Ham Radio General Class Practice Test

What is the reason for the occurrence of reflected power at the point where a feed line connects to an antenna?

1. Operating an antenna at its resonant frequency

2. Using more transmitter power than the antenna can handle

3. A difference between feed-line impedance and antenna feed-point impedance

4. Feeding the antenna with unbalanced feed line

The correct answer is: A difference between feed-line impedance and antenna feed-point impedance

Reflected power occurs at the junction where a feed line connects to an antenna primarily due to a mismatch in impedance between the feed line and the antenna. When the feed line's impedance does not match the impedance at the feed-point of the antenna, a portion of the transmitted power is not absorbed by the antenna and instead is reflected back towards the transmitter. In this context, the feed line is typically designed to have a specific characteristic impedance, often 50 or 75 ohms, but if the antenna is designed to operate at a different impedance, say 30 ohms or 200 ohms, this mismatch leads to the reflection of power. The reflected power can create standing waves along the feed line, which can be detrimental to efficient operation and may even damage the transmitter if not addressed. Understanding this impedance concept is crucial for ensuring that maximum power is transferred from the transmitter through the feed line to the antenna, promoting optimal operation in amateur radio communications.