Antennas
5.11 NOISE RECEIVED FROM AN ANTENNA
Radio frequency beam forming
using a corporate
feed Antenna
elements
RF Amp.
RF Amp.
RF Amp.
RF Amp.
RF Amp.
RF Amp.
RF Amp.
RF Amp.
RF Amp.
RF Amp.
RF Amp.
RF Amp.
RF Amp.
RF Amp.
RF Amp.
RF Amp.
RF Amp.
RF Amp.
IF Amp.
IF Amp.
IF Amp.
IF Amp.
IF Amp.
IF Amp.
IF Amp.
IF Amp.
IF Amp.
IF Amp.
IF Amp.
A to D conv.
A to D conv.
A to D conv.
A to D conv.
A to D conv.
A to D conv.
A to D conv.
A to D conv.
Intermediate frequency (IF) beam forming
Local oscillator
(LO) Local
oscillator (LO)
Coherent oscillator (COHO) Radio
frequency
amplifiers Mixers Mixers Synchronous
detectors Digital beam forming
Combined radio frequency (RF)
output
Combined intermediate frequency (IF) output
Combined digital output
2 2
2 2
2 2
2 2
2 2 2
2 2
2 2
2 2
2
2 2
RF Amp.
Output signal to noise ratio, each amplifier ' Sin k T B
Arf Arf
Output signal to noise ratio, all amplifiers ' n² n
Sin k T B
Arf
Arf ' n Sin k T B
Sin n Sin nkTB
Figure 5.78 Radio frequency, intermediate frequency, and digital beam forming systems.
(5.106)
(5.107) When each element feeds a radio frequency amplifier, the signal-to-noise ratio at the output of each amplifier is
Each of the wanted signals ( volts) adds coherently and the sum is volts. The noise adds incoherently to give in voltage.
This is the same as (5.106) above so that there is no difference in performance between passive and active radio frequency combination. The signal-to-noise ratio is not changed by shaping the beam as noise is attenuated at the tips of the aperture function as well as the signal.
Distributing the transmitter and receiver components among many parallel elements in an antenna gives a characteristic known as graceful degradation. That is, if one transmitter-receiver element fails, the radar can continue in operation with slightly reduced, but acceptable, performance. Low sidelobe antennas depend on very accurate illumination functions so that the sidelobe performance will be affected to a much greater degree.
Te ' TT (L&1)
ATTENUATOR
SOURCE LOAD
TS Loss = L
Temperature=TT
TS = 0 K Attenuator contribution = TT( 1 - 1/L) TS K After attenuator becomes TS /L K
Contributions Source Attenuator Tout ' TS
L % TT 1&1 L
+
k TS B
k TS B L
k TT(1- 1/L) B Attenuator
loss=L
Attenuated noise
Attenuator component of noise
.
(5.108)
Figure 5.79 Arrangement of source, attenuator, and load.
(5.109)
Figure 5.80 Additional noise from an attenuator.
The noise at the output port of an antenna is the cosmic noise from outer space, the noise from the sun or radio stars in certain cases, the noise from the warm atmosphere, and the noise from the resistive losses in the antenna itself [25, p.
415]. It is assumed that there are no amplifiers and that the system is completely passive. The noise components add and are then attenuated.
The behavior of an attenuator with a source and a load is illustrated in Figure 5.79. If the source temperature is 0 K then the noise power delivered to the load is totally generated in the attenuator. Referring the noise temperature to the input [1], the load thinks that the effective source temperature T ise
The load sees an impedance which is partly that of the source with the remainder the resistive impedance of the attenuator. It is like looking through mist as the resistive component in the attenuator is itself a noise generator.
If the source has a temperature T , the noise power is kT B W. This is attenuated by the attenuator to give kT B/LS S S W so that the source temperature as seen by the load is T /L K. The total temperature at the output is S
The effects of this are shown in the block diagram Figure 5.80. The input noise is attenuated and a component generated in the attenuator is added.
