UNIT-4

Practical Antennas
Dr.K.Suman
 Helical Antennas
• The antennas that exhibit broadband characteristics can
also provide circular polarization which is used in many
applications.
• Helical antenna is an example of wire antenna and itself
forms the shape of a helix.
• This is a broadband VHF and UHF antenna.
• It used in extraterrestrial communications in which
satellite relays etc are involved
Frequency Range
• The frequency range of operation of helical antenna is
around 30MHz to 3GHz. This antenna works
in VHF and UHF ranges.
Construction:
• It consists of a conducting (thick copper) wire wound in
the form of screw thread forming a helix.
• It is used as an antenna in conjunction with a flat metal
plate called ground plate.
• One end of the helix is connected to the center
conductor of the cable and the outer conductor is
connected to the ground plate.
The images of a helix antenna
 Modes of Operation
The predominant modes of operation of a helical antenna are
• Normal or perpendicular mode of radiation.
• Axial or end-fire or beam mode of radiation

Normal mode
• The radiation field is maximum in the direction normal to the helix
axis and circularly polarized.
• This mode of radiation is obtained if the dimensions of helix are
small compared to the wavelength (NL <<λ)
• The radiation pattern of this helical antenna is a combination of short
dipole and loop antenna.
• It depends upon the values of diameter of helix, D and its turn
spacing, S.
Drawbacks
-- low radiation efficiency
-- narrow bandwidth.

Hence, it is hardly used.

It can be increased by increasing the size of the helix.
Axial Mode
• The radiation is in the end-fire direction along the helical
axis.
• The waves are circularly or nearly circularly polarized.
• This mode of operation is obtained by raising the
circumference to the order of one wavelength (λ) and
spacing of approximately λ/4.
• The radiation pattern is broad and directional along the
axial beam producing minor lobes at oblique angles.
• Because of this it is mostly used.
• By s= λ/4, and helix circumference C/ λ = 1 λ, the
axial mode of radiation is produced.
 Wide band characteristics of Helical Antennas
• It has inherent broadband properties having desirable pattern,
impedance and polarization characteristics.

• The phase velocity is naturally adjusted, so that the fields from each
turn add nearly in phase in the axial direction.

• This accounts for the persistence of the axial mode of radiation over
a nearly 2 to 1 range in frequency.

• If phase velocity constant as a function of frequency, then axial

mode patterns would be obtained only over a narrow frequency
range.

• The terminal impedance is relatively constant over the same

frequency range because of large attenuation of the wave reflected
from the open end of the helix.
Advantages
• Simple design
• Highest directivity
• Wider bandwidth
• Can achieve circular polarization
• Can be used at HF & VHF bands also

Disadvantages
• Antenna is larger and requires more space
• Efficiency decreases with number of turns
Applications
• 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 ballastic missiles and
satellites at Earth stations
• Used to establish communications between the
moon and the Earth
• Used in radio astronomy
Horn Antenna
The E-Plane pattern is much broader than H-Plane
pattern because of the flaring.
Antennas
UNIT-4
 Log Periodic Antenna
1. A frequency independent antenna may be defined as
the antenna for which the impedance and pattern
remain constant as function of frequency.
2. Expand or contract in proportion to the wavelength
or
if the antenna structure is not mechanically adjustable,
the size of active or radiating region should be
proportional to the wavelength.
3. This development on frequency independent concept
led to log periodic antenna.
4. They are broad band antennas.
bandwidth – 10:1 – easily achieved
100:1 – is feasible if theoretical design is
closely approximated.
5. The broad band characteristics - include both
impedance and
pattern.

6. Radiation pattern – bidirectional or unidirectional of low

to
moderate directive gain.

7. Geometry is so chosen that electrical properties must

repeat periodically with the logarithm of the frequency.

8. Frequency independence can be obtained when the

variation of the properties is very small.

9. The structure size changes with each repetition by a

 LPDA
� No. of dipoles of different lengths and spacings.
� Fed by a balanced two wire transmission line.
� Included angle is constant.
� Length=L and spacing = S or R
� Scale factor or design ratio or periodicity factor τ <
1
� Thus dipole length and spacing are related as
Inactive T.L region (L< λ/2)

• At middle of operating range, the elements are short.

• Elements present a relatively high capacitance.

• Element current is small and current leads the Vb by 90 deg

app.

• Element spacing is small (in wavelength)

• By transposition of transmission introduces 180 deg phase

shift
between adjacent dipoles.

