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I've been told that I have "RF in my signal". What does this mean? What are some potential causes of having RF in the signal?

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  • $\begingroup$ What mode? What frequency? $\endgroup$ – Phil Frost - W8II Jun 19 '17 at 12:48
  • $\begingroup$ USB on 20-meters $\endgroup$ – Jerry Jun 19 '17 at 15:31
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    $\begingroup$ 1. At what power level does this happen? 2. How close is the antenna to your radio? 3. A 4-turn air core balun is likely not enough for 20. See the bar charts on G3TXQ's page; the bars where the green falls on 20m is what you need. $\endgroup$ – Mike Waters Jun 19 '17 at 17:11
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    $\begingroup$ I'm using a Kenwood TS-520 at 100 watts. The radio is old, and I wonder if that's a factor. The antenna is about 60 feet away, fed by RG-213 coax. I'll try to improve my balun sometime soon, and I'll report the results back here. $\endgroup$ – Jerry Jun 20 '17 at 19:23
  • $\begingroup$ @Jerry The 520 should be OK. At 100w and 60', something else may be be causing this. For example, I have an MFJ-941C that is not completely shielded (it has plastic sides). Besides the TS-520, what other accessories are near the 520? $\endgroup$ – Mike Waters Jun 20 '17 at 19:30
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It most likely means that unwanted RF from your antenna system is getting into your transceiver's audio input (microphone), distorting your audio.

Some solutions are reducing any common-mode RF on your feedline, moving the radio and antenna further apart, or using ferrite and/or capacitors on the audio line.

A power supply that cannot supply enough peak current can cause a similar effect. On voice peaks, the voltage drops.

EDIT: As Scott nicely pointed out, the proper balun at the feedpoint can reduce common-mode RF (on the outside of the shield). What band(s) were you on when this happened? That makes a difference in which ferrite mix to use for the balun. My all-time favorite reference for common-mode chokes is K9YC's PDF.

"Ugly Baluns" (made from coax) are often recommended, but they are ineffective unless care is taken to use the correct number of turns wound on the proper diameter. G3TXQ explains this beautifully on his site.

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  • $\begingroup$ I should've given more specifics: I'm on the 20-meter band, and using a half-wave dipole. The SWR is very low across the band (1.4 max). I'm using a coax balun with the number of turns that was recommended in the ARRL Handbook (I think it was 4 for the 20-meter band, but I don't remember). Thanks for the PDF links. $\endgroup$ – Jerry Jun 19 '17 at 15:34
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One symptom of having 'RF in the shack' is that metal parts of the radio (or morse key, or microphone) will feel "hot" to the touch, and can give a nasty burn - or at least a bit of a surprise! Mike has already suggested several ways of reducing this, and I would add that when he talks about reducing common-mode RF on the feedline, this can sometimes be achieved inexpensively by the use of a balun. A good article explaining this can be found here.

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I've never heard of "RF in your signal". The signal is RF. It doesn't make any sense.

I wouldn't jump to the conclusion that "RF in the shack" is the problem. While common-mode current may be a consequence of your antenna design, a distorted signal is not the first symptom I associate with those problems. Many a ham has transmitted into a coax fed dipole with no balun, and it works well enough.

Or consider the case of a handheld radio, where all the antenna current must flow around the radio. If common-mode current near the transceiver caused notable distortion, how could these work at all?

More likely, "RF in your signal" just means your signal is distorted, somehow. Consider the qualifications of the person who gave you this report and treat it with the appropriate degree of skepticism.

Before making any conclusions, I'd try to corroborate that report, and get a more objective assessment of the distortion. Perhaps you can listen to yourself on the internet, or get a report from an experienced operator (that is, not most hams). The cause of the distortion could be something pedestrian, like the mic gain is too high.

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  • $\begingroup$ I agree. You might want to check settings on your rig that my cause distortion such as audio compression that when too high can overdrive and clip your audio signal resulting in distortion. I would also get someone to help you test your signal over the air and try different things and if possible consider testing with a different mic. $\endgroup$ – K7PEH Jun 21 '17 at 15:34
  • $\begingroup$ Phil, I fully agree that the distortion might not be caused by RF getting into his audio. However, I have seen countless cases where it was. That expression is incorrect as you state, but it's common. arrl.org/forum/topics/view/314 $\endgroup$ – Mike Waters Jun 21 '17 at 16:59

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