Figure 5.81 shows the effects of an attenuator at 290 K, the IEEE standard temperature, on the effective noise
0 100 200 300 400 500
100 200 300 400 500
Loss =
4 4 4 4
No loss Loss
= 1 dB Loss
= 2 dB Loss = 3 dB
TS K Tout K
0 50 100 150 200 250
2 4 6 8 10
Attenuator loss dB
Figure 5.81 The effects of an attenuator at 290 K on the effective noise temperature at the output.
Figure 5.82 The additional temperature component from an attenuator at 290 K.
temperature out. If the attenuator is warmer than the source, the attenuator adds noise. If the source is warmer, the attenuator reduces the noise temperature. At infinite attenuation the output sees a matched load.
Figure 5.82 shows the additional noise temperature produced by a resistive attenuator at 290 K according to (5.109).
The noise contributions for a ground radar from cosmic noise, the sun, the ground, and the antenna are shown in Figure 5.83 [27]. Noise components from the sun and outer space [26] are first attenuated by the atmosphere.
0°
1°
2°
5°
10°
30°
90°
30°
90°
0°
10°
Maximum galactic noise, 10×sun
Minimum galactic noise without the sun at 90°
100 MHz 1 GHz 10 GHz 100 GHz
Radio frequency 10 000
1 000
100
10
1
ELEVATION ANGLE
100 MHz 1 GHz 10 GHz 100 GHz
Radio frequency 10
1
0.1 100
Water resonance 22 GHz Oxygen
resonances at approximately 60 GHz
0°
1°
2°
5°
10°
30°
ELEVATION ANGLE
90°
1°
2°
5°
10°
30°
90°
Figure 5.83 Noise temperature of an idealized antenna as a function of frequency and elevation angle. [Source: Blake L. V., A Guide to Basic Pulse Radar Maximum Range Calc- ulation, U.S. Naval Research Laboratory Report, 23 December 1969, p. 126, Figure 11.]
Figure 5.84 Two-way atmospheric attenuation for a transit of the entire troposphere. [Source: Blake L.
V., A Guide to Basic Pulse Radar Maximum Range Calculation, U.S. Naval Research Laboratory Report, 23 December 1969, p. 73, Figure 21.]
This attenuation is shown in Figure 5.84.
Antenna Atmospheric loss Latm
Temperature Tatm
Noise from ground Tground Cosmic noise
Tcosmic
Isotropic level Antenna pattern
Noise from the sun Tsun
.
Latm
Tsun
TCosmic
Tground
Tantenna
+ +
Antenna surface Weighting
(gain) E" = 1
Antenna output Gain measurement
point Resistive
losses in the antenna
"sun
"cosmic
"atm
"ground
Lant
Atmospheric losses
Latm
Tatm
Tantenna surface ' "cosmic Tcosmic
Latm % "sun Tsun
Latm % "atm Tatm % "ground Tground Note: "cosmic % "ground ' 1
Figure 5.85 Noise components in the antenna.
Figure 5.86 How the noise components sum at the antenna output.
(5.110) The contributions made by the various noise sources [28, p. 152 et seq.] depend on the integrated antenna gain over the solid angle where the noise occurs. These are attenuated and added as shown in Figure 5.85 where the weighting factors
" in Figure 5.86 represent the various gain factors.
• "sun is the integrated sidelobe gain over the solid angle subtended by the sun;
• "cosmic is the noise gain (integrated over the solid noise beamwidth) of the antenna;
• "ground is the integrated sidelobe gain over the solid angle subtended by the ground.;
• "atm is the integrated gain for atmospheric noise.
The noise arriving at the surface of the antenna is given by
Rule of thumb values for " assume that "cosmic = "atm = 1 - "ground . Values for the component of Tground at 290 K for a number of values of " are [28, p. 171]:
Toutput ' Tantenna surface
Lantenna % Tantenna 1& 1
Lantenna (5.111)
"cosmic 0.975 0.924 0.876
"ground 0.025 0.076 0.124
Ratio dB 16.02 11.19 9.067
T g 7.25 22.04 35.96 K
The noise which arrives at the output is attenuated by the resistive losses in the reflector, the feed system and beam forming networks, and the waveguide or coaxial line leading to the output flange where the antenna gain measurement was made. This is