RF noise coming in when antenna is installed on the NMO mount

[OhSixTJ]

I’m trying to mitigate noise that comes through when the antenna is mounted. If I remove the antenna, I can see the noise stop.
Any suggestions?

 

[doctordialtone]

Any suggestions?

Is it interference coming from something nearby?

Try changing locations to see if it goes away.

 

[OhSixTJ]

Nope. It follows me wherever I go and only appears when vehicle is on and the antenna is attached to the mount.

 

[doctordialtone]

Ahhhhhhhso coming from the engine, alternator or on board computer?

 

[mmckenna]

Vehicle?
Frequency?
Antenna location?
Mount type?

 

[OhSixTJ]

Well it’s present with the key forward and the engine not running. So either computer or fuel pump or whatever other electronic item is active when the key is forward. The vehicle in question is a 2015 F250 with the Diesel engine.

it happens on most frequency except, weirdly,146.52.

antenna is mounted on the roof, NMO. With a browning mount. I know, cheap.

and it happens with any antenna. I’ve tried them all. A laird 1/4 wave, tram 5/8,tram 2/40, and laird 1/2 wave.

 

[mmckenna]

OK, Diesel, so not ignition.

Yeah, computer noise, fuel pump, etc.
Probably the easiest thing to do is take a hand held VHF radio with the antenna removed and start sniffing around and see if you can track it down.

I've got a 2018 with a Diesel and a VHF radio, no issues, but I'm sure there is some differences between those model years.

 

[K8EAA]

I have this same problem with a two meter rig in the car. The car is a Honda. They use a 3.58mhz clocking crystal in the ecu. Turns out that the
41st harmonic of 3.58 is 146.78. You may have a different brand of car but they all seem to use the came clocking in the ecu. It only bothers me
when trying to scan on the Icom rig. It causes problems with the local repeater on 146.76.
I will try clip on chokes on the input and out put of the ecu if I can get them to fit the various cables in question.

 

[Firekite]

The vehicle in question is a 2015 F250 with the Diesel engine.

What radio, and how is power supplied? I have an FTM-400XDR powered (for now) via the 12V socket in the center console of my 2011 F250 diesel, and I have not noticed any such issues.

 

[OhSixTJ]

OK, Diesel, so not ignition.

Yeah, computer noise, fuel pump, etc.
Probably the easiest thing to do is take a hand held VHF radio with the antenna removed and start sniffing around and see if you can track it down.

I've got a 2018 with a Diesel and a VHF radio, no issues, but I'm sure there is some differences between those model years.

thank you for the suggestion. I just tried it with and without the antenna and couldn’t find anything. Tried it on the same frequency that the mobile unit is showing the noise on. Swept under the hood and under the truck.

 

[OhSixTJ]

What radio, and how is power supplied? I have an FTM-400XDR powered (for now) via the 12V socket in the center console of my 2011 F250 diesel, and I have not noticed any such issues.

I have it wired to the battery’s positive post and have tried both the negative post and a body ground.

I can see and hear the noise on the radio when the vehicle is on.

It’s kinda weird because it looks like it comes in waves. The meter starts to climb to max and then goes back down.

 

[mmckenna]

thank you for the suggestion. I just tried it with and without the antenna and couldn’t find anything. Tried it on the same frequency that the mobile unit is showing the noise on. Swept under the hood and under the truck.

OK, lets start looking at the installation.

Is this a new install? Or and existing install that just started doing it recently?

How is the radio chassis grounded? Try running a short ground strap from the radio chassis to the vehicle body.

 

[OhSixTJ]

I have this same problem with a two meter rig in the car. The car is a Honda. They use a 3.58mhz clocking crystal in the ecu. Turns out that the
41st harmonic of 3.58 is 146.78. You may have a different brand of car but they all seem to use the came clocking in the ecu. It only bothers me
when trying to scan on the Icom rig. It causes problems with the local repeater on 146.76.
I will try clip on chokes on the input and out put of the ecu if I can get them to fit the various cables in question.

146.76 is one of the frequencies I see it on. I also see it on147.60, 146.14 and 444.60. Others too.