• Hence currents in the elements of these region are small

Active T.L region (L = λ/2)

• Impedance offered is resistive in nature.

• Element current is large and in phase with the Vb.

•Current below resonance – leading slightly

•Current above resonance – lagging slightly

• Element spacing is large, causing the phase in a particular

element to lead app. by 90 deg.
•For e.g., by the time field radiated from element Ln+1
reaches Ln, the phase of Ln advances by 90 deg.
•Its field add to the field of Ln+1, resulting in large field
towards left.
•Hence there is strong radiation towards left in between
direction and little towards right.
Inactive reflective region (L > λ/2)

•Impedance is inductive, causing currents to lag the base

voltage

• Base voltage is very small as almost all the energy

transmitted
down the line has been attracted and radiated by the
active
region.

• It presents a large reactive impedance to the line and thus

any
small amount of incident wave from active region is
reflected
Advantages
The following are the advantages of Log-periodic
antennas −
� The antenna design is compact.
� Gain and radiation pattern are varied according to
the requirements.
Disadvantages
The following are the disadvantages of Log-periodic
antennas −
� External mount.
� Installation cost is high.
Applications
The following are the applications of Log-periodic
antennas −
� Used for HF communications.
� Used for particular sort of TV receptions.
� Used for all round monitoring in higher frequency
bands.
 General Characteristics
� It is excited from the shorter length side or HF side for
one active region and at the center for two active region
log periodic antenna.

� They are fed by a balanced two wire T.L.

� For unidirectional antenna, the structure fires in backward

direction and forward radiation is very small or zero.

� For bidirectional log periodic antenna, the maximum

radiation is in broadside direction.

� T.L inactive region must have proper characteristic

impedance with negligible radiation.
� In active region, the magnitude and phasing of
currents should be proper so that strong radiation
occurs along backward direction and zero radiation
along forward direction in case of unidirectional and
broadside for bidirectional.

� In inactive reflective region, there should be rapid

decay of current
Microstrip Antennas

Dr.K.SUMAN
 Microstrip Antennas
• In high-performance aircraft, spacecraft,
satellite, and missile applications,
• where size, weight, cost, performance,
ease of installation, and aerodynamic
profile are constraints,
• low-profile antennas may be required.
History
1950’s –Concept by DESCHAMPS
1972 – Discovered by Howell J Q
Microstrip Antennas are used.
• Low profile.
• Conformable to planar and nonplanar
surfaces.
• Simple and inexpensive to manufacture using
modern printed-circuit technology.
• Mechanically robust when mounted on rigid
surfaces.
• Compatible with MMIC designs.
• Versatile in terms of resonant frequency,
polarization, pattern, and impedance.
• Microstrip antennas are also known as
printed antennas or patch antennas.

• Used at microwave frequencies.

• They are directly related to the wavelength at

resonant frequencies, they are also referred
to as narrow band wide beam antenna.

• It consists of radiating patch on one side of

the dielectric substrate and a GP on the other
side.
Different types of patches
 Basic principle of operation

• The patch acts as a resonant cavity (s.c

walls on top and bottom, o.c on the sides)
• In a cavity, only certain modes are allowed
to exist at different resonant frequencies.
• If the antenna is excited at a resonant
frequency a strong field is set up inside the
cavity and a strong current on the (bottom)
surface of the patch.
• This produces significant radiation.
• Assuming no variations of the electric field
along the width and the thickness of the
microstrip structure.

• Radiation may be mostly due to the

fringing fields at the o.c edges of the
patch.

• The fields at the end can be resolved into

tangential and normal components w.r.to
ground plane.
• Normal components - out of phase as the
patch line is λ/2 long.

• Far field produced by them cancel in the

broad side direction.

• Tangential components - parallel to the

ground plane and are in phase.

• Resulting field combine to give maximum

radiated field normal to the surface of the
structure.
 Basic Characteristics
• t << λ 0 , h << λ 0 ,0. 003 λ 0 ≤ h ≤ 0.05 λ 0
• It is designed in such a way that pattern
maximum is normal to the patch.
• It depends on the mode selection.
• For rectangular patch, λ 0 /3 < L < λ 0 /2
• The strip and ground plane are separated
by a dielectric sheet (substrate).
• Dielectric 2.2 ≤ εr ≤ 12
• Spacing between patch and ground plane
is
< λ 0 /10
• Most desirable – thick substrates εr – lower
range.

• They provide better efficiency, larger

bandwidth, loosely bound fields for radiation
into space, but large size.

• Thin substrates – higher εr (tightly bound

fields to minimize undesired radiation and
coupling, but small size.

