Help with Antenna Creating Squeal through Vehicle Speakers

[Ice_Cream_Man]

Hi Folks:

I have a real head scratcher here. I have a 2015 Nissan Frontier with a roof NMO hardmount (not magnetic), but every time I attach an antenna & get above 50 MPH it creates a squeal through the vehicle's speakers. This happens even when there is no ham radio connected or present in the vehicle! I've tried a few types of antennas, but they are all in the 40" length so the issue isn't the antenna itself.

However when I use a magnet mount with the same antenna(s) there is no issue with the squeal at any speed.

Anyone have any ideas?

 

[RFI-EMI-GUY]

That is pretty weird and I would suggest it can be fixed.

Are you sure the shield of the NMO mount is actually bonded to ground? If precipitation static is building up, it could be radiating into adjacent wiring if the mount and cable shield are not at DC ground.

Have you checked the VSWR? Is it OK. Check the shield to ground with ohmmeter any way.

Does the 2 way radio (CB??) have to be on for this squeal to occur?

Is the 2-way radio negative lead grounded to vehicle chassis ? I hope not going to the battery negative terminal?

Is the 2-way radio connected to the vehicle speaker or audio system in any way?

 

[mmckenna]

Are you sure it's not the antenna vibrating in the wind? I live fairly close to the ocean, and when the fog sets in, the antenna will sometimes start to sing with the extra resistance, and it's almost always speed dependent, as in it'll only do it above 50MPH or so. This has done it on several vehicles all with permanent mount NMO's on the roof. The magnet mount might be providing enough insulation from the vibration to keep it from transferring in to the cab.

As for the Nissan, we had some of those at work with a 1/4 wave VHF mounted on the roof as well as an 800MHz antenna. 100 watts VHF, 15 watts 800MHz, no issues with the vehicle speakers making any noise.

 

[Ice_Cream_Man]

Thanks for the replies folks. I finally bit the bullet and bought a ground strap & had my local installer remove the headliner to install the ground strap. That seemed to fix the issue as the squeal is barely noticeable at 78mph (which is a helluva lot faster than I normally drive). I picked up a ground strap from The Com (Antenna Ground Wire - TheCOM) in case anyone wanted to know.

Just to address the RFI-EMI-GUY's questions for the record:

Have you checked the VSWR? Is it OK. Check the shield to ground with ohmmeter any way.
Didn't have the equipment to make the check.

Does the 2 way radio (CB??) have to be on for this squeal to occur?
No, that's the weird part. It was happening when the antenna cable was totally disconnected. It's normally connected to an ICOM 2730A 2m/70cm unit.

Is the 2-way radio negative lead grounded to vehicle chassis ? I hope not going to the battery negative terminal?
Now it is! Prior to that it was just a standard NMO mount drilled into the roof.

Is the 2-way radio connected to the vehicle speaker or audio system in any way?
No, no connections to any electrical systems.

Guess we can mark this one down as improper ground on the mount and a lesson learned. Thanks for the assist guys!