Roof mount 2M antenna

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KLF

KLF

Frame waxer
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I'm about to send the cab of my pickup out for paint, but I'm trying to work up the courage to drill a hole in the roof to install a permanent NMO mount for my Ham radio antenna. I have a really nice Larsen NMO2/70B antenna that I was using on a mag mount, but we all know that's only a temporary solution, and they scratch the paint no matter how well you protect it.

I bought the NMO mount kit, it requires drilling a 3/4" hole. I'm concerned about how thin the metal feels on the roof, and the chance that a slight hit on the antenna will bend/buckle the roof. I was planning to take the antenna off during the occasional wheeling trip, probably put it back on the magnet stuck to the toolbox in the bed.

I was thinking about taking a piece of stiff sheetmetal and adding a ~6"x6" stiffener plate under the roof, probably drill a few holes at the corners and do some quick button welds to hold it in place so it can't rattle. Like I said, the cab is getting a complete repaint so no worries about that. I just don't want to find out later that I should have done this, which will ruin the fresh paint. Or, maybe I can just epoxy the plate in?

Thoughts?
 
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Punch or drill the hole now, before you paint for sure.

The great thing about the NMO mount is how quick and easy it is to change out antennas. Just carry something like a Flexi-Whip in the truck at all times (takes up almost no space), and swap it for the taller/stiffer antenna whenever there is a risk of overhead obstructions. It's only a 1/4-wave on 2M, but mine works so well from the roof top that I just leave it on there all the time.

http://www.sti-co.com/antenna-products/public-safety-antennas/flexi-whip
 
I like the stiffener plate idea. As you say, now's the time. It would do exactly what you want-to spread the load of a strike on the antenna.

THe Larsen whips are the best and toughest antennas in my book, especially if you install the Larsen spring in the base. That will mean retuning your antenna but will worth it and takes a lot of stress off the mount.

I use this one:
http://www.hamcity.com/store/pc/SPRINGB-p1950.htm
 
I've had this antenna since I got my ticket in '07, so there's no returning it.

I'm committed to drilling the hole, I just can't seem to bring myself to do it. I've had the antenna cleaned off the roof a couple of times when it was on the mag mount, and that was just going down the road. Once was on a low-hanging wire that I saw too late, as it was dark out. So I know it will get hit, being a lifted truck and a tall mast (~40").

I've got a bunch of scrap 22ga steel in the garage, I may try the stiffener idea on a test piece.
 
I run a larsen NMO-2/70 SH which comes with the spring. It's a tough antenna and CAN take the hits. It took so many on one trip (lots of low hanging scrub on some tracks in the oz bush) that the spring/whip was pulled out of the spring base adapter. Mine is mounted on a plate bolted to the rack on my patrol so that the lower spring part is protected by the rack's front edge and allow the antenna to fully fold over in case of a low hit.

On returning to the US, I sent an email to Larsen and within a week they mailed me a complete new antenna for $0. I HIGHLY recommend that you mark the position of the spring whip, then undo the allen grub screws, grind a flat onto the whip where one of the allen lock screws bites in and then reassemble with loctite on the grub screw threads. That's my only complaint with the unit.

mq_ant.jpg


cheers,
george.
 
Not a lot of trees in the SW, but I've had an NMO in the roof of my '84 Xcab for about a decade. No stiffener of any sort, and no issues. I have a very short (~12") antenna that I use mostly as a moisture cap around town leaving the taller antenna for trips. I have hit trees with it, hard enough to bend the whip right at the top of the sleeve in the base that it fits into. Still no issues with the roof or with leakage.
 
The Comet C213NMO mount only needs a 3/8" hole IIRC



Here is a picture of the mount in a trick tab;





The mount would allow a stiffener plate and still go through the body metal. I'm not a huge fan of RG316 but it is thin and would be easy to route under the headliner.
 
Thanks for your input. Here's what I ended up doing... after really over-thinking this.

I cut out a piece of 22ga sheet metal, 4"x4", drilled a 3/4" hole in the middle. Then, I worked up the guts, and drilled the 3/4" hole in the roof. No turning back now...

I was in Lowes yesterday and found a 3M tape called "Extreme Mounting Tape". OMG is this stuff STICKY and strong. I was worried that welding the plate in, even with short plug welds, would cause distortion in the roof, and eventually make dimples as the metal flexed from the torque of the antenna. But, I also know that the stiffener has to be electrical connected to the body, so I couldn't just tape or epoxy it in, so the mount gets a ground.

I tinned a small spot next to the hole on the underside of the roof and the stiffener with a propane torch and some solder. Then I applied the tape only around the perimeter ~1/2" of the stiffener, and stuck it to the bottom of the roof, being careful to align the hole. I used a #8 screw/nut with some washers to pinch the metal together right at the edge of the hole, and quickly sweated them together with the torch. The roof did buckle up a little bit, but as soon as it cooled back down it went back flat.

