UNIT-4

Long Wire, V and Rhombic Antenna, Yagi-Uda Antenna, Turnstile Antenna, Helical
Antenna- Axial Mode Helix, Normal Mode Helix, Biconical Antenna, Log Periodic
Dipole Array, Spiral Antenna, Microstrip Patch Antennas. Antenna Measurements:
Radiation Pattern Measurement, Gain and Directivity.
Applications: Design of a 3-element Yagi guda Antenna for given specifications
 Long-wire Antennas
• Long wire antennas are formed by using a number of dipoles. The length of the wire in
these type of antennas is n times λ/2
𝜆
𝐿=𝑛
2
• Where, L is the length of the antenna, n is the number of elements, λ is the wavelength As
‘n’ increases, the directional properties also increase.
• Long wire antennas are further classified as V antennas, inverted v antennas, and rhombic
antennas etc.
• Long wire antennas are divided into two types namely − Resonant Antennas and Non-
resonant Antennas.
Long wire resonant antenna:
• The resonant antennas are periodic in nature. They are also called as bi-directional
travelling wave antennas, as the radiated wave moves in two directions, which means
both incident and reflected waves occur here. In these antennas, the length of the
antenna and frequency are proportional to each other. Resonant Antennas are those for
which a sharp peak in the radiated power is intercepted by the antenna at certain
frequency, to form a standing wave. The radiation pattern of the radiated wave is not
matched with the load impedance in this type of antenna.
• The figure below represents the resonant type of antenna:

• Here the length of the antenna is directly proportional to the operating frequency.
• Radiation Pattern
• The figure below represents the radiation pattern of long wire resonant
antenna:

• Here the bidirectional nature of the radiation pattern shows the existence of
standing waves.
Long wire non-resonant antenna:
• Non-resonant Antennas are those for which resonant frequency does not occur.
The wave moves in forward direction and hence do not form a standing wave.
The radiation pattern of the radiated wave matches with the load impedance
in the non-resonant antennas.
• These non-resonant antennas are non-periodic in nature. They are also called as
Unidirectional travelling wave antennas, as the radiated wave moves in
forward direction only, which means that only incident wave is present.
• The frequency and length are inversely proportional to each other.
• The figure below represents the non-resonant type of antenna:
Radiation Pattern

• Now see the radiation pattern of the non-resonant type of antenna:

• The unidirectional radiation pattern shows that due to the properly terminated
load, standing waves do not exist in this type of antenna.
Advantages
• Long wire antennas are low-cost antennas.
• It offers ease of construction.
• The gain and directivity are improved with the use of a long wire antenna.
• It offers multiband operation.
Disadvantages
• These antennas are more prone to local interference.
• It requires proper matching system so as to have better results.
• As its orientation is considered with respect ground; thus, it needs proper connectivity
from the ground all the time.
Applications of Long Wire Antenna
• Thus from this discussion, it is clear that such long antennas can be used in point-to-
point long-distance communication due to their structural simplicity such as in sky
wave propagation.
 V-Antenna
• Definition: V antenna is a type of long wire antenna whose structure is designed in
the form of a V.
• This antenna is formed by arranging the long wire in a V-shaped pattern. The end
wires are called as legs. The frequency range of operation of V-antenna is
around 3 to 30 MHz. This antenna works in high frequency range.
• It is generally a resonant antenna that provides bidirectional radiation. However,
proper impedance matching leads to provide a non-resonant antenna also that
produces unidirectional radiation.
 Construction & Working of V-Antennas
• Two long wires are connected in the form of a horizontal V, fed at the apex, It is
denoted by 𝜃0 as shown in fig
• Here 𝜃0 is called as included angle.
• If the angle between the two sides of the V is twice the angle that the cone of
maximum radiation of each wire makes with its axis, then the two cones will add up
in the direction of the line bisecting the apex angle of V, and produce a maximum
lobe of radiation. The two long wires are excited with 180˚ out of phase. As the
length of these wires increases, the gain and directivity also increases.
• The gain achieved by V-antenna is higher than normal single long wire antenna.
The gain in this V-formation is nearly twice compared to the single long wire
antenna, which has a length equal to the legs of V-antenna.

• In case of these antennas, if the length of each leg is 8λ, then it can provide a gain
of about 12 dB. It is to be noted that the apex angle shows dependency on the
length of the leg. Thus, for the length of each leg of about 2λ to 8λ, the apex
changes between 36° to 72°.

• The radiation pattern of a V-antenna can be unidirectional or bidirectional .The

radiation obtained on each transmission line is added to obtain the resultant
radiation pattern.
 Radiation Pattern of V Antenna
To achieve the unidirectional characteristics, the wires of the V antenna must be non
resonant .This can be accomplished by minimizing reflections from the ends of the wire.
One way of terminating the V antenna will be to attach a load, usually a resistor equal in
value to the characteristic impedance of the leg.
• The figure given is representing the radiation pattern
for the resonant type of V antenna:

• Now, have a look at the radiation pattern of the non-

resonant type of antenna, showing unidirectional
radiation pattern:
Advantages of V-antenna
• Construction is simple
• High gain
• Low manufacturing cost

Disadvantages of V-antenna
• Standing waves are formed
• The minor lobes occurred are also strong
• Used only for fixed frequency operations

Applications of V-antenna
• Used for commercial purposes
• Used in radio communications
Inverted V antenna
• The operating frequency of V antenna is limited. This can be modified by using another
antenna, which is a non-resonant antenna or a travelling wave antenna. A travelling wave
antenna produces no standing wave. The radiation pattern of inverted V-antenna is uni-
directional pattern.
• The frequency range of operation of an inverted vee antenna (or V-antenna) is around 3 to 30
MHz. This antenna works in high frequency range.
Construction & Working of Inverted V-Antenna
• The maximum radiation for an inverted V-antenna is at its center. The antenna is placed in the
shape of an inverted V, with its two transmission lines or legs bent towards the ground making
120° or 90° angle between them.
• The angle made by one of the legs with the axis of
the antenna, is known as the tilt angle and is denoted by 𝝓.

