Radio Feedback into Cellphone Handsfree Kit

[john hearne]

I just finished an install that included a Motorola Astro Radio (110 watt VHF) and a Motorola Handsfree kit for the 557 phone. Whenever I key the radio, I get nasty feedback out of the handsfree speaker. This happens whether the antenna is not yet hooked to the handsfree kit.

I can't imagine I'm the first this has happened to - any suggestions or possible fixes?

Edited to add:
The handsfree kit is installed in a 2006 Crown Vic. The power and ground for the handsfree kit are coming directly from the wire that fed the power point in the factory ashtray. The ashtray was removed and the factory connector was removed. The Radio is an Astro Spectra that is trunk installed. Power is from the factory power point. Ground is to the body with a star washer for a better ground.

[Birken Vogt]

I have my Motorla HFKs make noise when the radio transmits too. Depends on frequency. One that was really severe I put ferrite snap-together chokes on all the wires going to the juction boxes, the others I have just left alone because they are not bad.

It's just the age of mediocrity....

Birken

[Ranger522]

I have found that the feedback is directly related to the antenna.

-With a Wilson trucker (short, trunk-lip model) I have so much feedback that I cannot use the radio without disconnecting the phone first (dispatchers complain).

-With the same antenna (only the longer mount, above the roofline) I have very little feedback.

Also note that we have only experienced this on certain VHF digital frequencies (John and I work together).

I will have to try the ferrite chokes and see if it makes a difference.

[Birken Vogt]

The ferrite chokes did not completely quiet the bad one but they made it not noticeable any more. Before that I tried switching antennas on the radio (QW152 was on there and I put on a B132S and I think a 5/8 wave IIRC) and also I was using the Wilson dual band antenna on a roof mount and if I know myself I would have tried a QW800. All of these things made different results, and we use frequencies from 150 to 174 and everywhere in between, and the frequency used made a difference too. All conventional FM. Also one vehicle has the antennas comparatively close together but no feedback at all.

One thing I have noticed is most Wilson antennas tend to be tuned for the 700 MHz range as they come from the factory, per the network analyzer. The trucker style are adjustable I just have never bothered to do it since they usually exhibit an acceptable SWR per the AEA antenna analyzer when installed. I ought to try that sometime when I get some free time, yeah right

Birken

[Tom in D.C.]

It's not "feedback" in the classic sense. It's RF getting into
the front end of the phone's receiver, plain and simple. Your Spectra's 110 watts is a LOT of power for any other receiver in the
area to ignore and it probably is simply being overpowered
by the RF transmitter of the Spectra.

Ferrite beads on power leads, shielding the phone, relocating
various pieces of equipment, better grounding (like with woven
metal straps instead of wires) etc. etc. make problems such as
this an interesting crap shoot. I would start with the ferrite
beads and be prepared to spend some time putting them in
different locations to test the results. Good thing about them
is they're pretty inexpensive.

[Ranger522]

What's interesting is that we just changed from Nokia phones and car kits, never had a problem, to the Moto V551/557's when all of the problems began. Does the Nokia phone/kit have better shielding or is it the change from TDMA to GSM?

[Birken Vogt]

I think the problem is actually RF getting into the audio amp of the junction box, not the phone itself, though it may be carried down the antenna wire.

Birken

[john hearne]

I installed another handsfree kit in an almost identical vehicle. This time, I clipped the power wire forward of the factory fuse block. The handsfree kit does not exhibit any signs of RFI at all.

During my experiments with making the kit work in my car, I found that by not coiling the power cord, it worked better. I suspect that the power cord needs to be as short as possible to avoid these problems.