Unit -IV

UNIT IV

SPECIAL ANTENNAS

Dr.T.V.Padmavathy Professor/ECE

RMKCET

Presentation Outline

 Rhombic Antenna

 Yagi Uda Antenna

 Helical Antenna

 Biconical Antenna

 Log Periodic Dipole Array

 Microstrip Patch Antenna

Rhombic Antenna

 Highest development of the long wire antenna is the Rhombic

antenna.

 It is a broad band directional antenna

 It consists of four conductors joined to form a rhombus or diamond shape

 Antenna is placed end to end and terminated by a non inductive resistor to produce unidirectional pattern

 Widely used for long distance, high frequency transmission and

 Maximum radiation from a rhombic antenna is along the direction of main axis

Rhombic Antenna

Design of Rhombic Antenna

The performance parameters of rhombic antenna are,

Length of the legs(L)

Tilt angle(θ)

Height above ground(h)

The field intensity of rhombic antenna can be expressed by,





Where,

β = Angle of radiation θ = Tilt angle (ie θ = 90-β)

h = Height of rhombic antenna from ground L = Length of legs

λ = Wave length

Rhombic Antenna

L and h in the maximum field intensity design is given by

Advantages

It is useful over a wide range of frequency It is frequency independent

High gain and low noise Low cost

Input impedance and radiation pattern are constant.

Disadvantages

Power loss occur at the terminal resistors It needs large space for installation

The radiation pattern contains more number of side lobes which reduces the operating efficiency.

Applications

Used as a broad band antenna in microwave applications Used in radio communications

Yagi Uda Antenna

 It is just a dipole with other parasitic dipoles nearby

 It is an example for parasitic array

 It consists of a driven element, a reflector and director

 driven element is usually a resonant dipole or a folded dipole

 reflector is slightly longer than the driven element

 The impedance of reflector is inductive

 director is shorter than the driven element or active element

 Yagi Uda antenna works under the “optical principle”

 Driven element, reflector and director may be considered as

source, mirror and the lens

respectively.

 Driven element radiates energy from a transmission line to free space

 Reflector receives the radiation from dipole

 Director increases the energy

or increases the gain as a lens

 Radiates greater power in one, or more directions allowing for increased performance on transmit and receive

Helical Antenna

 Helix antenna is exited by

electric field generated by monopole in near field.

 Flux is captured by helix

antenna and change of flux with time produced induce current in helix.

 This induce current produces

inductance in the system.

 Strong electric field generated by monopole antenna in near field create capacitance in the system.

 Monopole antenna and helix are properly adjusted for tuning

 If helix pitch length is small and rectangle length is large it

produce large inductance

 Height of monopole antenna determine resonance behavior of

antenna

 This antenna design is for naval communication in the frequency of

18.6 MHz.

This antenna can be operated in two different modes  Normal Mode

 Axial Mode

Helical Antenna

 Circular Polarization

 ¾<C/λ<4/3

 C/λ=1:near optimum

 S= λ/4

 Half-Power Beam width: 50 x 50 degrees

 Directivity:

 Typical Gain: 10dB

 Bandwidth: 52%

 Frequency limit: 100MHZ to 3GHz NS

C

2 3

52

3 2

15



S C N

NORMAL MODE

Helical Antenna

Radiation Pattern for Normal Mode

Radiation pattern similar to linear

dipole

The dimensions of the helix are small

compared to the wavelength

Narrow in bandwidth

Radiation efficiency is small

Rarely used

Satellite communication.

Space communication & space probes.

For telemetry applications.

Biconical Antenna

Biconical antenna configuration is one of many configurations that can be

used to achieve broadband

characteristics

The configuration of biconical antenna fed by coaxial cable

Cone top radius is and cone bottom radius is the radius of co-axial cable.

Flare angle between two cone is

   

 

2 sin l

Biconical Antenna

Log periodic Dipole Array

Similar configuration to the Yagi– Uda antenna is the log-periodic antenna

It produces a similar end-fire radiation pattern and directivity

typically between 7 and 15 dBi to the Yagi–Uda

It has a much wider bandwidth

than the Yagi–Uda.

log-periodic dipole antenna (LPDA) consists of many dipoles

This antenna is divided into active region and inactive regions.

 If length, L, is around half of the wavelength, it is an active dipole

and within the active region

If length is greater than half the wavelength, it is in an inactive region and acts as a reflector

If length is smaller than half the wavelength, it is also in an inactive region but acts as a director

 The highest frequency is basically determined by the shortest dipole length

 The lowest frequency is basically determined by the longest dipole length

Log periodic Dipole Array

N

L

1

Log periodic Dipole Array

Design Equations

n

element

of

poles

the

between

Gap

g

n

element

of

Diameter

d

n

and

n

elements

between

Spacing

S

N

n

and

n

element

of

Length

L













)

1

(

...,

,

2

,

1

,

1

2











g

g

d

d

S

S

L

L

L

L



 There are three independent variables in log-periodic antenna

design

Apex angle

Directivity

Length of the antenna

Advantages

 It is broadband antenna.

