Explain how RF gets into audio equipment, what each sound means, and why ground wires and bonding matter.
Syllabus: Interference to consumer electronics; grounding and bonding
Compendium articles on the ideas in this lesson, beyond what the exam asks.
Practise the 12 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".
Cables pick up your RF as common-mode current: the same RF on every conductor. Two fixes:
Shorting the center conductor to the shield or grounding it would also short the audio. Extra insulation does nothing.
RememberChoke the cable, or bypass the RF to ground.
SSB has no steady carrier, so the audio device detects a distorted version of the speech. CW switches the carrier on and off, so you hear on-and-off humming or clicking. A wide range of frequencies points to arcing at a poor electrical connection.
RememberGarbled speech: SSB. Keyed thumps: CW. Wideband noise: an arc.
At RF a wire has impedance, not just resistance. If the ground wire has high impedance at your frequency, RF voltage appears on equipment and can cause RF burns. A resonant ground wire is the worst case: it presents high impedance at the equipment and puts high RF voltage on enclosures.
RememberKeep RF ground wires short, and never resonant on a band you use.
| Question | Answer |
|---|---|
| Ground loops (hum on transmitted audio) | Bond equipment enclosures together |
| RF "hot spots" | Bond all enclosures together |
| Why ground every metal enclosure | hazardous voltage cannot appear on the chassis |
| Lightning-protection grounds | No solder: a strike's heat can destroy the joint |
RememberBond everything together; ground it for safety; never solder a lightning ground.