• The terminating resistance is about 400 W.

Advantages of inverted V-antenna
• Occupies less horizontal place
• No standing waves are formed
• High gain
Disadvantages of inverted V-antenna
• It has considerable undesired minor lobes
• Minor lobes create horizontally polarized waves
Applications of inverted V-antenna
• Used in tuned circuit applications
• Used in radio communications
• Used in commercial applications
Rhombic antenna
• Rhombic Antenna is a combination of two V-antennas. It is an equilateral
parallelogram shaped antenna. Generally, it has two opposite acute angles. The tilt
angle, rhombic antenna works under the principle of 𝝓 is approximately equal to 90°
minus the angle of major lobe(𝜃0 ) travelling wave radiator. It is arranged in the form
of a rhombus or diamond shape and suspended horizontally above the surface of the
earth.
• The frequency range of operation of a Rhombic antenna is around 3MHz to
300MHz. This antenna works in HF and VHF ranges.
Construction of Rhombic Antenna
• Rhombic antenna can be regarded as two V-shaped antennas connected end-to-end to
form obtuse angles. Due to its simplicity and ease of construction, it has many uses −
• In HF transmission and reception
• Commercial point-to-point communication
• The construction of the rhombic antenna is in the form a rhombus, as shown in the
figure.
 The two sides of rhombus are considered as the conductors of a two-wire
transmission line. When this system is properly designed, there is a concentration of
radiation along the main axis of radiation. In practice, half of the power is dissipated
in the terminating resistance of the antenna. The rest of the power is radiated. The
wasted power contributes to the minor lobes.

• The length of equal radiators vary from 2 to 8 𝜆.

• The tilt angle, 𝜙 varies between 45𝑜 𝑎𝑛𝑑 75𝑜 .
• The terminating resistance is about 800Ω.
• The input impedance of Rhombic antenna lies
between 650 to 700Ω.
• The directivity of Rhombic antenna varies
between 20 and 90.
• The maximum gain from a rhombic antenna is along the direction of the main axis,
which passes through the feed point to terminate in free space. The polarization
obtained from a horizontal rhombic antenna is in the plane of rhombus, which is
horizontal.
• The resultant pattern is the cumulative effect of the radiation at all four legs of the
antenna. This pattern is uni-directional, while it can be made bi-directional by
removing the terminating resistance.
• By properly selecting the tilt angle, the rhombic antenna will give additive effect of
radiation pattern of each long wire antenna
• The radiation mechanism basically depends on three factors:
• Tilt angle (𝝓)
• Height above the ground (h)
• Length of the leg (L)
• Due to ground effect, the maximum radiation is elevated about an angel (𝛽)
The antenna is usually
terminated at one end in a
resistor, usually about 600–
800 ohms, in order to reduce
reflections.
Design equations
Let us assume that it is desired to design a rhombus such that the maximum of the main
lobe of the pattern, in a plane which bisects the V of the rhombus, is directed at an angle
𝛽above the ground plane. The design can be optimized if the height h is selected
according to
𝛽
Tilt angle, 𝝓 = 90– elevation angle
with m = 1 representing the minimum height.
The minimum optimum length of each leg of a symmetrical rhombus must be selected
according to
𝛽

The best choice for the included angle of the rhombus is selected to satisfy
𝛽

The main disadvantage of rhombic antenna is that the portions of the radiation, which do
not combine with the main lobe, result in considerable side lobes having both horizontal
and vertical polarization.
Advantages of Rhombic antenna
• Input impedance and radiation pattern are relatively constant
• Multiple rhombic antennas can be connected
• Simple and effective transmission
Disadvantages of Rhombic antenna
• Wastage of power in terminating resistor
• Requirement of large space
• Reduced transmission efficiency
Applications of Rhombic antenna
• Used in HF communications
• Used in Long distance sky wave propagations
• Used in point-to-point communications
 Helical Antenna
• A helical antenna generally consists of a wire wound in the form
of a helix placed above a ground plane.
• Helical antenna is an example of wire antenna and itself forms the
shape of a helix (corkscrew). This is a broadband VHF and UHF
antenna.
• The antenna is fed by a coaxial transmission line with the center
conductor attached to the helical wire and the outer conductor to
the ground plane.
• Typically the diameter of the ground plane is greater than 3λ/4.
• The helix antenna is a travelling wave antenna, which means the
current travels along the antenna and the phase varies continuously.
Frequency Range
• The frequency range of operation of helical antenna is around 30MHz
to 3GHz. This antenna works in VHF and UHF ranges.
• The unwrapped length L, of one turn of the helix is given
by
𝐿 = 𝑆2 + 𝐶2
• Another important parameter of the helix is the pitch
angle α, which is the angle between the tangent to the
helix and the plane perpendicular to the axis of the helix.
The pitch angle can be related to S and C by the following
relation
tan 𝛼 = 𝑆/𝐶 = 𝑆/𝜋𝐷