 It is unidirectional antenna.

 It is frequency independent antenna

Microstrip Patch Antenna

 Microstrip patch antenna consists of a

radiating patch on one side of a

dielectric substrate which has a

ground plane on the other side

 For good antenna performance, a

thick dielectric substrate having a low

dielectric constant is desirable

 In general Micro strip antennas are also

known as “Printed Antennas”

 These are mostly used at microwave

 The patch usually fed along the centerline to symmetry and thus

minimize excitation of undesirable modes.

 Micro strip antennas are easy to fabricate and comfortable on curved surface

 The directivity is fairly insensitive to the substrate thickness

 Micro strip patch antennas patches are in variety of shapes, such

as rectangular , square , triangular and circulator …etc.,

 A thicker substrate will increase the radiation power , reduce

conductor loss and improve Band width

 The most commonly used substrates are

 Honey comb(dielectric constant=1.07)

 Duroid (dielectric constant=2.32)

 Quartz(dielectric constant=3.8)

 Alumina(dielectric constant=10)

Advantages

 Low fabrication cost, hence can be manufactured in large quantities.

 Easily integrated with microwave integrated circuits (MICs).

 Capable of dual and triple frequency operations.

 Supports both, linear as well as circular polarization.

 Mechanically robust when mounted on rigid surfaces.

 High Performance

 Light weight and low volume

Disadvantages

 Narrow bandwidth associated with tolerance problem  Lower Gain(Nearly 6db) .

 Large ohmic losses in feed structure of arrays  Inherently low impedance bandwidth.

 Low efficiency

 Most microstrip antennas radiate into half-space  Low power handling capacity

Applications

 Used in mobile satellite communication system.  Direct broad cast telivision(DBS).

 Wire less LAN’S

 Missiles and telementry  GPS system

Optimizing the Substrate Properties for Increased

Bandwidth

 Print the antenna on a thicker substrate

 Decrease the dielectric constant of the substrate

 Stack two patches on top of each other separated by a dielectric

Antenna Measurements

Basic Methods in Antenna Measurements are

Antenna ranges and anechoic chambers.

Measuring far-field patterns

Gain

Directivity

Radiation efficiency

Input impedance

Antenna Measurements – Antenna Ranges

 The antenna measurement sites are called antenna ranges (AR)

 They can be categorized as

outdoor ranges and indoor ranges

 According to the principle of measurement, they can be also categorized as

 Reflection ranges

 Free-space ranges

Antenna Measurements – Far-field Pattern

Measurements

 The far-field patterns are measured on the surface of a sphere of

constant radius.

 Any position on the sphere is identified by the directional angles θ and

ϕ of the spherical coordinate system

 The total amplitude pattern is described by the vector sum of the two

orthogonally polarized radiated field components:

2 2





E

E

 A simplified block diagram of a pattern measurement system is given below.

 The gain measurements require essentially the same environment

as the pattern measurements

 To measure the gain of antennas operating above 1 GHz, usually,

anechoic chambers are used.

 Between 0.1 GHz and 1 GHz, ground-reflection ranges are used.

 There are three gain-measurement techniques are used

 Two-antenna method,

 Three – antenna method

 Gain-transfer or Gain-comparison method.

 The two-antenna method is based on Friis transmission equation

and it needs two identical samples of the tested antenna.

 One is the radiating antenna, and the other one is receiving antenna

 The Friis transmission equation is

Antenna Measurements – Gain Measurements

                    t r dB P P R

G 20log₁₀ 4 10log₁₀

2 1   r t t r G G R P P 2

4 

Antenna Measurements – Directivity Measurements

 The directivity measurements are directly related to the pattern

measurements.

 Once the pattern is found over a sphere, the directivity can be

determined using the definition





 

 



















sin

,

,

4

d

d

F

F

D

is the power pattern of the test antenna

is the direction of maximum radiation

 

F

Antenna Measurements – Radiation Efficiency

 To calculate the radiation efficiency, the gain and the directivity must

be measured first

 Factors like impedance mismatch and polarization mismatch

have to be minimized during these measurements.

 The radiation efficiency is then calculated using its definition:

y

Directivit

Gain

ant



Antenna Measurements – Impedance Measurements

 The input impedance of an antenna is calculated via the reflection

coefficient at its terminals



If the magnitude and the phase of are known, then, the antenna input

impedance is calculated as



 is usually measured using a vector network analyzer (VNA).

 The VNA measures the complex S-parameters of microwave networks.

 The antenna is a single-port device, therefore,



11

S

Antenna Measurements – Polarization Measurements

 The polarization of an antenna is not the same in every direction, i.e., it depends on the observation angle.

 The polarization measurement methods are classified into three

general categories

 Partial methods give incomplete information about the polarization but are simple and require conventional equipment.

 Comparison methods yield complete polarization information; however, they require a polarization standard.

 The polarization-pattern method is a common partial method. It

produces the polarization ellipse parameters in a given direction of

radiation

 It cannot determine however the sense of rotation.

 A typical arrangement for the polarization-pattern measurement is

given below