NMO Mounting issues

[360mk2]

Hey guys,

I'm having some issues with some antennas that were mounted to headache racks. They were installed by mounting a Laird mounting plate that was screwed into the headache rack using self tapping screws. the screws holding the plates were snapping off. (welded back on to fix the issue) Now they are breaking the NMO mounts themselves. What would be the propper way to fix this issue?

 

[mmckenna]

Tell us more about the exact antenna model, exact NMO mount brand/model, and what is causing them to break.

I've installed a lot of NMO mounts and I've got a ton of them installed on various vehicles at work. I've never seen one break before.

"A picture is worth a thousand words" applies here, if you can toss up some photos.

If they are physically hitting something, you may need to address that. If you are using inexpensive mounts, that may be an easy fix. Installation issues can be a problem.

Also, mounting antennas on headache racks can cause issues with providing proper ground plane for the antennas, which can impact performance.

 

[SurgePGH]

They? The drivers? Start charging them every time they break one. That will bring about a quick change in their actions when you hold them responsible. Aside from that I would go with a roof mount (ideal) or fender mount (less than ideal). Do you have overall pictures of the scenario?

 

[360mk2]

I can grab a couple of photos. unfortunately I made a bad call and agreed to mount these on the headache rack . I'm responsible to fix them per the terms of my agreement ( install warranty) I could claim its their fault and not mine. But they are a rather big account for me and do not really want to fight them on the issue.

They are driven in tough remote logging roads and I agree that its completely ridiculous that they are breaking. The first one was a broken retaining ring. I have not seen the second as of yet.

As for the antennas they were re-used from the decommissioned vehicles. and are almost all different (trust me I tried to get them to buy new ones) I have not received any complaints on the performance.

For the install I used Laird MB8UP cable and a Laird mounting plate. I have been considering installing the MTPM800.

 

[mmckenna]

OK, so decent mounts.
I'd love to see a photo of the broken ones, just to see how/what they did.

Looks like abuse, to me. With the coil down below the bar on the headache rack, that ~should~ protect them. You may want to add a spring to the whip base to absorb some of the energy. I'm using a Laird 1/2 wave VHF with a spring at the base of the whip on a Polaris Ranger UTV. I run a lot of logging roads, so I understand the risks. Even running down trails at 30+ MPH, the antenna has taken all the abuse I've given it.

I'd be concerned about:
-using the antennas as a grab handle.
-using the coil as a tie down.
-Carrying loads in the back that are hitting the coils and breaking them.

It would seem that any low branch that would take the antenna off would either leave marks on the roof/rack, or something. If they are truly running these off road through the underbrush, maybe mounting something just in front of the antenna to deflect some of the energy would help. I think a spring at the base would be wise.

Either that, or relocate them entirely. It'll impact performance, but reality is nothing impacts performance more than a broken/missing antenna.

Or….
If they are VHF, UHF, etc. move to a 1/4 wave whip antenna.

One other thing I'd point out, the NMO mounts you are showing on the rack are designed for going through the roof of the vehicle where the coax connection point is inside a warm/dry vehicle. Having those exposed outside like that is going to result in corrosion and water ingress in the coax. That's going to destroy them pretty quickly. You either need to use a sealed mount, mount them through the roof, or get under there with some silicone sealer PDQ.

I think the lesson here is to never guarantee performance on a system you don't have 100% control over. But I'm sure you've figured that out.

 

[jeepsandradios]

Ive yet to see a NMO mount broken except on the roof of a car, and normally the roof rips out first. I use the Larsen NMO especially on brackets as the base is more protected. I would think they would bend the bracket rather than break the tabs off. Even a gain antenna should have a spring. I've had 1/4 waves flattened on roofs of ambulances but not broke off the mount.

 

[360mk2]

Thanks for all of the replays i appreciate the help!

So i really messed up this one. I would love to just do roof mounts and be done but that would take a lot of time calling back the vehicles removing the prisoner cage, dropping the headliner and re routing the wire. (and would delay other work)

Lesson learned....i need to learn how to say no.

is there a sealed mount that you guys would recommend? im not really a radio guy and 99.8% of the time i just do standard roof mount.

 

[mmckenna]

I've been installing on Ford and Chevy 1/2, 3/4 and 1 ton trucks for years and have never had to drop a headliner. It might take longer up front, but it'll save you time in the long run.

As for the NMO mounts:

https://www.theantennafarm.com/catalog/index.php?main_page=product_info&cPath=264_266_291&products_id=1125

Slide a piece of marine grade heat shrink tubing over the crimp where the coax enters the NMO mount. The marine grade heat shrink has a hot melt adhesive on the inside that will melt when heated and flow to seal everything up. You can buy lengths of it pretty cheap on Amazon. If you are in a real jam, you can pick up kits of it at Harbor Freight that will have a few suitably sized pieces. Make sure you use a good heat gun to shrink it, not a Bic lighter.

While you are at it, install some over the coax connector crimp at the radio after you install it. It makes for a really nice strain relief.

 

[mmckenna]

https://www.amazon.com/WindyNation-20-Feet-Black-Adhesive/dp/B07LB977TP/ref=sr_1_1?dchild=1&keywords=WindyNation+3%2F8%22+20+Feet+Black+3%3A1&qid=1613183743&s=industrial&sr=1-1

The 3/8" size works nicely.

 

[R8000]

If you are running VHF and above, STI-CO flexi-whip antennas may be the answer. The mount won't clear the bar as far as a gain antenna, but the small interaction is better then no antenna at all. I have 3 of them and have been quite happy.

STI-CO Flexi-Whip Antennas