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NMO Grounding

Joined
Feb 19, 2019
Messages
10
#1
I've got an issue I cannot figure out for the life of me. I've got a NMO mount mounted on the front fender of my pickup. I ran a ground strap from one of the bolts to a bolt on the firewall. I usually have my WS1040 plugged into my truck's stereo using the AUX Jack. Whenever I used my mag mount, the audio would play through both sides of the truck. But whenever it's hooked into the NMO, it cuts the audio out on one side. I was thinking there was a short in the wire or the mount. I checked for continuity between the center conductor of the RG58 and the ground strands and there was no continuity. I recently had a BNC connector crimped to the cable and I'm thinking maybe the guy who installed it, shorted the ground with the center conductor. But if that was the case, wouldn't there be continuity? I can't figure it out and it's giving me a headache.
 

KB4MSZ

Member
Premium Subscriber
Joined
Mar 12, 2018
Messages
220
Location
Tampa, Florida
#2
In using the two different antennas, does the resting position of the radio change depending on which antenna is connected? Maybe the NMO antenna cable routing requires you to angle the radio differently when at rest and it puts a strain on the phono cord that the mag mount doesn't? That's all I can think of.
 
Joined
Feb 19, 2019
Messages
10
#3
No. It's in the same position. I can unplug the NMO and just touch the BNC on it to the scanner and it will cut the audio out.
 
Joined
Feb 19, 2019
Messages
10
#5
It still cuts it out. Antenna on the mount, antenna not on the mount. If I unbolt the mount from ground, then it doesn't do it. But as soon as the mount is connected to ground, it cuts out.
 

KB4MSZ

Member
Premium Subscriber
Joined
Mar 12, 2018
Messages
220
Location
Tampa, Florida
#6
Question, does the phono cord plug to the head unit have two or three conductors? In other words, mono or stereo?
 
Joined
Feb 19, 2019
Messages
10
#7
It's stereo. But the audio plays through both left and right sides of the speakers when the scanner isn't hooked into the nmo. When it is, only the left side plays.
 

KB4MSZ

Member
Premium Subscriber
Joined
Mar 12, 2018
Messages
220
Location
Tampa, Florida
#8
My guess is that the jack inside the radio is mono and it is being shunted to ground do to one of the conductors on the stereo plug riding on ground during use. I looked at the owner's manual for that radio but it doesn't specify what type of jack it is. Evidently the two upper most conductors on the stereo plug are connecting to the audio signal when the radio is isolated from ground. The magnet mount wouldn't provide this path to ground, whereas the NMO is hard grounded to the vehicle.

If my theory is right you would need a mixing adapter to send the mono from the radio to both channels on the head unit. I don't advise using the radio with this shorted condition if it is indeed taking place. It could damage the audio circuit in the scanner.
 
Joined
Feb 19, 2019
Messages
10
#9
I'm not really worried about using the head unit for audio as much as I am harming the scanner. But like you said, the scanner having a mono audio jack is the only thing I could think of. Thanks for the help!
 
Joined
Nov 1, 2005
Messages
327
Location
JoCo, KS (SoDak native)
#11
I assume that the vehicle is negative ground....

Since the only obvious difference in the antennas is that the MNO grounds the shield and the mag mount does not, you may have ground loop. The outside contact on the speaker jack may not be at chassis ground. Ground loop?

something to check.....

$.02
 
Joined
Mar 30, 2005
Messages
3,099
Location
So Cali
#12
One good precaution, a Ground Loop Isolater on the audio cable. Some audio "ground " connections are NOT ground. So you use a audio transformer, ie ground loop isolater, to isolate the radio/scanner audio from the car radio input.
 
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