563489578934

Problems 665
★ 8–26 A TV set is connected to an antenna system as shown in Fig. P8–26. Evaluate
(a) The overall noise figure.
(b) The overall noise figure if a 20-dB RF preamp with a 3-dB noise figure is inserted at
point B.
(c) The overall noise figure if the preamp is inserted at point A.
A
125 ft of 300 ohm transmission
line with loss 3.0 dB/100 ft B TV set
F=16 dB
Figure P8–26
8–27 An Earth station receiving system consists of a 20-dB gain antenna with T AR = 80 K, an RF
amplifier with G a = 40 dB and T e = 30 K, and a down converter with T e = 15,000 K. What is the
overall effective input-noise temperature of this receiving system?
8–28 A low-noise amplifier (LNA), a down converter, and an IF amplifier are connected in cascade.
The LNA has a gain of 40 dB and a T e of 25 K. The down converter has a noise figure of 8 dB and
a conversion gain of 6 dB. The IF amplifier has a gain of 60 dB and a noise figure of 14 dB.
Evaluate the overall effective input-noise temperature for this system.
8–29 A geosynchronous satellite transmits 13.5 dBW EIRP on a 4-GHz downlink to an Earth station.
The receiving system has a gain of 60 dB, an effective input-noise temperature of 30 K, an
antenna source noise temperature of 60 K, and an IF bandwidth of 36 MHz. If the satellite
is located 24,500 miles from the receiver, what is the (CN) dB at the input of the receiver
detector circuit?
★ 8–30 An antenna with T AR = 160 K is connected to a receiver by means of a waveguide that has a
physical temperature of 290 K and a loss of 2 dB. The receiver has a noise bandwidth of 1 MHz,
an effective input-noise temperature of 800 K, and a gain of 120 dB from its antenna input to its
IF output. Using MATLAB find
(a) The system noise temperature at the input of the receiver.
(b) The overall noise figure.
(c) The available noise power at the IF output.
8–31 The Intelsat IV satellite uses 36-MHz transponders with downlinks operating in the 4-GHz band.
Each satellite transponder has a power output of 3.5 W and may be used with a 17° global coverage
antenna that has a gain of 20 dB.
(a) For users located at the subsatellite point (i.e., directly below the satellite), show that (CN) dB =
(G AR T syst ) dB - 17.1, where (G AR T syst ) dB is the Earth receiving station figure of merit.
(b) Design a ground receiving station (block diagram) showing reasonable specifications
for each block so that the IF output CNR will be 12 dB. Discuss the trade-offs that are
involved.
★ 8–32 The efficiency of a parabolic antenna is determined by the accuracy of the parabolic reflecting surface
and other factors. The gain is G A = 4phAl 2 , where h is the antenna efficiency. In Table 8–4
an efficiency of 56% was used to obtain the formula G A = 7Al 2 . In a ground receiving system for