• If α = 0◦, the helix reduces to a loop of N turns and for α =

90◦, the helix is the same as a straight wire of length NL
• The radiation characteristics of the antenna can be varied by controlling the
size of its geometrical properties compared to the wavelength.
• The input impedance is critically dependent upon the pitch angle and the size
of the conducting wire, especially near the feed point, and it can be adjusted by
controlling their values.
• The general polarization of the antenna is elliptical. However circular and
linear polarizations can be achieved over different frequency ranges.
• The helical antenna can operate in many modes; however the two principal
ones are the normal (broadside) and the axial (end-fire) modes.
Normal Mode Helix
• A helical antenna operates in the normal mode if its
dimensions are small compared to the wavelength, that
is D <<λ and NL < λ. The radiation of a helical antenna
operating in the normal mode, it radiates minimum along
the axis of the helix and is maximum in a plane normal to
the axis.
• The radiation pattern is very similar to that of a dipole of
length l < λ or a loop of radius a < λ. It is observed that if
we choose 𝐶 = √2𝑆𝜆 or tan 𝛼 = [(𝜋𝐷)/(2𝜆)], the
helical antenna radiates circularly polarized waves.
• The geometry of the helix reduces to a loop of diameter D when the
pitch angle approaches zero and to a linear wire of length L when it
approaches 90◦. Since the limiting geometries of the helix are a loop
and a dipole, the far field radiated by a small helix in the normal
mode can be described in terms of Eθ and Eφ components of the
dipole and loop, respectively.
𝑗60𝜋𝐼𝑠𝑖𝑛𝜃 𝑆
𝐸𝜃 = and
𝑟 𝜆
120𝜋2𝐼𝑠𝑖𝑛𝜃 𝐴
𝐸𝜑 =
𝑟 𝜆2
• Where , A=area of loop, I=retarded current, r=distance, 𝜆=wavelength

• In normal mode N small loops and N short dipoles connected

together in series. The fields are obtained by superposition of the
fields from these elemental radiators. The planes of the loops are
parallel to each other and perpendicular to the axes of the vertical
dipoles. The axes of the loops and dipoles coincide with the axis
of the helix.
• The ratio of the magnitudes of the Eθ and Eφ components is defined here as the axial
ratio (AR), and it is given by

• By varying the D and/or S the axial ratio attains values of 0 ≤ AR≤∞. The value of AR =
0 is a special case and occurs when Eθ = 0 leading to a linearly polarized wave of
horizontal polarization (the helix is a loop). When AR=∞, Eφ = 0 and the radiated wave
is linearly polarized with vertical polarization (the helix is a vertical dipole). Another
special case is the one when AR is unity (AR = 1) and occurs when

When the dimensional parameters of the helix satisfy the above relation, the
radiated field is circularly polarized in all directions

• The radiated fields of an axial mode helical antenna are highly directional and circularly polarized.
• Because of the critical dependence of its radiation characteristics on its
geometrical dimensions, which must be very small compared to the
wavelength, this mode of operation is very narrow in bandwidth and its
radiation efficiency is very small. Practically this mode of operation is limited,
and it is seldom utilized.
 Axial Mode Helix
• A more practical mode of operation, which can be generated with great ease, is the
axial or end-fire mode. In this mode of operation, there is only one major lobe and
its maximum radiation intensity is along the axis of the helix.
• To excite this mode, the diameter D and spacing S must be large fractions of the
wavelength.
• In the axial mode of operation, a helical antenna radiates maximum energy along its
axis.
• C = λ and S = λ/4.
• The pitch angle, α, of the helical
antenna operating in the axial mode
is usually between 12◦ and 15◦.
Design equations of axial mode helical antenna

The relations are valid for 12◦ < α < 15◦, 3/4 < C/λ < 4/3(with
C/λ0 = 1 near Optimum), and N >3. a ground plane, whose
diameter is at least λ0/2
Problem

• Consider an 8-turn helical antenna with S = 0.25λ and C = 1λ. The pitch angle
is found to be 14.04◦. This represents a helical antenna operating in the axial
mode. The directivity of the helical antenna can be computed as 30 or 14.77
dB.
Advantages
• The following are the advantages of Helical antenna −
• Simple design
• Highest directivity
• Wider bandwidth
• Can achieve circular polarization
• Can be used at HF & VHF bands also
Disadvantages
• The following are the disadvantages of Helical antenna −
• Antenna is larger and requires more space
• Efficiency decreases with number of turns
 Applications
• Because an elliptically polarized antenna can be represented as the sum of two
orthogonal linear components in time-phase quadrature, a helix can always receive a
signal transmitted from a rotating linearly polarized antenna. Therefore helices are
usually positioned on the ground for space telemetry applications of satellites, space
probes, and ballistic missiles to transmit or receive signals that have undergone
Faraday rotation by traveling through the ionosphere.

