NMO Mount with LMR-240 Ultra

[Tinkertek]

Hello,

I’ve searched for hours and can’t seem to locate a 3/4” NMO mount that has a fully enclosed base connection that uses LMR-240 Ultra Flex coax. I would think this should be a very common thing, as NMO mounts are everywhere and who wouldn’t want to use some nice low loss LMR-240 Ultra Flex coax along with it. That coax is nearly exactly the same size as RG-8X, but has lower loss. Everyone sells RG-58 with those mounts, why?

I have found it very difficult to waterproof the under side of the NMO antenna connection, as water always gets to it and corrodes badly. My NMO mount is fabricated into my F-150 stake bed mounting hole, so it’s all exposed to the elements. Hoping to find a completely sealed NMO mount with attached coax that is at least RG-8x and preferably LMR-240UF

Ideas?

Thanks!
Paul

 

[mmckenna]

While there is cable loss differences between the cable, it's very slight when looking at the length of cable you'd use on any mobile install.
RG-58 is common because it's inexpensive, easy to install, and works just fine on mobile installs, even up into the GHz range. The amount of loss differences is in the fractions of a Decibel and would not be something you could hear.

But, there are other options.
I ended up with an NMO mount with LMR-200 once. Couldn't tell the difference in performance between any of the mounts with RG-58. It was also a stiffer cable, so it was more difficult to run. Yeah, I know you are looking for LMR-240 ultraflex, but it's still not going to be a joy to run.
You can get LMR-195 cabled mounts, RG-58 dual shield, RG-8X, etc.
Or, you can just get one of these and run whatever cable you want: https://www.theantennafarm.com/catalog/laird-technologies-vtpm800-1156

I run an 800MHz trunked system that has about 450 radios on it. About 50 of those radios are mobiles. RG-58 has been fine on all those installations, including buses which can have longer runs depending on where the radio gets mounted.

TL;DR: Don't get hung up on the cable loss for short runs.

 

[Tinkertek]

@mmckenna - thanks for the reply. I have RG-8x currently in place so I’d hate to step down to RG-58. I‘m hoping to at least find a sealed cable mount NMO. That way I could at least fix the corrosion issues.

 

[n0nhp]

If you are having corrosion problems, the O ring is not being seated properly.
Silicone Dielectric compound on the O ring will some times help but biggest thing is drill your hole in as flat a spot as possible to avoid leakage.
All else fails, a big blob of RTV before you set the NMO in the hole.

Bruce

 

[a417]

<snip> My NMO mount is fabricated into my F-150 stake bed mounting hole, so it’s all exposed to the elements. Hoping to find a completely sealed NMO mount with attached coax that is at least RG-8x and preferably LMR-240UF

NMOs are designed to be mounted thru holes in body panels, so waterproofing is accomplished by the body panel and gasket. Mounting them outside of the body is essentially off-label use , and comes with the issues you describe.

I would think you can take some tin snips, some light gauge aluminum sheeting and make a 2 x 2 x 2 cube which has a hole for the NMO mount on one side (which the mount secures it to the bracket) and a grometted hole for the cable on the other, and fill it with silicone and make your own little waterproof enclosure of your own. There is the other option, as well...

 

[Tinkertek]

@a417 Thats a good idea, i will have to try that. I just ordered a new Laird NMO mount, so I’ll solder that up and try to include sone sheet metal.
Thanks !

 

[mmckenna]

@mmckenna - thanks for the reply. I have RG-8x currently in place so I’d hate to step down to RG-58. I‘m hoping to at least find a sealed cable mount NMO. That way I could at least fix the corrosion issues.

That's easy to fix. Get the Larsen NMOHF style mounts. The coax enters a port on the bottom of the NMO mount and crimped in place. Get yourself some 3/8" or so Marine Grade heat shrink tubing cut a piece 2 or 3 inches long and slide it over the coax up to the mount. When you heat up the tubing to shrink it, the hot melt adhesive on the inside will flow and seal everything.

The older style mounts were never designed to be mounted with the underside exposed. They were purely designed to go through a hole in the roof of the vehicle. Using them on bracket mounts will let the water in and corrode everything out, like you noticed. I've been pointing this out for years, but people seem to miss it.

RG-58 Dual Shield, good for high frequencies:

https://www.theantennafarm.com/catalog/mobile-antenna-mounts-264/permanent-hole-mounts-266/nmo-3-4-hole-mount-no-connector-289/

LMR-200, even better, but larger/stiffer cable:

https://www.theantennafarm.com/catalog/larsen-nmokhf200noconn-8452

Neither of these mounts have connectors installed. If you need a preinstalled connector, get one of the Larsen mounts with the FME style connector on the end of the cable. Those are small enough that the heat shrink should slide over it. Better to install your own connector, but make sure you have the proper tools for the size cable you are using.

Harbor Freight sells little kits of various sized marine grade heat shrink for a couple of bucks. If you don't have one there, check at West Marine or a marine supply store.

 

[Tinkertek]

Yeah having it somewhat open under that NMO mount causes all this corrosion. When the reception is poor, I think about that antenna, grrrrr ! Thanks for the tips folks !