Ham Radio → O.k. grounding problem on antenna mount

At least that's what I think is the problem.

2009 Honda Accord using a Diamond K400NMO antenna mount connected to a Diamond CR620BNMO triband mobile antenna.

Tried using my Kenwood TH-D7AG on it and got very poor performance. Hooked it up to an MFJ 269 analyser and the SWR was over 3.

This sound like a grounding problem?

Here is a link to a newer version of the same antenna:

»www.diamondantenna.net/c ··· nmo.html

And here is the mount.

»www.diamondantenna.net/k ··· nmo.html

Could be, but could also be the antenna is not resonant. Have you tried reinstalling the mount and tightening the screw down tight until they penetrate the paint on the vehicle? You may have to do this several times to get through some of today's modern paint.

Also this antenna has a 6.5 foot pigtail. If you are using an additional length of coax to connect to it are you certain that the coax is good and the connector are installed properly? Make sure that you have continuity on the connector to connector on the braid, the inner conductor and that there is an open when checking from inner conductor to the shield.

The version of the mount doeasn't have the pigtail.

And I did try and redo the mount and did go to bare metal.

I go back and check the continuity of the cable.

O.k. I checked the cable for continuity and it was good.

Should I get any continuity from the mount into the vehicle?

Yes, with the antenna removed you should see continuity from the outer ring of the NMO mount to the vehicle. There should be infinite resistance from the center NMO contact to the vehicle.

I've got one of those mounts, and when I first installed it, the VSWR on 70cm was OK, but 2 Meters was over 4:1.

The problem I found was that the only "ground" connection it has to the body of the car is through the two set screws that clamp it on to the lip of whatever body panel you have it clamped to. If the screws don't penetrate the paint, you won't have a good ground.

On 70cm I think there's enough capacitance to ground for it to work OK, but 2 Meters is different.

What I wound up doing was to make a couple of bonding strips out of flat braid, like the shield of a piece of coax. I drilled and tapped two places in the mount to attach the braids to, and then mounted the other end of the braids to a couple of handy screws that held the hinges for my hatch to the body.

A friend of mine had a similar problem, but he drilled two small holes in the car body, and attached the braid to the car body with some small sheet metal screws.

Even through an Ohmmeter might show DC continuity between the outer ring of the NMO to the car body, if you have a poor RF ground, the mount won't work properly.

Well, I am not seeing that so grounding is the issue.

Now to figure out how to take care of it.

moonpuppy - as DrJim also pointed out, there is DC ground and RF ground. The DC ground may not be effective at RF. Did you try a grounding strap from the mount to the vehicle body? Does the antenna require pruning to resonance? I ask because I don't have the information on this antenna in front of me.

Not yet.

Now, here is something I forgot to mention. There is continuity to the NMO portion of the connection but NOT to the rest of the mount itself. Is this an issue?

Yes it is. If the NMO does not have an effective connection to the mount then you do not have a ground connection to the vehicle.

moonpuppy - did you get the antenna working ok?

Not yet. Going to try tomorrow again (since wife and child will be away for the day.)

I think there is a continuity problem between the antenna connection and the mount itself.

O.k. I took the mount off the car and found out there is no continuity to the four grounding screws. The screws themselves have continuity but there is too much paint on the mount that the surfaces are not getting enough contact and the screws are not contacting the surface enough.

Sounds like you are making progress. One would think that they would make this simple check at the factory. I do hope you get this resolved and will soon be on the air. Good luck.