DIY Elevated Feed 70cm

[TrevorK]

I have a Laird B4502NS (70cm 1/2 wave) that's currently on a custom bracket hose clamped to the rollcage of my jeep in the drivers side rear corner of the vehicle. It works perfectly fine for me there, except I cant use it with any kind of soft top that wraps around the back of the cage.

I'd like to keep it up above the "roof line" of the vehicle. I was thinking an elevated feed antenna mounted to the rear corner could be an interesting idea. The options there seem to be few and far between, not tall enough, or expensive. Now I'm thinking through the concept of putting an NMO mount on the end of a tube, running coax through the tube, and using my Laird antenna.

Here's my question: I cant seem to find any NMO antenna mounts where the cable exits straight out the bottom, inline with the antenna element. I get why that's an oddball request... NMO mounts are intended for through roof applications... a low profile 90 degree cable exit is perfect for that. Is anyone aware of a straight NMO mount that I could crimp/solder directly to my coax? Is my only option to use one of those NMO-N adapters and have that extra connection?

Thanks.

 

[prcguy]

There are many NMO mounts with an N, SO-239 or even an SMA connector in line with the mount.

 

[Firekite]

https://www.theantennafarm.com/catalog/index.php?main_page=product_info&cPath=264_1103_1136&products_id=1156

https://www.theantennafarm.com/catalog/index.php?main_page=product_info&cPath=264_1103_1136&products_id=1157

https://www.theantennafarm.com/catalog/index.php?main_page=product_info&cPath=264_1103_1136&products_id=1158

 

[TrevorK]

Cool. Thanks for the feedback. If anyone out there knows of something like this crimp/solder NMO connector but with the cable exiting straight out the bottom I would love to see it, but it looks like there are several options for NMO-X adapters that would work for me.

https://www.theantennafarm.com/catalog/index.php?main_page=product_info&cPath=264_1103_1135&products_id=4287

 

[Firekite]

If anyone out there knows of something like this crimp/solder NMO connector but with the cable exiting straight out the bottom I would love to see it

It’s not magic. Pick one up, give that tab a bend, and now it’s exiting straight out the bottom.

I’m not sure what you mean about a tube, though. For weatherproofing? How do you picture it mounting? An NMO mount is going to need a flat surface of some sort to clamp through, like a tab or sheet metal bracket or something. You don’t need to enclose all NMO mounts. The N type versions I linked above (first and last link) are probably your best bet for avoiding corrosion and water intrusion, but it still needs a flat piece of grounded metal to clamp through. Just keep an eye on it occasionally and keep it clean. Otherwise get the Larsen NMOKHFUD mount, slide some quality marine heat shrink over the crimped collar to seal it up, and again just keep an eye on it and keep it clean or any road salts and grime.

https://www.theantennafarm.com/catalog/index.php?main_page=product_info&products_id=1125

Here’s what one looks like, enclosed on the underside.

 

[mmckenna]

Cool. Thanks for the feedback. If anyone out there knows of something like this crimp/solder NMO connector but with the cable exiting straight out the bottom I would love to see it, but it looks like there are several options for NMO-X adapters that would work for me.

https://www.theantennafarm.com/catalog/index.php?main_page=product_info&cPath=264_1103_1135&products_id=4287

Attaching it to the riser tube will be difficult if you take the unterminated NMO mount like that.
You're better off using one with an N connector and feeding that down the tube. Notch the tube and use a hose clamp to hold the connector. Not ideal, but it'll give you what you are looking for.

 

[TrevorK]

It’s not magic. Pick one up, give that tab a bend, and now it’s exiting straight out the bottom.

True. I would have to come up with a decent way to relieve the strain on that little tab. It doesn't look like it would be as strong as a crimped ferrule on a typical connector. That NMO mount in your photo is similar to the one I am currently using on my homemade roll bar bracket. I put little adhesive heat shrink over the crimp but other than that its out in the elements.

I'm still thinking through it. It may be difficult to explain without making a sketch, so forgive me. I'd like to protect the coax inside something just to keep if from getting snagged.

One thought is to cut a small metal disc for the nmo adapter to mount through. It would be a little larger in OD than the nmo threads. Enough for the o-ring on the bottom of the antenna coil to seal against. I could braze that to a piece of tube to enclose the N or SMA connector. I would want to step that tube diameter down then... either through finding some conveniently sized reducers or just a few decreasing diameters of tube. At some point there should be a threaded connection to access the connector. It would need to be large enough for the connector and cable to pass through.

If it was metal all the way down to jeep tub that would probably get bent on a tree branch eventually or rip the whole thing off of the jeep. It would be cool to somehow switch to a material with some more flexibility. Maybe this metal bit could be epoxied to a composite tube. I've used garolite G-10 in the past but that's a little expensive for this. Maybe some garolite CE would be worth a try. The phenolic resin is not quite as strong but might work. I would want to try and replicate the flexibility of the fiberglass CB antennas that have been used for years on jeeps.

On the other hand I have plenty of scrap pieces of steel tube. If I could incorporate a spring at the base that could make it survive the occasional branch, but then I would have to figure out how to pass the coax through the spring and still mount it. Im guessing most of the antenna springs I could find would be too small in diameter for any through hole large enough for RG58. That would put me in the size range more for something like RG174. I'd want to work out the loss between the length of that and all the connections. Maybe I could pass the cable through the side of the tube before the spring, only exposing a small length near the jeep tub where it's not likely to get caught on anything, then run it to the transceiver.

And now its gotten pretty complicated, as most of these kinds of thought exercises do.