The following are the applications of Helical antenna −

• A single helical antenna or its array is used to transmit and receive VHF signals
• Frequently used for satellite and space probe communications
• Used for telemetry links with ballistic missiles and satellites at Earth stations
• Used to establish communications between the moon and the Earth
• Applications in radio astronomy
 Yagi-Uda antenna
• The original design and operating principles of this radiator were first described
in Japaneseby S. Uda, one of Professor Uda’s colleagues, H. Yagi, described the
operation of the same radiator in English. it should be called a Yagi-Uda antenna.
• The Yagi-Uda antennas typically operate in the HF to UHF bands (about 3 MHz
to 3 GHz)
Parasitic Array
• Array antennas can be used to increase directivity.
• Parasitic array does not require a direct connection to each
element by a feed network.
• The parasite elements acquire their excitation from near field
coupling by the driven element.
• A Yagi-Uda antenna is a linear array of parallel dipoles.
• The basic Yagi unit consists of three elements:
1. Driver or driven element
2. Reflector
3. Director
• The structure contains one driven element and a reflector
while directors can be more than one.
• Develops an end fire radiation pattern.
• There are three types of element within a Yagi antenna:
1. Driven element: The driven element is the Yagi antenna element to which
power is applied. It is normally a half wave dipole or often a folded dipole.
2. Reflector : The Yagi antenna will generally only have one reflector. This is
behind the main driven element, i.e. the side away from the direction of
maximum sensitivity. Further reflectors behind the first one add little to the
performance. However many designs use reflectors consisting of a reflecting
plate, or a series of parallel rods simulating a reflecting plate. This gives a
slight improvement in performance, reducing the level of radiation or pick-
up from behind the antenna, i.e. in the backwards direction. Typically a
reflector will add around 4 or 5 dB of gain in the forward direction.
3. Director: The director or directors are placed in front of the driven element,
i.e. in the direction of maximum sensitivity. Typically each director will add
around 1 dB of gain in the forward direction, although this level reduces as
the number of directors increases.
 • The reflector is present at one of the ends of the metallic rod and has length
around, 5% greater than the length of the driven element. While the directors are
almost 5% shorter than the driven element (i.e., λ/2 at the resonant frequency) and
are placed at the other side of the dipole as these are used to provide maximum
directivity to the antenna.
• So, for 3 element aerial, the lengths of the elements can be considered as:

• The spacing between antenna elements vary between 0.1 λ to 0.2 λ.

• With parasitic elements the impedance reduces less than 73 Ω and may be even less than 25 Ω.
• A folded λ/2 dipole is used to increase the impedance.
• System may be constructed with more than one director. Addition of each director increases the
gain by nearly 3 dB. Number of elements in a yagi is limited to 11.
• Basic Operation: The phases of the current in the parasitic element depends upon the length
and the distance between the elements. Parasitic antenna in the vicinity of radiating antenna is
used either to reflect or to direct the radiated energy so that a compact directional system is
obtained.
• A parasitic element of length greater than λ/2 is inductive which lags and of length less than
λ/2 is capacitive which leads the current due to induced voltage. Properly spaced elements of
length less than λ/2 act as director and add the fields of driven element.
• Each director will excite the next. The reflector adds the fields of driven element in the direction
from reflector towards the driven element.
• The greater the distance between driven and director elements, the greater the capacitive
reactance needed to provide correct phasing of parasitic elements.
• The antenna exhibits a directional pattern consisting of a main forward lobe and a number of
spurious side lobes. The main one of these is the reverse lobe caused by radiation in the
direction of the reflector.
A Yagi system has the following characteristics.

1. The three element array (reflector, active and director) is generally referred as “beam antenna”
2. It has unidirectional beam of moderate directivity with light weight, low cost and simplicity in design.
3. The band width increases between 2% when the space between elements ranges between 0.1λ to 0.15λ.
4. It provides a gain of 8 dB and a front-to-back ratio of 20dB.
5. Yagi is also known as super-directive or super gain antenna since the system results a high gain.
6. If greater directivity is to be obtained, more directors are used. Array up to 40 elements can be used.
7. Arrays can be stacked to increase the directivity.
8. Yagi is essentially a fixed frequency device. Frequency sensitivity and bandwidth of about 3% is
achievable.
9. To increase the directivity Yagi’s can be staked one above the other or one by side of the other.
Advantages
• Yagi-Uda antenna offers very high gain.
• It possesses a highly directional characteristic because of the use of directors.
• It is a low-cost antenna.
• Yagi-Uda antenna shows suitability towards high-frequency operations.
• It is light in weight and feeding mechanism is also simple.
• It is power efficient.
• Along with all the above-defined advantages, it also offers ease of construction and handling.
Disadvantages
• These antennas are highly affected by atmospheric conditions.
• Noise is the major factor that disturbs the overall performance of the antenna.
Applications of Yagi-Uda Antenna
• These antennas are widely used in the field of TV signal reception, as it has excellent
receiving ability. Even astronomical and defence related applications make use of Yagi-Uda
Antenna. Also, radio astronomy utilizes these antennas.
Log periodic dipole array(Log periodic antenna)
• The log periodic dipole array consists of a number of dipole elements.
These progressively reduce in size from the back to the front – the direction of
maximum radiation is from the smaller front.
• All the electrical properties of the antenna (such as impedance and radiation
pattern) are repeated periodically with the logarithmic of frequency.
• Each dipole element of the LPDA is fed, but the phase is reversed between
adjacent dipole elements – this ensures that the signal phasing is correct
between the different elements. It also means that a feeder is required along the
length of the antenna.
• The element at the back of the array where the elements are the largest is a half
wavelength at the lowest frequency of operation - the longest element acts as
a half wave dipole at the lowest frequency. The element spacing also
decrease towards the front of the array where the smallest elements are
located. The upper frequency is a function of the length of the shortest
element.
• Not all the antenna array is active at any given frequency. The active
region, i.e. the sections of the antenna that are contributing to the transmission
or reception vary with frequency, and only about three may really contribute to
the radiation at any given frequency. There is also a smooth transition of the
active region of the LPDA along the array as the frequency of operation
changes.
 Design of LDPA
The parameter that defines here the relation between the
length of the antenna elements and spacing between adjacent
elements is called design ratio or scale factor. This is
denoted by τ. It is sometimes referred as periodicity factor,
having a value less than 1.
The length of the dipoles (L) and spacing (R or S) between
them are related in the following way:

