Resonant Antennas

• Antennas that have dimensions near to a half-wavelength or any of its multiples.

• The impedance and bandwidth vary significantly with small changes in the operating frequency; the impedance vary from negative reactance, thru pure resistance to positive reactance as the frequency changes from below to above resonance.

• Characteristics of the antenna are strongly frequency dependent.

Non-resonant antennas

• The radiation pattern and the input impedance are not so variable with frequency.

• Referred to as broadband antennas

Microwave Antennas or Antennas with Parabolic Reflector

Terrestrial Microwave Antennas for Point-To-Point Communication

• Terrestrial microwave antennas generate a beam of RF signal to communicate between two locations.

• Point-To-Point communication depends upon a clear line of sight between two microwave antennas.

• Obstructions, such as buildings, trees or terrain interfere with the signal.

• Depending upon the location, usage and frequency, different types can be utilized.

Because a line of sight radio link is made, the radio frequencies used occupy only a narrow path between stations (with the exception of a certain radius of each station).

Antennas used must have a high directive effect; these antennas are installed in elevated locations such as large radio towers.

Parabolic Reflector Antenna

• Structure – parabolic reflector and the feed mechanism

• The feed mechanism– house the primary antenna(dipole/dipole array) which

radiates EM waves toward the reflector

• The reflector– reflects the energy into a concentrated, highly directional

emission eave

Features of a parabolic antenna

• Extreme high gain and directivity – used in μ-wave and satellite link

• Beam width (Θ ) Θ = 70λ/D

where λ = wavelength D = antenna mouth diameter Θ = beam width of half power point ( degrees )

Gain

Front to Back Ratio (F/B)

Radiation Pattern Envelope

Cross-Polar Discrimination (XPD)

Return Loss (VSWR)

Interport Isolation (IPI)

Electrical Performance Parameters

Ga (dBi) = 10 log10 h [ 4 p Aa / l2 ]

Where:

Ga = Antenna Directive Gain (Catalog spec)

h = Aperture Efficiency (50-55%)

Aa = Antenna Aperture Area

l = Wavelength (speed of light / frequency)

Parabolic Antenna Gain

Power gain of Parabolic Antenna

• Power gain for a transmit parabolic antenna (Pgain)

Pgain = η(πD/λ)2

where Pgain= power gain referring to isotropic antenna

D = mouth diameter of parabolic reflector(meters)

λ = wavelength(meters)

η = antenna efficiency (typical 55%)

• Power gain in terms of dBi (assume η = 55%)

Pgain (db) = 20 log f(MHz) + 20 log D(m) + 42.2

Illustrated Example

• A 2-m diameter parabolic reflector with 5W of power radiated by the feed mechanism operating at 5GHz with transmit efficiency of 55%

• Beam Width Θ Θ = 70λ/D = 70(3 x 108)/[(5 x 109)(2)] = 2.1o

note how narrow the beam is !

• Transmit power gainPgain(dB) = 20 log 5000 + 20 log 2 + 42.2 = 37.78 dBi

Typical ParabolicAntenna Gain in dBi

Antenna Diameter

Fre

qu

ency

2 ft(0.6m)

4 ft(1.2m)

6 ft(1.8m)

8 ft(2.4m)

10 ft(3.0m)

12 ft(3.7m)

15 ft(4.5m)

2 GHz 19.5 25.5 29.1 31.6 33.5 35.1 37

4 GHz 25.5 31.6 35.1 37.6 39.5 41.1 43.1

6 GHz 29.1 35.1 38.6 41.1 43.1 44.6 46.6

8 GHz 31.6 37.6 41.1 43.6 45.5 47.1 49.1

11 GHz 34.3 40.4 43.9 46.4 48.3 49.9 51.8

15 GHz 37 43.1 46.6 49.1 51 52.6 N A

18 GHz 38.6 44.6 48.2 50.7 N A N A N A

22 GHz 40.4 46.4 49.9 N A N A N A N A

38 GHz 45.1 51.1 N A N A N A N A N A

Radiation Pattern Concept

Antenna Under Test

Source Antenna

Antenna Test Range

Radiation Pattern(a) Polar plot (b) Rectangular plot

Radiation Patternabout Bore sight

Cut Through Radiation Pattern at Boresight

Front-to-Back Ratio

• Ratio of the signal level at beam peak to that directed behind the antenna

• Considered in intra-system interference calculation (hop overreach)

• Expressed in dB

Co-Polarization and Cross-Polarization

• Co-Polarization– Where Transmit & Receive Antennas have

the Same Polarization– Either Horizontal or Vertical (HH or VV)– In your System, The Wanted Signal

Co-polarized antenna pattern

Azimuth AngleR

ela

tiv

e P

ow

er

X-polarized patttern

• Cross-Polarization– Where Transmit & Receive Antennas

have Different Polarizations

– Either HV or VH.

