whose gain is to be measured in place of standard gain antenna. The
attenuator dial is adjusted such that receiver indicates the same previous reading as was with standard
gain antenna. Let the attenuator dial setting reading be 𝑊2 and the power bridge reading 𝑃2 .
We know that, G = KD
Now two cases arise.
VARIOUS METHOD:
Case 1. When 𝑃1 = 𝑃2 .
1) Direct comparison method (Gain transfer, Gain comparison) (HF measurement)
If 𝑃1 = 𝑃2 , then no correction need to be applied and the gain of the subject antenna under measurement
2) Use and calibration of gain “standard”
w.r.t standard gain antenna is given by
(a) Absolute gain of identical antennas (two antenna method) (first method)
(b) Three arbitrary antennas (second method) 𝑾𝟐
Power Gain = (𝑮𝒑 ) = ( )
𝑾𝟏
1) DIRECT COMPARISON METHOD:
where 𝑊2 and 𝑊1 being relative power levels. Expressing equation in decibels, we have
As defined above, gain is a comparison of two antennas and hence gain measurement by comparison is done. At
higher frequencies the comparison method is one which is commonly used. In comparison method, log 𝐺𝑝 = log𝑊2 – log 𝑊1
measurement of gain is done by comparing the signal strengths transmitted or received with the unknown-gain
or 𝑮𝒑 (𝒅𝑩) = 𝑾𝟐 (𝒅𝑩) – 𝑾𝟏 (𝒅𝑩)
antenna and a standard gain antenna. A standard gain antenna is that antenna whose gain is accurately known
so that it can used in measurement of other antenna. Electromagnetic horn antenna at microwave frequencies is Case 2. When 𝑃1 ≠ 𝑃2 .
mostly used as standard gain antenna. The secondary antenna may be an arbitrary transmitting antenna and it is
not necessary to know its gain. In place of primary antenna there will be two antennas one the subject antenna If, however, the power level is changed during the measurements and 𝑃1 ≠ 𝑃2 , then actual power gain
under test and the other a standard antenna at a considerable distance so that coupling or interaction between of subject antenna can be obtained by multiplying 𝐺𝑝 with ratio 𝑃1 /𝑃2 . Let
two antennas could be avoided. The experimental set up is like that of pattern measurement, shown in Fig. The
𝑃1
distance between primary and secondary antennas must satisfy the condition r ≥ 2𝐷2 /λ and reflections between 𝑃2
= P (say)
them should be minimised to the extent possible.
Therefore 10log 𝑃1 /𝑃2 = P (dB)
The attenuator pad is inserted to the input of receiver, in order that standard and subject antennas work into a
𝑃1 𝑊 𝑃
matched load and receiver look into a constant matched load. It is important that the secondary antenna and Hence, Power gain = G = 𝐺𝑝 ×
𝑃2
= 𝑊2 . 𝑃1
1 2
associated transmitter should be stable in respect of frequency. There should not be any variation in the
frequency of radiated power in the direction of primary antenna throughout the measurement. For this purpose, G = 𝑮𝒑 . 𝑷
the power bridge or other power level indicating device is used as a check on the stability of the transmitter.
Following procedure is adopted for gain measurement: or 𝐺(𝑑𝐵) = 𝐺𝑝 (𝑑𝐵)+𝑃(𝑑𝐵)
 G(𝒅𝑩) = 𝑾𝟐 (𝒅𝑩) – 𝑾𝟏 (𝒅𝑩) + 𝑷(𝒅𝑩) If the effects of direct and indirect rays are considered, then it can be shown that

The comparison method basically compares the power gains of the antenna but directivities may
𝟒 𝝅 𝒓 𝑾𝒓
also be determined, if antenna radiation efficiency factors are known. 𝑮𝒐 = √
𝝀 𝑭 𝑾𝒕
2) USE OF CALIBRATION OF GAIN “STANDARD”
where, F = propagation constant and arise due to the interference between direct and indirect rays.
Just, standard gain antenna was used in the measurement of gain in comparison method. The standard
gain antenna is nothing but an antenna with known or calibrated gain, which may be any antenna with Procedure:
which subject antenna could be compared. There are two methods with which the calibration of standard
 Orient the both antennas for maximum signal. The input to the transmitting antenna is adjusted to
gain antenna can be done. In first method use of two identical antennas are made whereas in the second
an appropriate level and corresponding receiver reading level is recorded. The attenuator dial
method three arbitrary antennas are used. The gain can be determined without its prior knowledge.
setting and the power bridge reading are recorded, say they are 𝑊𝑡 and 𝑃𝑡1 respectively.
(a) First method (or Absolute Gain of Identical Identification): The identical antennas having  Now the transmitter is disconnected from the antenna and is connected to the receiver through
distance r is shown in figure. pads. The attenuation dial is adjusted until the receiver reads the same previous levels. The
attenuator dial setting and the power bridge reading are noted, say 𝑊𝑟 and 𝑃𝑡2 .
 If 𝑃𝑡1 = 𝑃𝑡2 , then no correction would be needed due to the transmitter levels fluctuations and
gain is calculated. However, if drifting of power is involved, then correction would be needed.
(b) Three Antenna Method:
This method consists of three unknown antennas.

