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NMO extension cable?

This forum is dedicated to helping people with questions about installing radio equipment in vehicles. This can include antenna installs, electrical wiring questions/problems, and mounting systems. Pictures of installs are welcome.

Note: Discussions regarding lighting, sirens, and other equipment now has its own forum in the 'off-topic' section below.

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515
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NMO extension cable?

Postby 515 » Wed Jun 12, 2013 11:00 am

I'm considering obtaining a slide-in camper for my truck. Most of these have the bed that goes over the cab, which wouldn't work with the VHF NMO antenna I have mounted on the center of the roof.

Since the camper would only be on the truck when needed, I'm thinking about ways to temporarily move the VHF antenna to the roof of the camper. I don't want to have any coax exiting the truck's door or some other cheesy way, so I'd rather not use a typical mag-mount. Some type of coaxial extension cable that screws to the NMO mount on the roof and then has a magnet or permanent NMO mount on the other end (to mount on the camper roof) would be ideal.

I don't think anyone makes anything like that, but it doesn't sound too hard to homebrew... Anyone know of any NMO antenna bases that would be fairly easy to take apart and solder some RG-58 to the internal connections? I think a lot of these are glued together or otherwise pretty sealed up, though.

Of course I'm open to any other ideas that I haven't thought of...

RFguy
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Re: NMO extension cable?

Postby RFguy » Wed Jun 12, 2013 11:08 am

There is a test adapter that will screw onto the NMO base and give you a PL259 female jack. You can make up a cable with a PL259 plug to connect to the NMO base (via the NMO adapter) and run up to a new NMO mount on your camper.

Image
Last edited by RFguy on Wed Jun 12, 2013 1:45 pm, edited 1 time in total.

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515
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Re: NMO extension cable?

Postby 515 » Wed Jun 12, 2013 1:30 pm

That will probably do the trick! Thanks!

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Re: NMO extension cable?

Postby 515 » Tue Aug 06, 2013 6:49 am

Finally picked up a camper... Now just trying to figure out where the best place to mount an NMO would be...

The roof is a rubber membrane of some kind, so I really don't want to drill into that. I'd like to run a VHF quarterwave, as it's plenty tall already and I'm sure I'll be hitting plenty of tree branches as it is.

I'm not sure if there's enough ground plane for a quarterwave VHF to work well anywhere, though. I was thinking about an L bracket on the top front corner, but I'm not really sure what's behind the aluminum siding, or how well it'll seal up.

I could mount an NMO base to the luggage rack, but I bet I'd have to use one of those taller 'no groundplane' VHF antennas there.

Anyone have any ideas? Here's a pic of it:

Image

thebigphish
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Re: NMO extension cable?

Postby thebigphish » Thu Aug 08, 2013 12:54 pm

pie plate (or sheet metal disc) as a ground plane, glued like hell to the membrane roof should suffice for your antenna...although that will get mighty big as your frequency drops..
"How do you plan to outwit Death?"
"With a knight and bishop combination; I will destroy his flank.
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Bill_G
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Re: NMO extension cable?

Postby Bill_G » Thu Aug 08, 2013 4:26 pm

Get a foldable marine antenna that doesn't require a ground plane, and it can be stowed. Concept!

tvsjr
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Re: NMO extension cable?

Postby tvsjr » Fri Aug 09, 2013 9:24 am

I wouldn't do a thing to that roof. No holes, no glue, nothing. Clamp a no-ground-plane antenna to the rail, rear ladder, etc.

Will
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Re: NMO extension cable?

Postby Will » Fri Aug 09, 2013 3:06 pm

We use this antenna on applications like yours.

Comtelco A1812B-44 http://www.comtelcoantennas.com/PDF/A1812.PDF

I cut the ball off the antenna rod, and add the spring, so it does not catch in trees, ect.....

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515
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Re: NMO extension cable?

Postby 515 » Tue Aug 13, 2013 8:59 am

Thanks for the tips... I'm thinking something mounted to the rail is the way to go.

The top of the rail on the back is about 11.5' off the ground when it's on the truck, so I still want to try and get a quarter wave VHF on there. Maybe I could add some groundplane radials to it, since I don't plan on putting any luggage up on the roof... If that doesn't work, I'll do a 'no-groundplane' one like Will suggested.

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escomm
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Re: NMO extension cable?

Postby escomm » Thu Aug 15, 2013 7:15 am

Put a ball mount on the side

hehe

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Bill_G
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Re: NMO extension cable?

Postby Bill_G » Thu Aug 15, 2013 8:37 am

At UHF, you just need the spring.

!

Birken Vogt
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Re: NMO extension cable?

Postby Birken Vogt » Sat Aug 24, 2013 10:33 am

Mirror bracket NMO mount on the rail, and a quarter, half, or 5/8 wave with spring will give you a fine match. It is very similar to what we do to big trucks with fiberglass cabs.

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nukedude
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Re: NMO extension cable?

Postby nukedude » Tue Feb 25, 2014 1:31 pm

A marine grade stainless mount on the side or rear with a gland fitting for the cable would be good. As a previous poster said, do not poke holes in that rof if you don't want leaks.

NMO mounts work well on steel vehicle roofs, but do sometimes let water through. My work used to hire out vehicle installs and every one of them leaked until some sealant was applied.

My personal vehicles have never leaked, after 22 years for one vehicle, with no sealant. What was used to cut that hole, how precise it was installed, and if it was a new NMO mount or used can make a different.

Marine grade hardware, 1-14 thread marine swivel mount, Comprod or Morad antenna will outlive the camper.

Jim202
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Re: NMO extension cable?

Postby Jim202 » Mon Mar 03, 2014 5:43 am

nukedude wrote:A marine grade stainless mount on the side or rear with a gland fitting for the cable would be good. As a previous poster said, do not poke holes in that rof if you don't want leaks.

NMO mounts work well on steel vehicle roofs, but do sometimes let water through. My work used to hire out vehicle installs and every one of them leaked until some sealant was applied.

My personal vehicles have never leaked, after 22 years for one vehicle, with no sealant. What was used to cut that hole, how precise it was installed, and if it was a new NMO mount or used can make a different.

Marine grade hardware, 1-14 thread marine swivel mount, Comprod or Morad antenna will outlive the camper.



If the NMO mounts are leaking, the installer didn't apply any grease to the small O ring. Just poor training on the installers education on how to do the job the right way the first time. Have been installing these type of mounts for some 35 plus years. Never had one leak.

One trick that I do use is to not use the silicone grease supplied with the mount. I use "high vacuum grease" for all the O rings I install. The high vacuum grease doesn't migrate and turn into a slime when it gets hot. It is still a silicone based product, but much better for this application. Don't forget to put some on the threads of the mount. Keeps the corrosion from kicking in and preventing the mount parts from being able to be taken apart down the road several years later.

Jim


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