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RallyX Weenie
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672 Posts
Discussion Starter #1
Here's the setup:
Yaesu FT2900 wired directly (fused) to battery.
Magmount antenna on roof
Comet 1/4 wave dual band antenna

Aviation Headsets
Homebrew (repin + impedance change or something...) Adapters
Terraphone Plus, wired directly (fused) to battery.

On transmit (only tried at 5W), getting a fairly loud "taptaptap" heard through the intercom. It's louder if we've got the patch cable running from the radio to the audio in, but it still occurs if unhooked - just quieter.

It'll be manageable for our use (running 00 at Idaho), but would be preferable to eliminate if possible. Any ideas?

(Any suggestions on eliminating an engine-speed whine [alternator] coming through the intercom will be appreciated as well...)
 

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Unshielded or broken/non-continuous shield on intercom wiring? Coax to 1/4 wave antenna has shield damage, so signal is escaping inside the vehicle? I am not familiar with the Terraphone's construction or circuitry, but any amplifier circuit should be well shielded (large ground planes surrounding circuit board) and should have a metal enclosure for the components. Antenna line runs parallel to signal carrying lines for intercom? Shielding should keep all signals separate, but.... While not "high" voltage, your 2900's signal could certainly induce a new signal in the intercom lines - if shield and grounding is inadequate. The fact that you describe additional volume being generated when you plug in the audio patch cable, which often have light shielding, suggests some of what I am guessing at.

Then again, the "taptaptap" sound that you describes makes me think of power relays chunking in with commensurate possible voltage spikes or sags, etc. Is the sound continuous as you transmit?
 

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RallyX Weenie
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672 Posts
Discussion Starter #3
the noise is continuous.

Shielding issues on the intercom wiring (or the intercom itself!) certainly seems likely - not sure what's going on in the homebrew adapters. On a Peltor/terraphone pro, are all 4 wires connected at both end, or is one of the four connections the shielding (and thus, only connected at one end?)?

I suppose it could be in the coax - it's pretty new though a year old, with about 6 weeks (plus two rally weekends) of being closed in a doorjamb (that's rarely opened).

First thing that we tried was moving the antenna coax away from the intercom, but no change there.
 

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which left?
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Try powering one off of a separate battery (even just to test). If the problem goes away, you need to isolate the power better. It is not uncommon for things like rally intercoms (or rally odos), which aren't mass-produced, FCC certified devices to put off or pick up a lot of excess RF. Separating the power supplies as much as possible helps.
 

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There are 2 possibilities: ground loop noise, and noise due to RF interference (or some combination thereof). I'd try doing the test suggested by Mark above i.e. try powering either the intercom or the radio off a completely separate isolated battery (e.g. separate standalone battery, or battery from another vehicle next parked next to it), so that you can try & determine how much of the noise is due to any potential ground loop issues, and how much is due to any potential RF interference issues.

Also, as Mark pointed out, limited production non-FCC type approved devices, like rally intercoms, aren't guaranteed to not put out or pick up interference. The truth of the matter is you are pretty much at the mercy of the designers & build of the device, and probably won't be able to eliminate all of the interference; but if you can reduce it a little to a manageable level, then that would be worthwhile.

I haven't opened up my old Terraphone, but I have opened up my Peltor, and IIRC it's just a molded plastic case with the circuitry inside i.e. no real metal shielding inside. Also, the 2 long headset cords coming out of the intercom may act as 2 long antennas picking up RF interference. The Peltor headset cables have 4 wires: mic signal & mic ground, and headset signal & headset ground (i.e. separate grounds for mic & headset). The Terraphone headset cables have 3 wires: mic signal, headset signal, common ground. I haven't cut either of the cables open, so I no idea whether those 3 or 4 wires, depending on which system you are using, have any shielding around them.

I do know that I have been in several rally cars where we have used a Peltor FMT110 powered off its own internal 9V battery (i.e. not connected to the car battery or ground in any way at all), and when we are sitting at the start of a stage waiting for our minute, we still often get brief chirping & buzzing in our headsets connected to the intercom when the ham operator at stage start radios to net control that we or the car(s) in front of us are about to start, or is relaying some other message. So it is unlikely you'll be able to completely eliminate the intercom picking up any RF interference from a very nearby radio transmitting at 50+ W.
 
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