Thus, is given as:

Sometimes written as:

• While it can also be written in terms of another constant K, given as:

• where τ < 1 and is known as the scale factor. Another parameter viz., the spacing factor, σ, of
the array is defined by

• So, due to this, the two ends of the dipoles will be present along two straight lines, meeting at
angle α at one end while converging at the other end of the structure.
• The value of α is generally 30° with τ = 0.7.
• The three parameters τ , σ and α of a log-periodic dipole array are related to each other.
• As it is clear from the structure that there is repetitiveness in the structure of the antenna that
leads to provide repetitive behavior of characteristics.
• A log-periodic antenna operates only in the active region rather than the whole structure.
The various region of LPDA is classified as:
• Transmission line region: In this region of the structure of LPDA, the length of the
elements is shorter than the half-wavelength (i.e., resonant length). These elements provide
high capacitive impedance. Also, the spacing between the elements is small.
• The current flowing through the elements in this region is of small magnitude and leads the
supplied voltage by 90°. Thus, the small current through the element results in small
radiation in the backward direction.
• Active region: This structural region of the LPDA has element length equal to half
wavelength and offers resistive impedance. Here the current through the element is large
and in phase with the supplied voltage.
• Also, sufficiently large spacing exists between the dipoles. This is the region which is
responsible to provide maximum radiation.
• It is to be noted here that, the frequencies where the length of the longest and shortest
dipole is λ/2 is said to be the cut-off frequency.
• So, for maximum frequency, the active region will be towards the apex, for
intermediate frequency, the region will shift towards the middle region of the
structure. While for minimum frequencies it will be near the region of longest
elements.
• This means that for LPDA, the phase center shifts from one end to the other
according to the shift in frequency from lowest to highest.
• Reflective region: The region having elements with length greater than resonant
length (i.e., λ/2) becomes the inactive region. Thus, the impedance is inductive in
nature that leads to generate a current that lags the supplied voltage.

Radiation Pattern
These antennas offer bidirectional as well as unidirectional
radiation pattern depending upon the log periodic structures.
Basically, a structure with a single active region, there will
be a unidirectional radiation pattern. But if there exist two
active regions, there will be a bidirectional radiation pattern.
Characteristics
• The excitation to the LPDA is provided at the shorter length side in case of the
single active region while at the centre in case of two active regions log
periodic.
• To have a unidirectional radiation pattern, the structure must radiate towards
the shorter element (i., left direction) and negligible towards the right.
• The transmission line inactive region must possess desired characteristic
impedance with very small or negligible radiation.
• In the active region, to have strong radiation, the magnitude and phase of the
currents must be proper.
• a typical log periodic antenna might provide between 3 and 6 dB gain.
Advantages
• It offers a compact structure.
• It provides adjustable gain according to the requirement.
• These offer negligible loss of power when terminated.
Disadvantages
• The mounting platform must be of sufficient strength to hold the elements.
• It is quite expensive than other antennas.
Applications of Log Periodic Antenna
• It is used majorly for high-frequency communication purposes. Also, it is
used for TV signal reception and for signal monitoring applications.
Turnstile antenna
• A Turnstile Antenna (also know as a crossed-dipole antenna) consists
of a set of two identical dipole antennas placed at right angles (90º)
to each other. These antennas are fed with an in phase quadrature
signal i.e the signal to each dipole is +/- 90 degrees out of phase with
each other.
• They usually operate at VHF and UHF frequencies from 30 MHz to 3
GHz.
• Turnstile antennas have an omnidirectional radiation pattern with
horizontal polarization.
Modes of Operation
• Turnstile antennas have two modes of operation:
• Normal Mode: In this mode of operation, the antenna radiates horizontally
polarized waves which are perpendicular to its axis.
• Axial Mode: In this mode of operation, the antenna radiates circularly
polarized waves along its axis i.e., parallel to the Earth's surface.

