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Author Topic: coax to NMO  (Read 1722 times)
KC2LVC
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Posts: 19




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« on: November 29, 2004, 10:23:32 AM »

I'm trying to connect RG-58 to an NMO connector. I just want to verify that i am doing it correctly, as I tried twice already, and my swr was horrendous. I wish I could find a picture online of the bottom of the NMO with coax connected, but have not been able to. I am connecting the  coax center conductor to the center pin (for lack of a better word) in the NMO. I figured the coax outer conductor(shield) was to be crimped in the piece which comes out to the side, from the center pin with the insulator on the NMO. Is this the correct way to do it? Does anyone have a photo by any chance, or know of one online with details?  Thanks in advance.
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N3ZKP
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Posts: 2008




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« Reply #1 on: November 29, 2004, 02:47:47 PM »

It sounds to me like you are doing it correctly. Have you checked for a short between the center pin and the ground side of the mount? Did you scrape the paint from the underside of the hole the mount is installed in to get a good ground connection?

I'm curious why you are having to attach the coax. Most NMO mount assemblies come with the coax attached.

What antenna are you putting on the mount? Do you know that the antenna itself is resonant on the desired frequency?

I have installed probably a hundred of these mounts over the last forty years and never had a problem with them, as long as they were properly installed.

Lon
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W7DJM
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Posts: 1




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« Reply #2 on: November 29, 2004, 03:31:32 PM »

I don't believe I'd just crimp the braid, solder it, too.

Sounds to me like you are describing what you are doing correctly, but I would assume NOTHING.

Do NOT assume that the cable, or antenna you have is good.

If you want to look hard enough, or spend enough money, there IS an adaptor that adapts the top (antenna connection) of an NMO to N or UHF, probably meant for attaching a dummy load.

I would first check the cable, and make SURE it it good, beyond all doubt.

Examine the NMO carefully, for any dirt/rust, braid, etc, and check with an ohmeter.

Where is it mounted, and what antenna are you using?

You really can't check most NMO antennas "off the rig", but you MIGHT run down to your local radio shop and get the chrome ring and a quarter wave whip--these are ALWAYS handy to have.

Now, at least, you can cut yourself a quarter wave whip and have THAT for testing, backup, etc.

Make sure the little contact finger in the NMO antenna is good and clean, and "springy"

Any possibility that the antenna you have needs to be trimmed?

Has the wrong whip installed?

Is it a used antenna, and maybe cut for the commercial band?  (way too high in freq)
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KC2LVC
Member

Posts: 19




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« Reply #3 on: November 30, 2004, 08:16:21 AM »

There's no paint on the mount, and the NMO is tight, so that is fine. I bought a separate NMO and coax because I was not sure how long I needed the coax to be, at the time. I bought a whip, that is supposedly resonant.( i bought it new at a hamfest this past Sunday)  I don't have another to test with. If all else fails, I will get one.   I have tried two different cables actually. That was my first thought, so I tired a second time with a different cable. Same results.  I'll try your suggestions. If it doesn't workout, I'll just get another that has a cable already installed, so I can check how it's done. Plus, i can use it for a second antenna. Thanks for the suggestions.
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W7DJM
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Posts: 1




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« Reply #4 on: November 30, 2004, 09:54:09 AM »

Buying one after the other is not the recommended procedure, here.

You should consider learning to actually troubleshoot.

What are you using for an SWR meter, any chance that it is screwed up?

Do you have a dummy load?

Have you removed the coax from the mount and tried to "jury rig" it to check?  Ohmeter?  

You say you "bought a whip."   Is this a base or center loaded antenna, or is it just a whip?

If it's just a whip, how long is it?

What model is this antenna supposed to be?  Do you have a sheet with it for cutting length?


These questions, and many more.,. .. . . . . ..
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KC2LVC
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Posts: 19




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« Reply #5 on: December 01, 2004, 09:34:19 AM »

no, no, you're right. Troubleshooting is first. I meant, if all else fails. But no, I don't have a dummy load. The swr meter is fine. I tested it on another antenna. I think the whole problem is just the way I had attached the coax. I did not solder the braid to that peice. Just kind of crimped it in. Only soldered the center piece. But I'll try soldering both and see what happens. Hopefully tonight. Just received a new radio, and eager to use it. Tongue
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WA4MJF
Member

Posts: 1003




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« Reply #6 on: December 01, 2004, 03:45:32 PM »

If all else fails, buy a Larsen
NMO mount with coax attached, run
it to where it needs to be,
leave about 6 inches extra, cut
off the excess RG-58/U and put
on the PL-259.

Happy Holidaze!

73 de Ronnie
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N3ZKP
Member

Posts: 2008




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« Reply #7 on: December 01, 2004, 08:30:13 PM »

I totally agree with just buying the preassembled mount. I disagree, however, with cutting the coax. The coax that comes with the mounts is 17' long. The signal loss in that length of coax simply isn't worth the effort of cutting the coax and reinstalling a connector, especially on a mobile installation.

Lon
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W7DJM
Member

Posts: 1




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« Reply #8 on: December 02, 2004, 09:37:48 AM »

And I disagree with THAT.  I've salvaged a number, perhaps as many as 8 or 10 NMO mounts that had been pulled from other vehicles,  and NEVER have I had trouble soldering the NMO to a new piece of coax,  although I'll admit that once in a while I've screwed up a connector.  

DO NOT USE RADIO SHACK CONNECTORS!!!!!!!!!

You HAVE to take this one step at a time, and check your work.  

We still don't know for sure

Where it is mounted?

 (center of roof, surrounded by ski rack? Bumper?  Trailer hitch?  Spare tire rack?)

The condition of the groundplane?

 (See above:  Describe how and where it is mounted)

What kind/how long an antenna?

(Quarter wave whip?  HOW LONG??  Did it come with a chart?)

What kind/ condition of the coax (brand? new?)

What kind (Radio Shack??) condition of the connector.

What you have done to eliminate all the variables?

What you are using for an swr meter? (brand, model?)

You said "There's no paint on the mount"   Are you referring to the NMO unit itself, or a separate "mount" like a trunk lip mount?    
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KC2LVC
Member

Posts: 19




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« Reply #9 on: December 06, 2004, 07:11:28 AM »

I cut the coax to start fresh, and resoldered to the NMO.  This time I soldered the braid also. Works ok now, so lesson learned. Solder the braid also. Smiley  Thanks
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KD7EZE
Member

Posts: 60




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« Reply #10 on: December 11, 2004, 07:29:00 AM »

ZKP--

  Lon,
    Every NMO mount that I've ever seen, never had the connector already soldered in place. If that were the case, you'd have to drill a 3/4 inch hole to route the coax. That's why they come with the connectors unattached. Another reason they come unattached, you might need to install a different connector than the one supplied.
 
  Another thing: it doesn't matter how much coax is supplied with the mount. After you mount the antenna and run the coax to the radio, cut off ALL of the excess coax, and attach the connector. The length of the coax has NO effect on the tuning or SWR.

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