Practice Problems

Practice Problems#

  1. A CIA operative with SOF support is trying to upload data to a Cessna flying at 1000 ft AGL in the People’s Republic of Pineland. He is using a VSAT (very small aperture antenna), a parabolic dish, to upload the data. The CIA operator’s antenna is 1 m in diameter, has an efficiency of 80%, and transmits 120 W at 3.7 GHz. The Cessna has a small parabolic dish that is 500 mm in diameter with an efficiency of 75%. Assuming the Cessna is 80 km away and requires at least 200 nW of received power, does the CIA operative have valid communications? If so, what is the received power at the Cessna? (Assume the CIA terminal is at 30 ft AGL.)

  1. A search and rescue helicopter is at 450 ft AGL due to clouds, while the airman they are searching for is on a 50 ft hill. The downed airman’s survival radio transmits 6 W of power at 2 MHz, with an antenna gain of 3. The helicopter’s radio has a gain of 3.2 and requires 3 µW of received power to communicate effectively. Will the airman be able to communicate with the helicopter at 50 km? What is the maximum communication distance?

  1. T / F: Decibels are fundamentally used to represent a voltage ratio.

  1. Two identical communication towers are to be placed 40 miles apart. A parabolic dish antenna at the top of each tower will be used to relay telecommunication signals.

    a. Assuming relatively flat terrain, how high must the towers be to establish line-of-sight communications?

    b. For a transmit frequency of 3.5 GHz, each antenna requires a gain of 1986. What radius dish should be used? (Assume an efficiency of 91%.)

  1. A reconnaissance UAS is flying over flat terrain at 800 ft AGL and attempting to detect radio traffic from an insurgent group. The insurgent camp is on a plateau 25 ft above the surrounding terrain. The insurgents transmit 120 W at 24 MHz using an antenna with a gain of 1.5. The UAS uses an antenna with a gain of 3.0 and requires 80 nW of received power to detect the signal. If the UAS is 80 km away, will it detect the transmission?

  1. A HC-130 is flying a search pattern at 10,000 ft AGL. Your radio transmits a 4 W signal at 243.0 MHz with an antenna gain of 2.8. The HC-130 has an antenna gain of 3.0 and requires 496.8 aW of received power to detect the signal.

    a. How far away can the HC-130 detect and process your signal?

    b. At that distance, what is the signal-to-noise ratio (SNR) in dB if the noise power (\(P_{\text{noise}}\)) is 66.24 aW?