Vehicle RFI Issue

[KQ4RBK]

Hello,

I recently installed an FTM-200dr with a Comet 2X4SR on an NMO roof mount in my 2011 Chevy Tahoe and am having an RFI issue I'm trying to track down. Video here:

A few things I've noted:

• There are two distinct sounds that are overlaid on each other. One is a loud chugging noise that sounds like a large diesel truck idling. This sound does not change with engine RPM. The second is a high pitched whine that does vary with RPM. If you listen close you can hear it in the video as I change engine speeds. I assume the latter is alternator whine.
• It is loud enough to break the squelch even when the squelch is set on its highest setting. This makes scanning difficult and annoying.
• I have noticed this same issue before when running a Baofeng HT off of battery power with a mag mount antenna on the roof, so I'm reasonably sure it is RFI coming in through the antenna and not the power leads.
• I only notice it in specific spots on the 2 meter band. Looking at the band scope, I can see it repeats at approximately every 35khz across the band. The frequencies it is occurring at do shift around a bit over time.
• It only occurs when the engine is running, and goes away immediately upon shutting the engine off. However it doesn't come back immediately upon restart. It seems to take a few minutes to kick in and can come and go over the course of a half hour drive.

I'm guessing it has something to do with the GM Regulated Voltage Control/PCM charging system, but don't know how to go about narrowing it down and fixing the problem. Has anyone had to deal with something similar? Any help is appreciated.

 

[RFI-EMI-GUY]

The chugging sound is probably CANBUS or other module with a CPU. You need to make a sniffer with a coil of wire at the end of a coax cable that you plug into the receiver. Move the coil around on end of a wooden dowel to locate modules or cables radiating the signal.

 

[mmckenna]

Where, exactly, are you sourcing your 12 volt power from for the radio?

 

[KQ4RBK]

The chugging sound is probably CANBUS or other module with a CPU. You need to make a sniffer with a coil of wire at the end of a coax cable that you plug into the receiver. Move the coil around on end of a wooden dowel to locate modules or cables radiating the signal.

I like the sniffer idea. Any specifics on the coil of wire?

 

[KQ4RBK]

Where, exactly, are you sourcing your 12 volt power from for the radio?

Positive is run directly to the positive battery terminal. Negative is run to a factory grounding location on the front side of the firewall. But as I said, I have gotten the same noise when using an HT on battery power with a mag mount antenna on the roof.

 

[mmckenna]

OK.

The whine is often the in-tank fuel pump. You might be able to search on that. I think I've read some having luck installing some extra shielding and possibly a capacitor to drain off noise.

CANBUS stuff can sometimes be picked up by wire routing.

 

[pb_lonny]

If your hand held is the same, you might be out of luck...

I had a work van that was terrible on the 160MHz band.

 

[RFI-EMI-GUY]

I like the sniffer idea. Any specifics on the coil of wire?

A few turns that create a short from center conductor to the shield. You are looking to make the coil fairly insensitive, less of an antenna and more of an inductive sensor. Plug one end into a portable radio as the receiver and then probe in and around the wiring harnesses and the modules.

 

[RFI-EMI-GUY]

OK.

The whine is often the in-tank fuel pump. You might be able to search on that. I think I've read some having luck installing some extra shielding and possibly a capacitor to drain off noise.

CANBUS stuff can sometimes be picked up by wire routing.

That could explain the delay. Though the pump should charge at key on. Maybe the duty cycle becomes relevant at speed.

 

[nokones]

Where, exactly, are you sourcing your 12 volt power from for the radio?

Both the battery + and ground leads? Where did you buy your NMO roof mount from?