• But more losses, less efficient and small

bandwidth.
 Microstrip T.L
• Approximate field distribution is transverse
to the direction of propagation.
• Electric field go from microstrip line to the
GP.
(Most of the lines are concentrated
underneath the microstrip)
• Magnetic field lines encircle the microstrip
line and extend above the substrate.
• Some originate on charges on the edges
and top of the microstrip and partially
extend into freespace.
• Thus the wave is no more TEM.
• The presence of field lines in air reduces
the effective dielectric constant seen by
waves propagating along the lines.
• If all the fields existed between the line
and GP, the dielectric constant would be
that of substrate.
• (Instead it is somewhat less)

• Less depends on w, h and εr

Design of Microstrip Antennas
Design a rectangular microstrip antenna
using a substrate (RT/duroid 5880) with
dielectric constant of 2.2, h = 0.1588 cm
(0.0625 inches) so as to resonate at 10 GHz.

Solution: The width W of the patch is

The effective dielectric constant of the patch is
The extended incremental length of the
patch ΔL is

The actual length L of the patch is found by

Finally the effective length is
Parabolic Reflectors

Dr.K.suman
 Reflector Antennas
• They are used to modify the radiation
pattern of a radiating element.
• E.g., the backward radiation from a
antenna may be eliminated with a plane
sheet reflector of large dimensions.
• In general case, a beam of predetermined
characteristics may be produced by
means of a large, suitably shaped, and
illuminated reflector surface.
Several reflector types are

Large, flat sheet reflector near a

linear dipole antenna to reduce the
backward radiation.
With small spacing between the
antenna and sheet this
arrangement also yields a
substantial gain in the forward
radiation
Small flat sheet preserves the
desirable properties of the sheet
reflector with reduced size

The properties of the large

sheet are relatively
insensitive to small
frequency changes. The
thin reflector element is
highly sensitive to
frequency changes.
•With two flat sheets
intersecting at an angle α
(<180◦).

•A sharper radiation pattern

from a flat sheet reflector (α =
180◦) can be obtained.

•It is also called an active

corner reflector antenna

•Aperture size is 1λ or 2λ
•A corner reflector without an
exciting antenna can be used
as a passive reflector.
•Used for radar waves.
•α = 90◦.
•Aperture may be of many
wavelengths.
•An incident wave is reflected
back towards its source.
•The corner acts as a
retroreflector.
•Parabolic reflectors are
highly directional antennas.

•They reflect the waves

originating from a source at
the focus into a parallel
beam
 Parabolic Reflectors
• Parabolic reflectors are Microwave
antennas.
• To understand these antennas the concept
of parabolic reflector has to be discussed.

• Frequency Range
• The frequency range used for the
application of Parabolic reflector antennas
is above 1MHz.
• These antennas are widely used for radio
and wireless applications.
 Principle of Operation
• The standard definition of a parabola is -
Locus of a point, which moves in such a
way that its distance from the fixed point
(called focus) plus its distance from a
straight line (called directrix) is constant.
• F-Focus; V-vertex

• Parabola is a two dimensional plane

curve.
• The following figure shows the geometry
of parabolic reflector.
• The point F is the focus (feed is given)
and
• V is the vertex.
• The line joining F and V is the axis of
symmetry.
• PQ are the reflected rays
• where L represents the line directrix on
which the reflected points lie (to say that
they are being collinear).
• Hence, as per the above definition, the
distance between F and L lie constant with
respect to the waves being focussed.
OF – Focal length(f)
K – constant (depends on the shape of parabola
FV – axis of parabola
D – aperture
By definition
FP1 + P1Q1 = FP2 + P2Q2 = FP3 + P3Q3 =
constant(k)

Equation of parabola --- y2 = 4fx

• The reflected wave forms a collimated
wave front, out of the parabolic shape.

• The ratio of focal length to aperture size

(ie., f/D) known as “f over D ratio” is an
important parameter of parabolic reflector.

• Its value varies from 0.25 to 0.50.

• The law of reflection states that the angle
of incidence and the angle of reflection are
equal.
• i.e., ∟i = ∟r
• All the waves originating from focus will be
reflected parallel to the axis.

• All the waves reaching the aperture plane

are in phase
• Hence rays are parallel to the axis
because they are perpendicular to the
wavefront.

• The principle of equality of path lengths is

maintained between all rays of two
wavefronts.

• If there is path length difference between

the two rays then cancel each other.
• Hence the geometrical properties of
parabola provide excellent microwave
reflectors that lead to the production of
concentrated beam of radiation.

