NMO Mount Grounding

[xodeuce]

I had an NMO mount installed in the roof of my Toyota Tacoma by the wireless shop in town that does the public service vehicles. They neglected to sand the underside of the mount to expose metal, by the look of it. I popped out the dome light and inspected their work as soon as I got home. Anyway, I have a Larsen 2/70SH on it. The guy there was checking something with a Bird meter, and he said the SWR looked good. However, the radio was on the memory stations that it was on when I dropped it off, so I feel fairly certain that if he did in fact check SWR it was only on 2M, on the two local repeater inputs 146.280, 146.380, and not 440.

Anyway, my question is this. Does the underside of the mount need to be sanded in order to achieve a proper ground plane? Is that something I can check with a DC volt meter for continuity b/t the coax sleeve at the radio and another part of the truck body?

I assume the answer is "Well, stick an SWR / Wattmeter, or an antenna analyzer on it and find out."

But I figured I'd ask...

 

[rescue161]

Unscrew the coax from the back of the radio and connect one lead of a multimeter set to Ohms Ω to the outer conductor (shield) of the coax connector and place the other lead on the body of the car (somewhere that is not painted). You should get a reading, hopefully close to 0Ω (less than 1&#937. If it reads OL, then you'll have to check into why. Sanding is not necessary due to the teeth on the NMO that will dig into the metal. In fact, sanding will just invite rust.

 

[xodeuce]

Cool. I'll do that. I had similar thoughts about the rust thing. Glad to hear it's not necessary.

 

[xodeuce]

Sweet, my meter showed 0.7-0.9Ω. Sounds like it is good.

Thanks for the help!

 

[fineshot1]

The grips in the "3/4 inch hole" nmo mount once tightened bite into the metal creating a solid ground.

No sanding is ever necessary.

 

[Fast1eddie]

Presuming the installer used a mobile radio industry specific hole saw (Such as is made for Motorola), the underside of the blade scrapes away the paint. No additional paint removal is necessary. If you are using a decent dual band 2/70 whip, your swr is fine.

 

[wyomingmedic]

While it has already been said , let me say it again.

No need to sand off the paint. I have installed thousands of these boogers and never once sanded the underside.

The most common mistake installers make is the pinch the o-ring or get the mount not centered in the hole. I have even seen techs snap off one of the brass feet and screw the mount up. And if they do not use the appropriate bit, they can drill the hole too big and make the whole mount very unstable. I even saw a feller "drill" the hole by using an arc welder to punch a nasty opening into the metal. Disgustingly enough, it worked!!!

But despite all of that, never had a problem with paint on the back.

And he may not have been able to check the UHF SWR as he may have not had a slug for the appropriate frequency. Especially if it was a small shop.

WM

 

[reedeb]

Many years ago [when I lived in Maine] I had a pickup truck and had the antennas installed on the roof . The guy whoi did my holes drilled them out and then put 2 coats of primer let it dry and 2 coats of paint to cover up the uncoated metal
Then after the paint was dry he mounted the antenna. I lived and worked near the ocean and this kept the sea air out of them and would keep it from corroding. All my tranciever antennas had great matches.