Communication Systems Flashcards

What VSWR reading indicates a perfect impedance match between a VHF comm transmitter and its antenna?

Answer: 1:1

A 1:1 VSWR means no power is reflected, so the load matches the line impedance exactly.

An antenna system shows a VSWR of 3:1. Approximately what percentage of forward power is reflected?

Answer: 25%

Reflection coefficient = (3-1)/(3+1) = 0.5, and reflected power = 0.5² = 25%.

Which piece of test equipment should be connected to a transmitter's output instead of the antenna when testing power on the bench to avoid radiating a signal?

Answer: Dummy load

A dummy load is a non-radiating 50-ohm resistive load that absorbs transmitter power safely.

What is the characteristic impedance of the coaxial cable normally used for aircraft VHF communication antennas?

Answer: 50 ohms

Aircraft RF systems such as VHF comm use 50-ohm coax like RG-58 or RG-400.

A technician suspects a break somewhere along a long coax run in the fuselage. Which instrument best locates the fault distance?

Answer: Time-domain reflectometer

A TDR sends a pulse down the cable and times the reflection to pinpoint the fault's distance.

Which connector is most commonly used on VHF comm transceiver coax connections in general aviation aircraft?

Answer: BNC

BNC bayonet connectors are standard for VHF comm and nav coax on many aircraft.

An in-line RF wattmeter shows high reflected power immediately after a VHF antenna was replaced. What is the MOST likely cause?

Answer: Poor antenna ground plane bonding

A monopole blade antenna needs a good bond to the airframe ground plane to present the correct impedance.