Question T9B02
From subelement T9 - T9B
What is the most common impedance of coaxial cables used in amateur radio?
Why is this correct?
50 ohms is correct because most amateur radio transmitters have 50-ohm output impedance and most antennas have input impedance near 50 ohms. To minimize signal loss, the feed line impedance should match both the transmitter output and antenna input. The other choices are wrong: 8 ohms is speaker impedance, 600 ohms is used for some balanced lines like ladder line, and 12 ohms has no standard application in amateur radio feed lines.
Memory tip
Look for impedance matching patterns: when three components connect (transmitter-feedline-antenna), they should all have the same impedance for maximum power transfer. This 50-ohm standard simplifies amateur radio system design and ensures compatibility between manufacturers' equipment.
Learn more
The 50-ohm standard creates a complete ecosystem in amateur radio. Your transceiver outputs 50 ohms, your coax carries 50 ohms, and your antenna presents 50 ohms—this impedance matching maximizes power transfer efficiency. Compare this to cable TV systems that use 75-ohm coax, which works but creates impedance mismatches in amateur stations, reducing signal strength and potentially causing standing wave ratio issues.
Think about it
Why do you think amateur radio settled on 50 ohms instead of 75 ohms like cable TV systems, and what would happen to your signal strength if you mixed different impedance components?