1. Using antenna 1 as transmitter and antenna 2 as receiver, the received power W1 is measured. Let
the transmitter power be 𝑃1 .
2. Replacing antenna 2 by antenna 3, the received power 𝑊2 is measured for the same transmitted
Let , Wt = Transmitted Power power (𝑃2 = 𝑃1 ).
3. When antenna 2 is used as transmitter and antenna 3 is used as receiver, receiver power 𝑊3 is
Wr = Received Power measured. Let the transmitter power be 𝑃3 . Then we have
Aet , Aer = Effective apertures of transmitting and receiving antennas. 𝑊1 4𝜋𝑅 2
𝐺1 𝐺2 = ( ) ( )
𝜆2
𝑃1 𝜆
Since antennas are identical so, Aet = Aer = 4𝜋 𝐺0
𝑊2 4𝜋𝑅 2
𝐺1 𝐺3 = ( )( )
From the Frii's transmission equation, 𝑃2 𝜆
𝑊𝑟 𝐴𝑒𝑟 𝐴𝑒𝑡 𝜆2 𝜆2 1
= = [ 4𝜋 𝐺0 ] [ 4𝜋 𝐺0 ] 𝜆2 𝑊3 4𝜋𝑅 2
𝑊𝑡 𝜆2 𝑟2 𝑟2 𝐺2 𝐺3 = ( ) ( )
𝑃3 𝜆
where, r = Distance between transmitting and receiving antenna (far field distance).
The above expression in dB are given
𝜆 = wavelength
𝑊1 4𝜋𝑅
𝐺1 + 𝐺2 = 10𝑙𝑜𝑔 ( ) + 20𝑙𝑜𝑔 ( )
𝑊𝑟 𝐺𝑜 𝜆 2 𝑃1 𝜆
=( )
𝑊𝑡 4𝜋𝑟 𝑊2 4𝜋𝑅
𝐺1 + 𝐺3 = 10𝑙𝑜𝑔 ( ) + 20𝑙𝑜𝑔 ( )
𝑃2 𝜆
𝑊𝑟 𝐺𝑜 𝜆
√ = 𝑊3 4𝜋𝑅
𝑊𝑡 4𝜋𝑟
𝐺2 + 𝐺3 = 10𝑙𝑜𝑔 ( ) + 20𝑙𝑜𝑔 ( )
𝑃3 𝜆
𝟒 𝝅 𝒓 𝑾𝒓 𝑊 𝑊 𝑊
𝑮𝒐 = √ Knowing ( 𝑃 1 ) , ( 𝑃 2 ) , ( 𝑃 3 ) , λ and 𝑅 , 𝐺1 , 𝐺2 and 𝐺3 are obtained.
𝝀 𝑾𝒕 1 2 3
9. MEASUREMENT OF RADIATION PATTERN: However, pattern in only one plane provides sufficient information. For example, horizontal
plane pattern is of interest in earth to earth communication and broadcasting services, whereas
Radiation pattern of a transmitting antenna is described as the field strength or power density at a the vertical plane patterns are of interest in communication system between earth and space, or
fixed distance from the antennas as a function of direction. The radiation pattern of an antenna is vice versa. For example, radio astronomy, radar, and space communications.
a three dimensional figure and it needs measurement of field intensity all over the spatial angles.
Hence for radiation pattern of antenna under test, the various spatial (space) angles must be
FUNDAMENTAL PROCEDURE:
specified. The test antenna is assumed to be placed at the origin of spherical co-ordinate. XY It is always necessary for measurement of radiation pattern to have two antennas. Out of two
plane is horizontal plane and XZ plane is vertical plane. The radiation pattern is accordingly antennas, one antenna under test and the other at some distance away for illuminating the former.
taken either along latitude (or polar angle θ) as a function of azimuth angle (ϕ) or along azimuth The one will be transmitting and the other will be receiving. By reciprocity theorem measured
angle (ϕ) as a function of polar angle (θ) depending upon the application and information radiation pattern is same for both whether antenna id transmitting or receiving. Sometimes,
needed. discussions to follow, the antenna under test may also be called as primary antenna and the other
as secondary antenna irrespective which antennas receives or transmits.