• They are most often used for FM & TV broadcasting, military and general
satellite communication applications.
Biconical antenna
• Biconical antennas (also known as Bowtie or Butterfly
antennas)
• One simple configuration that can be used to achieve
broadband characteristics is the biconical antenna formed by
placing two cones of infinite extent together.
• The application of a voltage Vi at the input terminals will
produce outgoing spherical waves, which in turn produce at
any point (r, θ = θc, φ) a current I along the surface of the
cone and voltage V between the cones.
• These can then be used to find the characteristic impedance of
the transmission line, which is also equal to the input
impedance of an infinite geometry.
Biconical antenna acts a guide for spherical waves

Input impedance of Biconical antenna

Spiral antennas
• Spiral antennas belong to the class of "frequency independent" antennas;
these antennas are characterized as having a very large bandwidth.
The fractional Bandwidth can be as high as 30:1. This means that if the lower
frequency is 1 GHz, the antenna would still be efficient at 30 GHz, and every
frequency in between.
• Spiral antennas are usually circularly polarized. The spiral antenna's radiation
pattern typically has a peak radiation direction perpendicular to the plane of
the spiral (broadside radiation).
The Log-Periodic Spiral Antenna
• In 1954, Edwin Turnur started messing with a dipole antenna.
Instead of leaving the arms straight, he wrapped them around
each other, forming a spiral. This was the beginning of the
spiral antenna. We can define the arms of a spiral antenna
using simple polar coordinates and polar functions. The log-
periodic spiral antenna, also known as the equiangular spiral
antenna, has each arm defined by the polar function:
• In above equation is a constant that controls the initial
radius of the spiral antenna. The parameter a controls the rate
at which the spiral antenna flares or grows as it turns.
Equation [1], in English, states that the spiral antenna radius
grows exponentially as it turns. In Figure 1, a plot of a planar
Log-Periodic Spiral Antenna is shown.

Source: https://www.antenna-theory.com/antennas/travelling/spiral.php
• 1. Total Length of the Spiral, or the outer radius This determines the lowest
frequency of operation for the spiral antenna. The lowest operating frequency of the
spiral antenna is commonly approximated to occur when the wavelength is equal to
the circumference of the spiral:

• 2. The Flare Rate (a) - The rate at which the spiral grows with angle is the flare rate.
If it is too large, the spiral is tightly wrapped around itself. In this case, it will
behave more like a capacitor, with closely coupled conductors, giving poor
radiation. If the flare rate is too small, the spiral acts more like a dipole as it doesn't
wrap around itself. A commonly used value is a = 0.22.
• The highest frequency in the spiral antenna's operating band occurs when the
innermost radius of the spiral (i.e. where the spiral starts after the feed structure) is
equal to lambda/4 (one quarter wavelength). That is, the highest frequency can be
determined from the inner radius
• 4. Number of Turns (N) - The number of turns of the spiral is also a design
parameter. Experimentally it is found that spirals with at least one-half turn up
to 3 turns work well, with 1.5 turns being a good number.
• Radiation Pattern
 The Archimedean Spiral Antenna
• Another common planar spiral antenna type is known as the
Archimedean Spiral antenna. Each arm of the Archimedean spiral is
defined by the equation: …………(1)
• Equation [1] states that the radius r of the antenna increases linearly
with the angle The parameter a is simply a constant that
controls the rate at which the spiral flares out. The second arm of the
Archimedean spiral the same as the first, but rotated 180 degrees. A
plot of the Archimedean spiral given in Equation [6] is shown in
Figure 2:
• In Figure 2, we have two arms of the Archimedean spiral antenna
flaring away from the center, as defined by Equation [1]. The feed of
the antenna (the voltage source), is placed directly across between Figure 2: Spiral antenna
the two arms of the spiral - the positive end to one arm and the
negative end of the feed to the second spiral arm.
Microstrip Patch Antenna
• The idea of microstrip patch antennas arose from utilizing printed circuit
technology not only for the circuit components and transmission lines but also for
the radiating elements of an electronic system.
• It was first proposed by Deschamps [2].
• The basic structure of the microstrip patch antenna is shown in Figure 1.12. It
consists of area of metallization supported above a ground plane by a thin
dielectric substrate and fed against the ground at an appropriate location. The
patch shape can in principle be arbitrary; in practice, the rectangle, the circle, the
equitriangle and the annular-ring are common shapes. Four feeding methods are
shown in Figure 1.13. They are: coaxial probe feed, microstrip line feed, aperture-
coupled feed and proximity feed. Electromagnetic energy is first guided or
coupled to the region under the patch, which acts like a resonant cavity with open
circuits on the sides. Some of the energy leaks out of the cavity and radiates into
space, resulting in an antenna.
Fig. 1.13 Four common feeding methods of microstrip patch antenna
Feed Methods for the Single Element
• 1.6.1 Coaxial Probe Feed
• This is perhaps the most common feeding
method. The geometry is shown in Figure
1.20. The coaxial probe usually has a
characteristic impedance of 50 ohms. As will
be shown in a later chapter, the input
impedance of the patch antenna varies with
the feed location. Thus the location of the
probe should be at a 50 ohm point of the patch
to achieve impedance matching. There are a
number of terms associated with the coaxial
probe. Type N, TNC, or BNC connectors are
Fig. 1.20 The geometry of coaxial probe feed microstrip patch
for VHF, UHF, or lower microwave antenna (a) top view and (b) side view.
frequencies. OSM or OSSM connectors can
be used throughout the microwave
frequencies. OSSM, OS-50 or K-connector
are for millimeter wave frequencies.