 

[OhSixTJ]

OK, lets start looking at the installation.

Is this a new install? Or and existing install that just started doing it recently?

How is the radio chassis grounded? Try running a short ground strap from the radio chassis to the vehicle body.

no the problem has been there since it was first installed, I’m just recently paying attention to it. The reason why is because I got a reply over the radio that said I was fading out. I don’t make a lot of contacts because of where I live.

the radio is grounded to a bolt on the cab and the cab is ground-strapped to the frame. The antenna connector is also supplying ground for the radio that keeps it powered when the ground wire is not connected to anything. I’m pointing this out because I’ve never had the radio on while the ground wire is disconnected. Honesty I’m not even sure if it’s supposed to behave like that. The radio is an ID-5100a.

I’ll run a wire from the body to the cab/chassis and see what it does.

 

[w2xq]

WAG. On the location where the NMO is sited, did you sandpaper or otherwise clean/clear the underside of the metal roof/fender/??? so the mount bites into metal and not paint or rustproofing or ?

I assume you have used an ohmmeter to ensure (1) continuity on both the center and the shield of the coax and (2) there is no short between the center and the shield.

Your dealer should have access to a supplemental service manual(s) that address electrical noise.

HTH a bit.

 

[mmckenna]

no the problem has been there since it was first installed, I’m just recently paying attention to it. The reason why is because I got a reply over the radio that said I was fading out. I don’t make a lot of contacts because of where I live.

OK, just wanted to see if there was something recently starting to fail.
I sort of like the 3.58MHz clocking crystal idea.

the radio is grounded to a bolt on the cab and the cab is ground-strapped to the frame. The antenna connector is also supplying ground for the radio that keeps it powered when the ground wire is not connected to anything.

OK, that means that you probably have the radio grounded well. As for the radio grounding through the antenna, that's OK, that means you probably have a pretty good ground at the NMO mount. It's not necessary to sand the underside if you tighten the mounts down properly. Even the cheap mounts have some teeth on them.

I’ll run a wire from the body to the cab/chassis and see what it does.

Probably not necessary if it's working OK. I got in the habit of grounding the radio chassis directly to a nearby grounding point. It's not usually required, but I've had it fix strange issues that were probably static electricity related. It won't hurt, though.

I seem to recall others complaining about this, but I could be mis-remembering a different conversation. If your handheld doesn't pick it up, that probably means it's not a strong signal. You might want to check on your coax and power wiring, make sure it's not running up tight agains other vehicle wiring.

 

[OhSixTJ]

WAG. On the location where the NMO is sited, did you sandpaper or otherwise clean/clear the underside of the metal roof/fender/??? so the mount bites into metal and not paint or rustproofing or ?

I assume you have used an ohmmeter to ensure (1) continuity on both the center and the shield of the coax and (2) there is no short between the center and the shield.

Your dealer should have access to a supplemental service manual(s) that address electrical noise.

HTH a bit.

I did not sand the underside but assume it’s properly grounded as it’s providing a sufficient ground to power the radio.

I haven’t checked with an ohmmeter since I’m not at home due to the holidays. I’ll do that when I arrive home.

somwthing odd I noticed: the noise goes away on some freqs and appears on others depending on the headlights being on or off. Both states affect different freqs. My headlights are regular halogen bulbs.

 

[OhSixTJ]

You might want to check on your coax and power wiring, make sure it's not running up tight agains other vehicle wiring.

I like this idea. I’ll check the coax routing in the morning. Although, I’m pretty sure it’s not near anything as I ran it diagonally across the roof to the b-pillar then down to the floor and across to the radio that sits under the drivers seat.

 

[w2xq]

I did not sand the underside but assume it’s properly grounded as it’s providing a sufficient ground to power the radio.

The antenna mount is providing the ground to power the radio? I don't understand.

 

[doctordialtone]

The antenna mount is providing the ground to power the radio?

the radio is grounded to a bolt on the cab and the cab is ground-strapped to the frame. The antenna connector is also supplying ground for the radio that keeps it powered when the ground wire is not connected to anything.