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Author Topic: Larsen NMO Mount problems.  (Read 3184 times)
N8FU
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Posts: 10




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« on: July 01, 2013, 02:06:58 PM »

I have a Larsen NMO-K mount installed in the roof of my Toyota 4-Runner. Generally I am very happy with the mount, however I wanted to be able to use the mount with a variety of antennas. I bought a few adapters to use the NMO mount with 3/8" studs as well as so-239 type antenna connections (primarily a Wilson 1000 that I cut for 10 meters).

My problem is this. The NMO-so239 adapter has a spring loaded pin for it's center contact. It seems that the spring is much too strong for the center conductor in the NMO mount. I noticed that when the antenna was screwed tight to the connector, I lost all reception. The center pin of the mount was pushed through the plastic insulator that keeps it separate from the ground. I can only assume that it cut the insulator enough to short out the connection. When I unhook the mount from the roof, push the center conductor up where it belongs and hook my 5/8 wave 2 meter Larsen antenna it works again, but as soon as I use that adapter it messes everything up again.

Has anyone had this problem??? I am surprised that I can't find anything else online about this, especially since I can't seem to find this kind of adapter with any other kind of construction. My 3/8" stud type adapter has a flat piece of copper that just touches the center conductor of the NMO, and that is how the Larsen antenna is made as well. With the center conductor of the mount shoved down into the insulator, they don't stand a chance of making contact, and I am getting really sick of removing my mount on a regular basis.
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K5LXP
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« Reply #1 on: July 01, 2013, 03:12:42 PM »

I can't see this as being possible with a genuine motorola NMO mount.  The contact is a button that's much larger than the hole that goes through the insulator to the coax side of the mount.

I would also question the adapter - it may be spring loaded but are you using up all the spring travel with this particular mount, causing the spring plunger to become a solid shaft forcing the NMO contact out like a drift?

Step one would be to replace the NMO mount, then verify the adapter has adequate plunger travel.


Mark K5LXP
Albuquerque, NM
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N8FU
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« Reply #2 on: July 01, 2013, 07:58:22 PM »

So I got out the headlamp and the soldering iron and pulled apart the NMO mount. Just to clarify this mount is a Larson, not a Motorola.

The center pin does indeed have a larger head on it that won't fit through the hole. However, the hole is beveled at the top, just big enough that the head of the center pin fits in the largest part of the bevel. This alowed the head of the pin to cut the insulating washer and make contact with ground.

I don't know why they beveled the hole, if it was all straight walled I don't think it could have done this...

Anyway, I pulled the pin out, and fabricated a new washer out of the plastic from a 20oz pop bottle lid. (the toughest seeming material I had on hand) I also left the small part of the original washer (the part pushed into the beveled recess) under the new washer. I figured this would fill the beveled area and prevent this from happening again.

It's all soldered back together and reinstalled. We'll see how it works.

Carl, AC8LQ

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K1CJS
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« Reply #3 on: July 02, 2013, 05:24:53 AM »

I have a Larsen NMO-K mount installed in the roof of my Toyota 4-Runner. Generally I am very happy with the mount, however I wanted to be able to use the mount with a variety of antennas. I bought a few adapters to use the NMO mount with 3/8" studs as well as so-239 type antenna connections....

If the adapters you bought weren't made by Larsen, that's the problem.  Also, the Larsen NMO mount is a knock-off of a genuine Motorola mount, and as such is probably of a poorer quality.  I've had experiences with both the NMO and the GE type mounts, and one thing I'll say is that the original mounts work and don't let go at all when compared to the knockoffs that are available in the general marketplace.
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N8FU
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« Reply #4 on: July 02, 2013, 11:48:17 AM »

Interesting, up until now I had been led to believe that Larsen was the go to company for a quality NMO mount and to leave the other brands alone.

I ended up dremeling off some of the adapters pin so that it isn't pushed as far, so I think my problem is solved for now. Before the dremeling it was still trying to punch the center pin through my new homemade washer.
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K1CJS
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« Reply #5 on: July 04, 2013, 06:50:46 AM »

The Larsen NMO knockoff may be better than the no name knockoffs, but the only way you're going to get good results is to use ALL Larsen parts, mount, adapters AND antennas.  There is a NMO knockoff system that requires only a 3/8 hole to mount--I forget the company name.  The system is totally incompatible (thread pitch) with any other NMO system there is, however.  If you try, all you get is a ruined mount and a ruined antenna base.  The differences can be that great.
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K1DA
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« Reply #6 on: July 14, 2013, 06:25:43 AM »

Larsen makes high quality stuff.  Seems to me there are Larsen mounts which come with a removable pin for certain applications. 
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N0MKC
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Posts: 68




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« Reply #7 on: July 15, 2013, 02:43:52 PM »

This .pdf from Larsen may help...

www.pulseelectronics.com/download/3235/nmohf_mounts_10_07pdf

Seems there are 2 different center pin configurations used in NMO mounts...  Your adapter may not be a proper match to the mount, maybe?
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