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• 1.6.2 Microstrip-Line Feed
• A microstrip patch can be connected directly to a
microstrip transmission line, as shown in Figure
1.21. At the edge of a patch, the impedance is
generally much higher than 50 ohm (e.g. 200
ohm). To avoid impedance mismatch, sections of
quarter-wavelength transformers can be used to
transform a large input impedance to a 50 ohm
line.
• As shown in a later chapter, the input impedance
is smaller at points away from the edge. Thus Fig. 1.21 The geometry of a direct microstrip feed
another method of matching the antenna microstrip patch antenna (a) top view and (b) side view.
impedance is to extend the microstrip line into
the patch, as shown inFigure 1.22.
• With the microstrip-line feed approach, an array
of patch elements and their microstrip power
division lines can all be designed and chemically
etched on the same substrate with relatively low
fabrication cost per element. However, the
leakage radiation of the transmission lines may
be large enough to raise the sidelobe or cross-
polarized levels of the array radiation.
Fig. 1.22 The geometry of a recessed microstrip line feed
microstrip patch antenna (a) top view and (b) side view.
Proximity-Coupled Microstrip-Line Feed
• An open-ended microstrip line can also be
used to feed a patch antenna through
proximity coupling, as shown in Figure
1.23. For example, the open end of a 100
ohm line can be placed underneath the
patch at its 100 ohm location. An open- Fig. 1.23 The geometry of a proximity coupled microstrip feed
microstrip patch antenna (a) top view and (b) side view.
ended microstrip line can also be placed in
parallel and very close to the edge of a
patch, to achieve excitation through
fringe-field coupling, as shown in Figure
1.24. Both these methods avoid any
soldering connection, which in some
cases, could achieve better mechanical
reliability.
Fig. 1.24 The geometry of a patch antenna fed by an
adjacent microstrip line (a) top view and (b) side view.
Aperture-Coupled Feed
• An open-ended microstrip line can be placed on one
side of the ground plane to excite a patch antenna
situated on the other side through an opening slot in the
ground plane, as shown in Figure 1.25.
• This slot-coupling or aperture-coupling technique can
be used to avoid soldering connection as well as to
avoid leakage radiation of the line to interfere with the
patch radiation.
• In addition, by using a thick substrate, this feed method
allows the patch to achieve a wider bandwidth (>10%)
compared to the coax probe feed. Still wider bandwidth
(about 20%) is obtained if a resonant slot is used. When
two resonators (slot and patch) having different but
closely spaced resonance, wider bandwidth is achieved.
The main disadvantage of this feeding method is the
back radiation from the slot.
Fig. 1.25 The geometry of an aperture coupled feed microstrip
patch antenna (a) top view, (b) side view (c) pictorial view.
Summary of Advantages and Disadvantages
of Feeding Methods
1.4 Advantages and Disadvantages of Microstrip
Patch Antennas
• 1.4.1 Advantages
• The advantages of microstrip patch antennas are:
• (1) Planar, which can also be made conformal to a shaped surface
• (2) Low profile
• (3) Low radar cross-section
• (4) Rugged
• (5) Can be produced by printed circuit technology
• (6) Can be integrated with circuit elements
• (7) Can be designed for dual polarization operations
• (8) Can be designed for dual or multi-frequency operations
• Microstrip Patch
Disadvantages

• Narrow bandwidth (1 to 5%)

• Low power handling capacity
• Practical limitation on Gain (around 30 dB)
• Poor isolation between the feed and radiating elements
• Excitation of surface waves
• Tolerance problem requires good quality substrate, which are expensive
• Polarization purity is difficult to achieve
• Size is large at lower frequency
Applications
• The low profile structure of microstrip antennas offers its wide use
in wireless communications. This is the reason these antennas show
compatibility towards handheld devices like pagers and mobile phones.
• Due to the thin structure of these antennas, these are used as
communication antennas on missiles.
• Satellite communication and microwave applications also make use of
microstrip antenna due to its small size.
• GPS i.e., the Global Positioning System is one of the major advantages of
microstrip antennas. As it offers ease in tracking vehicles and marines.
• These antennas also find applications in phased array radars that can
handle bandwidth tolerance up to some percentage.
Antenna measurements
• Antenna measurements are a part of the analysis of antenna
parameters.
• Analysis is the determination of output knowing the input and system
details.
• Design is the determination of system details knowing the input and
output parameters.
MEASUREMENT OF ANTENNA PATTERN
• Antenna pattern is also known as radiation pattern. It is defined as
the graphical representation of the radiation properties as a function
of space coordinates.
• In general, the radiation pattern is determined in the far-field region.
The radiation properties include electric field strength, radiation
intensity, phase and polarisation.
• The antenna patterns consist of radiation lobes.
• The radiation pattern of any antenna consists of one major lobe and a
set of minor or side lobes.
• Antenna patterns are of two types:
• 1. Field pattern
• 2. Power pattern.
• Field pattern is the variation of absolute f ield strength with q in free
space.
• That is,
• | E | Vs 𝜃 is field pattern
• Similarly, power (proportional to E ^2) pattern is the variation of
radiated power with q in free space. That is, P Vs 𝜃 or | E |^2 Vs 𝜃 is
power pattern.
Measurement procedure
Here, transmitting antenna is fixed and
antenna under test is rotated by the driving
unit. For each position indicated by the
position indicator, the received power is
noted from the indicator. The indicator can
be a power meter or a Micro ammeter.
Then, from the results obtained, field
(proportional to current) or power
(proportional to I^2) is plotted as a function
of 𝜃. This gives the desired patterns of
antenna under test. For pattern
measurements, the following precautions
should be taken.
• Precautions in pattern measurements
• 1. Distance between the transmitting antenna and the receiving antenna (AUT)
• must be

• 2. AUT should be illuminated uniformly.