There are two procedures for radiation pattern measurements in a particular plane:
a) Low frequency measurement: The primary antenna may be kept stationary whereas the
secondary antenna is transported around along a circular path at a constant distance. If the
secondary antenna is directional, then it is kept aimed at primary antenna so that only primary
antenna pattern will affect the result. Usually the primary antenna is transmitting, in this
procedure, although is not a necessary condition. The field strength readings and direction of the
secondary antenna w.r.t primary antenna are recorded along the circle at different points. From
the readings of field strengths at a number of points the plot of radiation pattern of primary
antenna is made either in rectangular form or in polar form.
b) High frequency measurement: Both antennas are kept in fixed positions having a suitable
spacing between them and secondary antenna beam aimed at primary antenna. Now the primary
antenna is rotated about a vertical axis (assuming both antennas in horizontal plane). Usually, in
this procedure, the secondary antenna is transmitting, so that field strength reading and direction
of the primary antenna w.r.t. secondary antenna can be made. The readings are taken at a number
of points, by stopping the rotation of primary antenna for recording the readings or a continuous
reading can be also be taken if “pattern recorder” is available. The rotation of antenna and chart
For most antennas, it is generally necessary to take radiation pattern in XY plane (Horizontal are synchronized with motor-generator units.
plane) and XZ plane (vertical plane). For horizontal antenna, two patterns are sufficient e.g.
In this method use of pattern recorder can be made and the distance between primary and
i. The ϕ component of electric field (horizontal) is measured as a function of ϕ in XY secondary antenna is fixed. Unlike primary antenna in which the said distance can be changed
plane (θ=90°). It is represented as 𝐸𝜙 (θ = 90°,𝜙 ) and is called as E-plane pattern. also. Usually, first procedure is followed in low frequency antenna measurements and the second
ii. The ϕ component of the field is measured as a function of θ in the x-z plane (ϕ = 0°). It procedure at high frequency antenna measurements.
is represented as 𝐸𝜙 (θ, ϕ = 0°) and is called as H-plane pattern. These two patterns
bisect the major lobe in mutually perpendicular planes and hence provide enough ARRANGEMENTS FOR RADIATION PATTERN MEASUREMENTS:
information for a number of applications. In this simplest form the arrangement for radiation pattern measurement is shown in Fig. having
Similarly for vertically polarized antennas: a transmitting antenna (primary antenna) and antenna under test (secondary antenna), a mount
for rotating the primary antenna (i.e. shaft for rotating), detector (i.e. receiver) and indicator for
a) The θ component of electric field is measured as function of ϕ in XY plane (θ = 90°).It indicating the relative magnitude of received field. The equipment may be entirely automatic or a
is represented as Ɵ 𝐸𝜃 (θ = 90°, ϕ) is called as H-plane pattern. point to point plot.
b) The θ component of the field is measured as function of ϕ in XZ plane (ϕ= 0°). It is It is usual to operate the antennas under test as receiver, placing it under proper, illumination by
represented as 𝐸𝜃 Ɵ (θ, ϕ = 0°), and is called as E-plane pattern. primary antenna. The primary antenna is fixed and the secondary is rotated on a vertical axis by
antenna support shaft. For 𝐸𝜙 (θ = 90°, ϕ) pattern measurement the antenna support shaft is
For circularly or elliptically polarized antenna, measurement of these four patterns would be
rotated with the both antenna horizontal and for 𝐸𝜙 (θ, ϕ = 0), pattern the antenna support shaft
needed. It is rather impossible to plot three dimensional patterns on a plane sheet of paper.
is rotated with both antenna vertical. Indictation may be on a direct reading meter calibrated in
 field intensity. If large number of patterns are to be taken, “automatic pattern recorder” may also The value of r may be calculated in terms of receiving aperture d and distance r. From fig.,
be used, which is commercially available.
𝑑
(𝑟 + 𝛿)2 = ( )2 + 𝑟 2 {since r ≥ 𝛿, 𝛿 ≤ 𝑑; 𝛿 = phase difference error
2

𝑑 𝑑
or 𝑟 2 +𝛿 2 +2𝛿𝑟 = + 𝑟 2 or 2𝛿𝑟 = therefore 𝛿 2 can be neglected
4 4

𝒅𝟐
or r = 𝟖𝜹

This shows that minimum distance requirements depend on receiving antenna aperture d and wavelength
λ. The value of ‘r’ may be calculated by replacing ‘δ’ width allowable phase difference (i.e. δ = λ/16
typical). Reduction in ‘r’ tends broader radiation pattern, higher minor lobes and vice-versa.