– In your System, The Unwanted Signal

XPD

Inter-port Isolation (IPI)

• Leakage of Signal Between Antenna Ports

• Internal Noise

• Expressed in dBTowards Reflector

Transmit Signal

Leakage Signal

Feed Horn

Voltage Standing Wave Ratio (VSWR)

• VSWR : 1.30 Reflection Coefficient : 13% (0.13)• VSWR : 1.20 Reflection Coefficient : 9.1% (0.091)• VSWR : 1.10 Reflection Coefficient : 4.7% (0.047)• VSWR : 1.08 Reflection Coefficient : 3.8% (0.038)• VSWR : 1.06 Reflection Coefficient : 2.9% (0.029)

VSWR = 1 + (Reflection Coefficient) 1 - (Reflection Coefficient)

Reflected Signal

Incident Signal

Return Loss The Amount of Energy Lost due to Reflected (Returned) Signal

• RL : 17.8dB Reflection : 13% (0.13) VSWR : 1.30• RL : 20.8dB Reflection : 9.1% (0.091) VSWR : 1.20• RL : 26.7dB Reflection : 4.7% (0.047) VSWR : 1.10• RL : 28.4dB Reflection : 3.8% (0.038) VSWR : 1.08• RL : 30.7dB Reflection : 2.9% (0.029) VSWR : 1.06

RL = -20 x log (Reflection Coefficient)

Basic Antenna Types

Standard Parabolic Antenna

Shielded Antenna

GRIDPAK® Antenna

Focal Plane Antenna

GRIDPAK® Antenna

• Grid Reflector

• Low Wind load

• Single Polarized

• Below 2.7GHz

• Shipped in Flat, Lightweight Package

Standard Parabolic Antenna

• Basic Antenna

• Comprised of

– Reflector

– Feed Assembly

– Mount

Focal Plane Antenna

• Deeper Reflector

• Edge Geometry

• Improved F/B Ratio

• Slightly Lower Gain

Shielded Antenna

• Absorber-Lined Shield

• Improved Feed System

• Planar Radome

• Improved RPE

Antenna Efficiency

Well-designed antennas have efficiency ratings of 45 - 65%

Efficiency Factor Affected By :

• Feed Illumination

• Aperture Blockage

• Reflector Surface Tolerance

Efficiency can never be 100%

f

D

f/D = 0.250

Antenna f/D Ratio

f

D

f/D = 0.333

Standard & Shielded Antennas

Focal Plane Antennas

Unwanted Signals

Scattering

Diffraction

Spillover

Front to Back Ratio

Direction of Signal

Direction of Signal

Direction of Signal

Shielded Antenna

Focal Plane Antenna

Standard Parabolic Antenna

Parabolic Reflector Beamwidth

0.3 m 0.6 m 1.2 m 1.8 m 2.4 m 3 m 3.7 m 4.5 m

2 GHz 35 17.5 8.75 5.83 4.38 3.5 2.84 2.33

6 GHz 11.67 5.83 2.92 1.94 1.46 1.17 0.95 0.78

8 GHz 8.75 4.38 2.19 1.46 1 0.88 0.71 0.58

11 GHz 6.36 3.18 1.59 1 0.8 0.64 0.52 0.42

14 GHz 5 2.5 1.25 0.83 0.63 0.5 0.41 0.33

18 GHz 3.89 1.94 0.97 0.65 0.49 0.39 0.32 0.26

23 GHz 3 1.52 0.76 0.51 0.38 0.3 0.25 0.2

38 GHz 1.84 0.92 0.46 0.31 0.23 0.18 0.15 0.12

Beamwidth in Degrees

Diameter

Fre

qu

en

cy

3dB

0

10

20

30

40

50

60

70

80

90

100

0 10 15 205 4060 80 100 120 140 160 180

Azimuth - Degrees from Main Lobe

An

ten

na

Dir

ecti

vity

dB

do

wn

fro

m M

ain

Lo

be

Radiation Pattern Envelope

Radomes

• Reduce Windloading on Tower

• Protection Against Ice, Snow and Dirt

Parabolic dish

antennas

Parabolic dish antenna with Cassegrain feed

Penzias&Wilson

New Jersey - Bell Laboratories

Horn

antennas