• 3. Ground and other ref lections should be avoided.
• 4. Measurements should be taken in shielded chambers like anechoic chambers
• to eliminate the effect of external EMI.
• 5. Automatic range equipment should be used to avoid manual errors.
• 6. The transmitting antenna should be able to produce a uniform wave front to
• reduce phase error of AUT.
• 7. The TX antenna should have high gain.
• 8. The side lobe level of TX antenna should be very small.
• 9. Horns, paraboloids or arrays of dipoles may be used as TX antennas.
Measurements of Antenna GAin
• antenna gain measurement. These are as follows:
• Gain transfer or Direct Comparison Method
• Absolute Gain Method
• Gain of an antenna is of mainly four types:
1. Directive gain 2. Directivity
3. Power gain 4. Relative gain
• 1. Directive gain It is defined as the ratio of radiation intensity in a given direction
to the average radiated power of the antenna. The average radiated power over
spherical surface is the ratio of total radiated power to 4pi. Therefore, directive
gain in a given direction is also defined as 4ip times the ratio of radiation intensity
in that direction to the total radiated power.

• 2. Directivity, D It is defined as the maximum directive gain. That is, D = (gd)max.

• 3. Power gain, gp Power gain in a given direction is def ined as 4 p times the ratio
of the radiation intensity in that direction to the total input power to the
antenna. That is,

• Relative gain of an antenna, gr It is defined as the ratio of the power gain in a

given direction to the power gain of a reference antenna in the reference
direction. That is,
Absolute Gain Method

• As we have discussed in the previous section

that the standard gain antenna is the one
whose gain is already known or calibrated.
However, we can calibrate the gain using two
or three arbitrary antennas.
• The figure below represents an arrangement
of two identical antennas for absolute gain
measurement:
 GAIN MEASUREMENT BY TWO ANTENNA METHOD
• In this method, two antennas, one for radiation and another for reception are
used. When both the antennas are polarisation matched, the expression for the
received power is given by Friis transmission formula. That is,
GAIN MEASUREMENT BY THREE ANTENNA
METHOD
• This method consists of:
1. Three unknown antennas.
2. Using antenna 1 as transmitter and antenna 2 as receiver, the
received power W1 is measured. Let the transmitter power be P1.
3. Replacing antenna 2 by antenna 3, the received power W2 is
measured for the same transmitted power (P2 = P1).
4. When antenna 2 is used as transmitter and antenna 3 is used as
receiver, receiver power W3 is measured. Let the transmitter power be
P3. Then we have
The above expressions in dB are given
 Direct Comparison Method
• In this method of gain measurement,
comparison between signal strengths of the
unknown gain antenna and the standard gain
antenna is made.
• The figure below represents gain
measurement by comparison method:
• Basically, the standard gain antenna is the
one with known gain. The standard antenna
and test antenna together form an
arrangement of the primary antenna. While
the secondary antenna in the arrangement is
simply an arbitrary transmitting antenna with
unknown gain. This is clearly shown in the
above figure.
• It is to be kept in mind that a considerable distance between standard and test
antenna is maintained so as to avoid the chances of coupling or any type of
interaction. Generally, an electromagnetic horn antenna is used as a standard
gain antenna.
• Also, as discussed in the pattern measurement that the distance requirement of
the arrangement is such that r≥2d2/λ. So, the distance between the primary and
secondary antenna must be properly maintained.
• As standard and test antennas are acting as a unit thus to have appropriate
matching with the load, an attenuator pad is placed at the receiver input.
• It is to be noted here that during the whole measurement there must not be
fluctuation in the frequency of the power radiated towards the primary
antenna. So, to ensure this, power level indicating device or power bridge is
used at the transmitter.
• Let now see what procedure must be followed to determine the unknown gain
through the direct comparison method. The steps are as follows:
• Initially, the standard antenna with a known gain is connected to the receiver
using the switch S and it is directed towards the direction of maximal signal
intensity of the transmitting antenna (i.e., secondary antenna). With the
properly applied input to the secondary antenna, reading of the receiver is
noted. Along with this, the reading of the attenuator dial (W1) and power
bridge (P1) is also noted down.
• Now, the switch is removed from the standard antenna and with the help of
same switch connection of the test antenna is made with the receiver. Further,
the reading of the attenuator dial is adjusted to get the same reading on the
receiver as it was in the case with the standard antenna. Let the dial setting be
W2 and power bridge reading is P2.
 • So, on this basis there will be two cases:
• Case I: If P1 = P2, then gain of the test antenna will be
In decibels,

Case II: If P1 ≠ P2, then the actual power gain of test antenna will be the product of Gp and P1/P2. So, let

Power gain,

On substituting the value of Gp:

In decibels,

This is the gain of the test antenna. In this way, by the comparison method, the gain of the antenna is determined.
DIRECTIVITY MEASUREMENT

• The directivity, D of an antenna is its maximum directive gain. It is

obtained from the field pattern of the antenna. From the measured
patterns and their beam width in both the principal planes, D is
obtained.
• The principal planes are E-plane and H-plane.
Procedure for the measurement of directivity

• 1. Obtain E and H-plane patterns of AUT as described in Section 8.8.

• 2. Find the half-power beam widths from the patterns of step 1.
• 3. Find the directivity of AUT from

• here (B.W)E = half-power beam width in E-plane (degrees)

• (B.W)H = half-power beam width in H-plane (degrees)
• This method is accurate when the patterns consist of only one main
lobe.