Odd RF problem

[Atomic_Dog]

I replaced the head unit in my 2001 because of bad radio reception, but it didn't fix the problem. Now my 2M VHF ham radio loses all reception whenever the engine is running. I don't have noise drowning out the FM signal. It's more like the signal is just being jammed. What could cause that much RF and not create a whine or other noise? Thx

 

[2TH4IST]

How are the power and ground for the ham radio, connected straight to the battery? Are the coax connections tight and not corroded? Is the antenna tight to the base? Is the coax shorted anywhere? Or is a large amount of excess coax coiled under the carpet? Do you use an external speaker, if so is it good and working?

How are the vehicles power and grounds?

 

[phattyduck]

Do you have any LED lights that might cause interference? I had issues with 2M reception with some Rigid LED fog light cubes... A big ferrite and some HF caps on the lights as a filter helped, but swapped up to some Baja Designs stuff fixed it for good.

-Charlie

 

[Atomic_Dog]

No LED lights. The radio is wired directly to the battery and is in the side compartment in the left rear. The coax goes up and through the roof to the antenna mounted on a large aluminum rack. Maybe a 6ft feed line. It has worked in this configuration for at least a year. I tried replacing the radio and antenna. Using a 5/8 wave antenna helped a little but not much. The receive goes dead quiet with the engine running even with a full quieting signal when the engine is off. FWIW, I still have receive on the stereo head unit and it's still not great, but it does work and hasn't changed. The 2M problem happened all at once. Thanks

 

[phattyduck]

Next - does it happen with key-on engine-off or ONLY with engine running?

Are there ANY other aftermarket things on/plugged in? (phone chargers, stereo, etc.)

Is the rack separately grounded to the vehicle chassis or is it only grounded via the coax?

You may also want to check engine/chassis grounds closely since it might be ignition related.

-Charlie

 

[2TH4IST]

Yes like phattyduck said does it happen with the key on engine off? I'm wondering if your alternators rectifier is failing and letting out a ton of AC voltage?

 

[phattyduck]

Yes like phattyduck said does it happen with the key on engine off? I'm wondering if your alternators rectifier is failing and letting out a ton of AC voltage?

Oh, that's a good thought. You can 'turn off' the alternator by unplugging the 3-pin connector on it or the 4-pin connector on the bottom of the fuse box towards the alternator. That would be another easy way to check for a cause.

-Charlie

 

[Atomic_Dog]

Disconnecting the alternator fixes the problem. I can hear a pulse on the audio when I lower the squelch, but it doesn't change with RPM. The Denso alternator is only about a year old. Thx

 

[Atomic_Dog]

Well ... Replacing the alternator did not fix the problem. Any thoughts? Thx

 

[rallypoint_1]

That’s some wild attenuation. Have you tried connecting a VNA to see if your coax and antenna check out? Maybe a different radio? Only since it’s not change with a new alternator.

I have a radio and a Sirio mounted on the roof. The body is bonded to the frame at 8 spots along the body. Also doors, hood and hatch to body.

Looking for RF gremlins is so frustrating.

 

[fourwd1]

Does the received signal strength indicator/meter on the VHF radio show a large increase when this happens?
If so, have you tried disconnecting the antenna from the radio to see if the noise level drops? If it does the noise source is likely radiating the noise rather than sending it thru the DC power.

And in case it's coming in thru the 12VDC, have you added any kind of filtering to the DC line close to the radio?