0195136047.pdf

684 COMMUNICATION SYSTEMS

(a) (b) (c)

x

y

z

x

y

z

x

y

z

E− E−

E−

Figure 15.1.10Polarizations of the electric field.(a)Vertical polarization.

(b)Horizontal polarization.(c)General linear polarization.

Noisy loss Lr

Antenna with antenna temperature Ta

Physical temperature TL

Nao

A

Available noise power kTsysBN

B Noisy

receiver

Available power gain Ga(f)

Center-frequency power gain Ga(f 0 )

Noise-free

loss Lr

(a)

TL

dNao

ABNoise-free

receiver

Ga(f)

kTL(Lr − 1) df kTR df

(b)

(c)

Lr

Ga(f 0 )

A

Noise-free

loss and receiver

Available power gain Ga(f 0 )/Lr

Noise bandwidth BN

Nao = kTsysBN

Figure 15.1.11(a) Receiving

system with noise.(b)Noise-

free model in a small frequency

banddf. (c)Noise-free model

with noise bandwidthBN.

Note that bothF 0 andT ̄Rare measures of the noisiness of an amplifier. The noise figureF 0 is

usually available, whereas the temperatureT ̄Rmay not be given. In an ideal noise-free unit,F 0 = 1

andT ̄R=0.

Thesystem noise temperature Tsys, which is the equivalent noise temperature of the antenna,

can now be introduced such that

Tsys=Ta+TL(Lr− 1 )+T ̄RLr (15.1.29)

and all available output noise power is emanating from the antenna, as illustrated in Figure

15.1.11(c). Now the available system noise power becomeskTsysBNat pointA; the noise-free

receiver has available power gainGa(f 0 )/Lrwith noise bandwidthBN; and the output noise

power is given by