UNIFORM ILLUMINATION REQUIREMENT:

The other requirement for an accurate field pattern is the primary antenna (transmitting) should produce
a plane wave of uniform amplitude and phase over the distance atleast equal to r. The interference
between direct rays and indirect rays (i.e. waves reflected from ground) should be avoided as far as
possible. Besides reflections from surrounding objects like buildings, trees, etc. should be avoided. For
example, test should be conducted in open plain area and antenna should be directional, installed on
DISTANCE REQUIREMENT: higher towers or tops of high buildings as illustrated in figure.

In order to obtain accurate far-field or Fraunhofer radiation pattern, the distance between primary and
secondary antenna must be large. If the distance between the two antennas is very much small, then near
field or Fresnel pattern is obtained. For accurate, Fraunhofer pattern measurements the secondary
antenna (i.e.antenna under test) should be illuminated by a plane wavefront and plane wavefront is
obtained only at infinite distance. Thus the limit specified is that the phase difference between the centre
and edge of the antenna under test should not exceed λ/16.

10. MEASUREMENT OF VOLTAGE STANDING WAVE RATIO (VSWR) (OR) SLOTTED LINE
METHOD FOR IMPEDANCE MEASUREMENT:

Slotted line method of impedance measurement is based on input impedance of the traveling waves, which is
determined form the Voltage or current Standing Wave Ratio (VSWR), the spacing between the voltage or
current minimum and the reference point at which the impedance is desired.

Under this condition, the distance between primary and secondary antenna should be

𝟐 𝒅𝟐
𝒓≥
𝝀
where, d = maximum linear dimension of either antenna

λ = wavelength

r = distance between transmitter and receiver

  A transmission line system is getting terminated using an antenna. A part of transmission line is replaced
by axial slotted line which consists of a length of transmission line i.e., coaxial cable with an axial slot,
along which moves a travelling carriage carrying a probe.
 A voltage (or power) measuring device may be a crystal detector and a micro-ammeter. A signal source
is connected to the left end and the right end is connected to the antenna to measure its impedance.
 The standing wave pattern is obtained by moving the probe along the carriage and observing the
resulting variation in the crystal detector output. The standing wave indicator is a voltage measuring
device.
 The probe in the slotted coaxial cable is moved and two consecutive points of Vmax and Vmin are noted.
Their ratio will give the VSWR and hence the line impedance.
 The high or low pass filter is used to avoid harmonics spurious signal sources and unwanted signals
between slotted line and signal source. The modulation source is for modulating signal with a square or
pulse.
 VSWR and  are used for measurement of load impedance by the slotted line method. When a load Z L
not equal to Z0, is connected to the transmission line, standing waves are produced.

By inserting a slotted line system in the line, standing waves can be traced by moving the carriage with a
tunable probe detector along the line. VSWR can be measured by detecting Vmax and Vmin in the VSWR meter.
𝑽𝒎𝒂𝒙 𝟏+
Voltage standing ratio (S) = = 𝟏−
𝑽𝒎𝒊𝒏
The ratio of the electrical field strength of reflected and incident wave is called reflection coefficient and it is
expressed in terms of impedance as,

𝐸𝑅 𝑍−𝑍𝑜
= = 𝑍+𝑍𝑜
𝐸𝐼
where, Z is the impedance at a point and
Zo is characteristic impedance.

The reflection coefficient in terms of VSWR,

𝑽𝑺𝑾𝑹−𝟏 𝑺−𝟏
| |= 𝑽𝑺𝑾𝑹+𝟏 = 𝑺+𝟏

𝟒𝐝
Angle of reflection coefficient, 𝛉 = 𝛑 − 𝟐𝛃𝐝 = 𝛑 (𝟏 − 𝛌
)

where, β = 2π/λ

d = spacing between load end and first minimum