No rattles, and I feel a lot better about mounting the antenna up there now.
 
My wildland truck has a Larsen steel antenna mounted through the roof. That thing has been beat to s*** on branches at all sorts of speeds. I know we don't put a stiffener plate in. There are no creases or damage to sheet metal. Here are some pics showing the antenna.

First truck is mine, the second just shows the antenna better. These things get abused and just keep on broadcasting.

HTH,
Brittain

Fire truck.webp


421 antenna.webp
 
The key with the Larsen antennas is to lock tight the set screws otherwise they loosen and you will lose and antenna on the trail. Ask me how I know.:o

Otherwise they are almost indestructible, even if you manage to bend the whip it can be straightened out.
 
rusty_tlc said:
The key with the Larsen antennas is to lock tight the set screws otherwise they loosen and you will lose and antenna on the trail. Ask me how I know.:o

Otherwise they are almost indestructible, even if you manage to bend the whip it can be straightened out.
Click to expand...

Yep, see my post #5.

I recommend removing the whip, grinding a flat for the set screw and using loctite on the screw. At least if the screw loosens a little, the flat will prevent the whip from sliding out.

The design with 2 grub screws is somewhat flawed, since due to the relative angles of the two screws, only one really does anything. When I lost my whip and Larsen replaced the whole antenna for free (nice folk), I mentioned the design flaw. Maybe they will change their screw locking scheme.

They also mentioned to use loctite on the replacement - of course I asked, why they didn't loctite them at the factory...

cheers,
george.
 
Unless there is something that I'm missing, I'll disagree on the second screw not doing much. There is no reason that both can't bind up the mast independently. Both are cup point type set screws, so they need to be tightened enough to bite into the mast to be effective. I tighten them right up to the point where I start to get concerned that I'm going to strip the threads in the antenna base or strip the hex in the soft stainless screw.
I've wondered about Lock-tite causing conductance issues so I have yet to use it on any of my multitudes of base loaded antennas. At least one has been like that bouncing around the desert chasing desert racers for over a decade.
 
Good idea on grinding a flat George.

@ntsqd the place you lose the Larsen antennas in when you are going through trees and brush. I never had a problem with mine in the desert but lost a whip the first trip through a trail in the pinion's.
 
ntsqd: The 2nd screw is useless. Tighten screw #1, then tighten screw #2. Loosen screw #1 and the whip falls out or is very loose... Been there, done that... I mentioned this to Larsen and they agreed that it wasn't a 'perfect solution'.

A flat ground into the whip ensures that the whip can't slide out unless the grub screw loosens quite a lot and loctite will prevent that. Larsen themselves recommend use of loctite, though they don't come that way from the factory. Larsen doesn't put a flat since they pre-adjust the antenna and it would take time for them to disassemble after tuning to then grind a flat and then reassemble.

Anyhow, this is from my experience, take it as you will. I like the Larsen antenna unit, it just could do with a little help to prevent losing the whip on a trip. The issue I had was with the antenna taking hit after hit over several days driving through thick scrub with low branches etc.

cheers,
george.
 
While not the East Coast or even mid-America, I've gone thru/under trees and tall bushes and smacked them many times without a problem. Do you walk the screws down, or do you fully tighten one and then the other? I believe that I've walked them down so that the mast progressively ends up in the "corner" opposite both of the screws.

I'll for sure use Lock-tite in future installs, and ponder doing the flats.
 
ntsqd: Yes, I tried various combinations of doing the grub screws, it's just a flawed scheme that gives a false sense of security that 2 screws are giving 2 times the holding strength. One flat and one grub screw and loctite will take care of all the problems.

I was in the bush for near 2 weeks with many days of driving and hitting branches/scrub for hours on end. It was on the 2nd last day that the whip 'disappeared' off the vehicle. So, it took quite some time in my case to finally slip out.

Mine was the 2/70SH so just the short whip with the spring base. Lots of hits will take the antenna near horizontal - and the scraping action of a branch will work to pull on the whip in the direction to pull it out of the spring base.

Here's an easy and fairly clear track just out of Perth - I'm holding the camera with one hand while driving.



Where I normally go camping in the western oz goldfields a lot of tracks are overgrown and have hardier trees and branches hanging across tracks. Those are the ones that give any antenna on the roof area a good workout.

cheers,
george.
 
A compression fitting similar to a drill chuck would have been a better choice but I suspect it would also drive the cost up.

But they make a great antenna and the whip issue is a small problem which is easy to correct.
 
Wow, I can't believe the debate on such a trivial thing, but it is appreciated. I really don't want to lose my whip!

So, I pulled my antenna out of storage to take a look, yes there are 2 small set screws in the base that hold the whip. They are about 120deg apart (not 90deg). I carefully marked the length of the whip, then pulled it out and filed flats where the screws are hitting. Re-installed with Loctite. Should be good to go!
 
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