RF getting into radio?

[WB4CS]

I had a rather strange thing happen and can't figure out the cause. I'm wondering if something is wrong with my mobile antenna, and possibly getting RF back into the radio.

I have a Kenwood TM 281 in my car. Not using the stock mic (using KMC 30 no DTMF mic.) For almost a year I've been using a Larsen 1/4 wave antenna that's mounted on the top of my car on a drilled NMO mount. Yesterday I swapped the 1/4 wave for a new antenna, Comet SBB5NMO.

After mounting the new antenna and trying it out, I can receive very well and signals are much stronger than they were with the 1/4 wave. The radio was transmitting and I could bring up repeaters, but I had no audio. Confirmed buy listening to my transmissions on a handheld. Started troubleshooting the issue this afternoon and now I'm confused.

First, swapped back to the stock mic and audio was good and loud, but there was some static that sounded like the mic cord was loose. I could even wiggle the mic cord and make the noise worse/better. Swapped back to the second mic, worked perfectly loud and clear. Swapped the mic cables between the 2 mics, and had the same issues. Second mic worked but stock mic was scratchy. This is odd considering the stock mic has hardly been used. Now it gets weird. Changed back to the 1/4 wave antenna and both mics worked perfectly, loud and clear. Swapped back to the new anteanna, and both mics are working perfect, loud and clear.

To those with experience and not guesses, does this sound like an RF issue? Issue with the radio? Or intermittent mic issue?

Thanks!

Edit: I don't have access to an SWR meter or antenna anaylser, but that's my next step if the issue returns.

 

[n0nhp]

Have you thrown a watt meter on the radio? High SWR can cause all kinds of problems including stray RF. Does it make a difference if you reduce the power levels?
My guess is that you have RF getting into your radio causing problems. Check to make sure the radio is grounding through the B- wire not the coax. Pull and clean the NMO base make sure you have a good ground there and that rust has not built up under the edges.
If you notice, many radio techs have a decided lack of hair on the tops of their heads, I am one of them ;-). I have spent many hours trying to remedy problems like you are describing.

If there was something that could go wrong with a radio installation I have experienced most of them.


Bruce

 

[LtDoc]

I think it is probably related to using an untuned antenna. At least checking with someone else's meter would be a good idea. As is, you're working 'blind', no way of telling exactly what the problem may be.
- 'Doc

 

[VE3CRO]

Sounds to me like an intermittent problem with the mike connection to the radio. Loose/bent pin, cold solder joint in the radio that is finally giving up or some such.

 

[WA0CBW]

You might check the center pin of the antenna. Not all NMO mounts and antennas are compatible Although the NMO is a standard not everyone makes the part that screws onto the MNO the same. Most NMO antennas have a spring contact or spring loaded pin. Some antennas require different lengths depending on if a rubber washer or "o" ring are used. Measure the height of the NMO mount and the depth of the pin in the antenna and see if they are mating securely. Also check to see that the diameter of the pin and the pad on the NMO are aligned and the same size.
BB

 

[HF-Mobile-Antenna]

Remember that a choke can be added to limit this problem.