0195136047.pdf

15.1 WAVES, TRANSMISSION LINES, WAVEGUIDES, AND ANTENNA FUNDAMENTALS 683

(a) (b)

Main

lobe

Main

lobe

0

Relative radiation

intensity (dB)

Side lobe

Side

lobes

Relative

radiation intensity

1.0

−0.5

z

x

θB θB

θ

θ

Figure 15.1.9Typical radiation pattern

in one principal plane as a function of

θ.(a)Polar-coordinate plot.(b)Linear-

coordinate plot.

(+z-direction). The radiation pattern, with linear polarization, exhibits a principal lobe in the

+z-direction. The other elements, calleddirectors, are designed to enhance radiation in the +z

direction. Gain increases with the number of elements and is often in the range of 10 to 20 dB.

The bandwidth is usually small.

Both conical-horn and pyramidal-horn (aperture-type) antennas are mainly used asillumi-

natorsfor large-aperture paraboloidal antennas, which are capable of generating very narrow

beamwidth patterns (with even less than 1° in angle-tracking radars). When a parabola is rotated

about thez-axis [see Figure 15.1.17(g)], a surface of revolution known as a paraboloid results.

With the source at the focus called thefeed, the radiation pattern is mainly a dominant lobe in

thez-direction with smaller side lobes. Paraboloidal antennas have found wide use as antennas

for radar and communications. The Cassegrain antenna [shown in section in Figure 15.1.17(h)]

is a variation of the paraboloid that gives improved system performance. The feed in this case

is moved to the rear of the antenna, and it illuminates a conducting surface (subreflectorin the

shape of a hyperboloid) placed near the focus.

Noise in receiving systemsexists in two broad categories: (i) that originatedexternal to the

system(i.e., the one generated by the antenna in response to random waves from cosmic sources

and atmospheric effects), and (ii)internally generated noise(i.e., the one generated within all

circuits making up the receiver, including transmission lines and amplifiers). It is common to

model internal noise as having been generated by an external source.

Figure 15.1.11(a) shows a typical receiving system with noise. The antenna is a source of noise

with effective noise temperatureTa, known as theantenna temperature. Whatever receiving path

components (such as transmission lines, waveguides, and filters) are present prior to the receiver’s

amplifier, their noise effect is represented by a noisy lossLr(≥1) between pointsAandB, while

the loss is assumed to have a physical temperatureTL. The noisy receiver is supposed to operate at

a nominal center frequencyf 0 , and have available power gainGa(f) as a function of the frequency,

withGa(f 0 ) as center-frequency power gain.Naorepresents the total available output noise power

in Figure 15.1.11(a).

Figure 15.1.11(b) shows the noise-free model in a small frequency banddf. Herekis the

Boltzmann constant (see Section 14.3), andTR(f)istheeffective input noise temperature(to the

noise-free receiver).

Figure 15.1.11(c) gives the noise-free model withnoise bandwidth BN(a rectangular passband

of widthBNin hertz centered onf 0 ). The actual receiver is replaced by an idealized one with the

same nominal power gainGa(f 0 ) and aconstant(average) effective input noise temperatureT ̄R,

which is related to theaverage standard noise figure F 0 ;

T ̄R= 290 (F 0 − 1 ) (15.1.28)