• A parabola converts spherical wavefront to

plane wavefront at the mouth of the
parabola.
 Construction & Working of a
Parabolic Reflector
• If a Parabolic Reflector antenna is used for
transmitting a signal,
---- the signal from the feed,
---- comes out of a dipole or a horn
antenna,
---- to focus the wave on to the
parabola. It means that, the waves come
out of the focal point and strike the
Paraboloidal reflector.
• This wave now gets reflected
as collimated wave front .
• It can be used as a receiver.
• When the EM wave hits the parabola, the
wave gets reflected onto the feed point.
• The dipole or the horn antenna,
---- which acts as the receiver antenna at
its feed, receives this signal,
---- to convert it into electric signal and
forwards it to the receiver circuitry.
• The gain of the paraboloid is a function of
aperture ratio (D/λ).

• The Effective Radiated Power (ERP) of an

antenna is the multiplication of the input
power fed to the antenna and its power
gain.
• ERP = I/P Power X Power gain
Advantages
• Reduction of minor lobes
• Wastage of power is reduced
• Equivalent focal length is achieved
• Feed can be placed in any location,
according to our convenience
• Adjustment of beam (narrowing or
widening) is done by adjusting the
reflecting surfaces
Disadvantage
• Some of the power that gets reflected from
the parabolic reflector is obstructed.
• This becomes a problem with small
dimension paraboloid.

Applications
• The cassegrain feed parabolic reflector is
mainly used in satellite communications.
• Also used in wireless telecommunication
systems.
 UNIT-4
Practical Antennas
Dr.K.Suman
Rhombic Antenna

• It is in the shape of
Rhombus.
• Also called diamond
antenna.
• It is an equilateral
parallelogram with two
opposite acute angles.
• It may be regarded as
2 inverted V antenna s
connected in series or
• 2 V connected end to
end forming obtuse
angles.
• The tilt angle θ = 90deg – angle of major lobe.
• Change in length causes change in the angle of the major lobe.
• A change of length from 4λ to 8 λ, the angle of major lobe changes from 17de
24 deg.
• It indicates that the antenna operates over a wide band of frequencies.
Construction:
Ø A rhombic antenna is horizontally installed over the
ground at a height h.
Ø When used for transmission , the input is fed through a
balanced line and the terminating non-inductive resistor
is adjusted so that travelling waves are set up in the 4
legs of the antenna.
Ø The maximum gain is along the direction of main axis
which passes through feed point to termination in free
space.
Ø Polarization is in the plane of rhombus i.e., Horizontal
pol.
Ø Presence of earth brings the elevation a bit in the
upward direction, keeping the polarization intact.
 ….contd.
• In practice, half of the power is dissipated in the
terminating resistance, while the rest is radiated.
• The wasted power represents the power that have
contributed to backward radiation if not terminated.
• R = 600Ω-800Ω.
• R = -800Ω, 1 watt non-inductive type can be used for
reception.
• The radiation pattern is unidirectional and is informed
due to reinforcement of 4 lobes of four legs on each.
• The other lobes formed on each of the leg get cancelled
as they are oppositely directed.
• If terminating resistance is removed than unidirectional
pattern becomes a bidirectional pattern.
• Rhombic antenna is fed by BALUN(balanced tunable
transformer system).
• Directivity – 20 to 90
• Power gain – 50 to 60
• Power lost 35 – 50% in RT
 Design of Rhombic Antenna

• The three independent parameters to be considered are

(i) Tilt angle(θ)
(ii) Leg Length(L)
(iii) Height above ground(h)

Two types of Rhombic design

1. Alignment design – Obtained when ‘h’ above ground is so

chosen that maximum of the main beam coincides with the
desired angle of elevation.

2. Maximum field intensity design or Maximum output design –

When height above ground is so chosen that the maximum E
for a constant current is obtained at the desired angle of
elevation.
 ….contd.
q If h – less than needed, alignment may be obtained by
increasing L.
q If h – constant and L decreases, then alignment may be
obtained by changing θ.
q If h and L – decreases, then alignment may be obtained
by
changing θ.

Thus a compromise design may be obtained by

modification of any of above 3 conditions but at the cost of
gain.
Advantages
1. Simple and cheap to erect.
2. Input impedance is twice that obtainable from a single
side radiator.
3. Vertical angle of radiation is low and hence used for
long F-layer propagation.
4. Input impedance and radiation pattern do not change
rapidly over a considerable frequency range.
5. It is highly directional broadband antenna.

