Bad NMO mounts

[W4KRR]

What would cause NMO mounts to suddenly go bad?

I had two NMO mounts installed on the roof of my Ford F-150 by a professional two way radio shop. I have installed NMO mounts myself before, the only reason I had a radio shop do this installation is because I didn’t know how to remove the trim to run the cable, unlike my past vehicles, there’s no visible screws or fasteners holding the trim in place.

Anyway, one mount was for a scanner; the other was for a VHF/UHF dual band amateur radio.

One day I noticed I could no longer access any of the repeaters I had programmed in. I confirmed that the radio was still transmitting; I just couldn’t access any repeaters. So, I tried removing the antenna from the mount, and putting it on an NMO magnet mount base. When I tried this arrangement, the radio worked flawlessly. A similar problem occurred with the scanner NMO mount, I couldn’t hear stuff that I should be able to hear easily, until I put the same antenna on an NMO magnet mount. Doing that made the radio come alive.

I looked at the NMO body mounts, no sign of corrosion or damage. I replaced the PL-259 connector, it made no improvement. The only thing I can think of is the coax is pinched or otherwise shorted, or the bottom of the NMO mount is shorting out against metal in the roof.

I’m looking for ideas or suggestions from the “pros” out there.

[Tom in D.C.]

I have the exact same setup you have, except that your scanner antenna is my cellphone antenna.

Long before I read your remark about pinching the cables, or a roof-to-NMO short, I thought the same things you did.

So far, after more than a year, mine work fine, except that the guy who did my roof cuts almost hit the front roof crossmember and had to sort of squeeze the connection on, so I'm waiting for the day it shorts out to the crossmember. If he'd pulled the headliner BEFORE he used the hole saw this wouldn't have happened, of course. Live and learn.

Good luck. Let us know how you solve the problem.

[W4KRR]

Well, under the NMO mounts, there is the back edge of either a cross brace, or an internal roof skin. It's close. Anyway, the NMO mount could be shorting against that. However, if that were the case, you would think it would of been shorted from the beginning.

The NMO mounts they used don't have those plastic boots that cover the area where the coax is soldered to the mount. Maybe the mount wouldn't fit with them on, who knows.

[apco25]

Sounds like the mounts are shorting out to ground especially if the don't have the plastic boot on.

That's one of the reasons I went to a connectorized NMO mount from maxrad the MMF.

I have had NMO go bad do to improper install where the center conductor has broken a few inches back from the mount itself causing and open.

[Znarx]

ok..some "professional" shops, to save money and increase profit, use offshore built NMO mounts...this in itself is not the problem..where the problem lies is with the REALLY cheap shops, which purchase the NMO with the cable already soldered & wired as a kit....THESE KITS ARE CRAP!!!!...BAD CABLE (poor shield)...AND CRAPPY SOLDER JOBS!!!...most of them have strands of shield flopping around and cold solder joints on the centre conductor...have a look at the solder job on the bottom of the NMO

...Z

[WB2IID]

I just had this problem with the Larsen NMO mount on my UHF antenna. This has been installed for about 3 years and worked great up until a couple of months ago.

Working on top of the roof I loosened the mount and pulled it out as far as it would come, about 2 inches. I found that the center conductor had detached from the antenna connection "button". Looked like a cold solder joint, so I resoldered it.

Works fine now. I had to do some apologizing to some repeater trustees, after accusing them of faulty equipment at THEIR end. Oh well.

[jim]

Pre-wired NMO mounts junk?

Um....I've installed hundreds of Antenex, Comtelco and Antenna Specialsits' pre-wired units and have never seen a bad one yet.

Never even seen one with a pre-installed connector have a bad crimp, either.

It sounds as if the "shop" did a hack job. It's possible that the assembly was yanked or was snagged during installation.It;s possible that the cable is on a sharp metal edge and got pinched.

Were the NMO mount rings tight? If loose, they can move or spin, damaging the coax.

This sounds as if it's an installation problem- not a QC problem from the manufacturer.
[/quote]

[Znarx]

I was talking about the off-shore knock-off stuff (not the ones built by and QC'd by the major antenna manufacturers)...the off-shore stuff IS crap..every once in a while, I bring one in to see if they've improved their technique...not yet...all the problems I've described above are always there as well as a few new ones I've seen....soldered braid twisted and folded back on itself and crimped to the NMO..CRIMPED not soldered...<shudder>

...Z

[apco25]

I have had factory antenex and larsen mounts fail. Their quality is high but like any product there is some failure rate among them.

[W4KRR]

I ended up removing and replacing both mounts, coax, and connectors and replacing them with Antenex brand. Everything is working great again.

I don't know what brand was installed originally. The old NMO mounts were tight, the coax looked okay, there was no visible damage. The only problem I had was fishing the coax. (Could car makers try any harder to put rougher edges and sharper objects behind trim? My hands looked like I stuck them in a blender!)

[USPSS]

One note, in Arizona I have had mounts placed in the roof fail due to the cable center onnector melting through the dielectric and make contact with the braid. It happens with the foam cables so I have always used teflon cable since heat cannot effect it. I always chack the connections prior to installation and belive the Hirschmann (Antenna Specialist) and Maxrad MMF mounts will solve a lot of the connection problems. It may cost a few dollars more but is it worth not seeing the vehilce again after install?

[Bat2way]

Good point from USPSS about roof heat damaging the antenna cable. Always check the bushing connection BEFORE every installation. A few seconds there is well worth it. Have repaired a few cold soldered center conductors straight out of the bag. Even the old Motorola supplied NMO mounts in the 80's had some problems. Good for some $$$'s when you knew what to look for.

[jim]

....it must be nice to live somewhere warm enough to melt RG-58!

We only have to worry about the insuation freezing and cracking.

[Radioshop]

I installed 100's of these on utilitie trucks. I had a few Chevy and Ford trucks used by a power company have the same problem. We found that on Ford trucks the floor at the trans. hump and firewall will get hot enough to melt carpet padding on the passanger side. ( Usually where the coax is run under the dash to the rear of the cab.). On the chevy truck it is on the driver side at the gas pedal. We have found Foam and teflon cable to go bad. With no sign that they have got hot enough to melt. They just shorted out. Also seen a lot of mechanical breakage on NMO mounts of the center connection. Mostly due to quick installation of mount and twisting of coax at mount.

[Max-trac]

Yup, all of the above!
Had cheap teflon rg58 center conductor migrate and short to shield where it ran near the hot heater duct.
Had vibration wear thru things, crack things..Sharp edges,
Had bad rg58 on 800 with so much loss, the internal cell anttenna was better...

[motorolamonster911]

I just had 2 Maxrad antennas this week have broken conductors on the NMO mounts, I never checked them when I put them on, now I learn a lesson..

Nick

[KitN1MCC]

i must say i have never had a problem with comtelco nmo's