Disadvantages
1. Large space for installation.
2. Large number of minor lobes
3. Efficiency is less as some power is lost in termination.
Applications
1. Broad frequency capabilities
2. All radio communication
3. Suitable for HF transmission and reception in
commercial point to point communication.
 UNIT-4

Yagi-Uda Antenna
 Folded Dipole Antenna

• The variation of λ/2 dipole is Folded dipole

antenna.
• AB – minimum current points
• C – maximum current point or minimum
voltage point.
Two half wave dipoles
1. Continuous
2. Split at the center and folded and joined
together in parallel at the ends.

• A transmission line (balanced) is fed at

the center of the split dipole.
• Two dipoles have same voltages.
• Radiation pattern is same for both λ/2 and
folded dipole.

Characteristics
1. I/P impedance is high(> λ/2 )
2. Bandwidth is high
3. Directivity is bidirectional
4. Current is of same magnitude and phase
5. Power flow is same
Proof:
If the total current at AA’ = I
Then each dipole will have = I/2
i.e., If = Ic / 2 or 2If = Ic

If = current in folded dipole

Ic = current of conventional dipole
• As I/p power is same in both
Pf = ½ If2 Rf Pc = ½ Ic2 Rc

Pf = Pc ½ If2 Rf = ½ Ic2 Rc R f = 4 Rc

• I/P impedance is 4 times greater than that of

λ/2 dipole.
• As T.L is delivering the same power at only
half the current,
• The i/p impedance of a folded dipole is

Rr = n2 X 73
= 292 (for folded dipole)
 Yagi-Uda Antenna or Array
• Most high gain antenna
• 3 elements: Driven element(DR)
Reflector(R)
Director(D)
• It is an array of 3 elements
• Driven element also called Active Element
• Power from transmitter is fed trough this
Driven element , or it feeds received
power to receiver.
Construction:
• They are metallic rods.
• D and R are parasitic elements i.e.,
passive element
(not electrically connected but electrically
coupled)
• They receive their excitation from the
voltages induced in them by the current
flow in the driven element.
• The driven element is a resonant λ/2
dipole usually a metallic rod at the
frequency of operation.
• The DR, D and R arranged in parallel and
at the same line of sight level.
• The phase and currents flowing due to
induced voltage depends on
(i) spacing between elements
(ii) reactance of the elements
• By tuning reactance can be varied.
• Spacing between elements - 0.1λ, 0.15 λ
• Front of driven element DR is Director –
5% less
• Back of driven element DR is Reflector –
5% more
• DR = λ/2 at resonant frequency
• Practically for 3 element array of yagi
antenna, the following formulae gives
lengths.
Reflector length = 500/f(MHz) feet
Driven element length = 475/f(MHz) feet
Director length = 455/f(MHz) feet

Note: 1 feet = 12 inches or 30.48 cms

• The parasitic elements are clamped on a
metallic support rod, because at the
middle of each element , voltage is
minimum (there exists a voltage node at
this point)
• If more number of elements are used then
the dipole impedance falls below 73Ω
• It may be 25Ω
• Shunt feed or folded dipole is used to
increase i/p impedance and to match the
feed cable.
• If directors are added at intervals of 0.15λ ,
increases the gain upto 12 dB.
• 3 element yagi antenna is suitable for TV
reception.
 Operation
• Spacing between the elements and
lengths of parasitic elements determine
the phases of the currents.
Length of parasitic parasitic element Action
element

≥ λ/2 Inductive Phases of the currents will

(Reflector) lag the induced voltage

< λ/2 Capacitive Phases of the currents will

(Director) lead the induced voltage
• More than one directors are used, then
each director will excite the next.

• It adds the fields of driven element in the

direction away from the D.E.

• Reflector adds up the fields of D.E in the

direction from reflector towards D.E. if
properly spaced.

• Gain increases with increasing directors

• Additional gain is achieved by using additional
directors.
• Distance between 2 elements is 0.1λ to 0.3 λ

• The greater the distance between driven and

director elements the greater the capacitive
reactance needed to provide correct phasing of
parasitic element.

• D.E radiates from front to rear.

• Part of this radiation induces current in parasitic
elements, which in-turn re-radiates virtually all the
radiation.
• If distance between D.E and parasitic
element is decreased, then it loads the
D.E.

• The radiated energy is added – front

direction
• cancel – backward direction

• Thus input impedance at i/p terminals

reduces.
• So a folded dipole is used as driven
General Characteristics