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NMO mount through CFRP?

ergbert

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I know there are a lot of professional LMR folks on this forum, so maybe this will catch someone with firsthand knowledge.

At one point, the LAPD had a small fleet of BMW i3's. The body on these cars is CFRP (carbon fiber reinforced polymer), so they probably are a pain to set up with mobile radio, especially for NMO mounts, not to mention the whole ground plane thing. Has anyone on here got any experience with drilling for an NMO mount through CFRP, and are there any concerns to be aware of? I'm not looking for anything crazy, just want a single NMO with a UHF/VHF dual band Larsen on top.

I don't quite have the nerve to just go ahead and drill it... yet.
 

mmckenna

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Good question.

There's two ways around it:

1. Use a half wave antenna. They do not require a ground plane under them to work, but they do work better with one. Done correctly the antenna will tune up and perform properly.

2. Add a ground plane under the antenna. This would require access to the underside of the roof. I've done this on UTV's before. Adhesive backed foil tape can make a ground plane under the antenna. Works well. Other option is to add a metal disk under the mount. For UHF and higher frequencies, there are premade units. Or, really simple to make your own.https://www.theantennafarm.com/catalog/index.php?main_page=product_info&cPath=481_1241&products_id=5384

It wouldn't be hard/difficult/costly for manufacturers to include a layer of conductive material under the panel, but I doubt they do.
 

mmckenna

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I'm not looking for anything crazy, just want a single NMO with a UHF/VHF dual band Larsen on top.
So, here's your challenge...
The disk I linked to above is only 6" in radius. That's fine for 70cm band. But it's too small for VHF. For VHF you want 19" or so. I have seen some dual band antennas that are some variation of 1/2 wave on VHF, so it should, theoretically, work with the 6" disk.

If it was me, I'd drop the roof panel and lay in some metal mesh, foil tape, etc. That way you know it's done right. You also may find some metal structural members up there. I'd doubt it solely relies on composites.
 

ergbert

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I've seen relatively heavy gauge (as tape goes anyway) copper foil tape...I expect an asterisk pattern of that material nearly the width of the roof would be useable on VHF. There is no sunroof to contend with, though I have not dropped the headliner to see if there are any other obstructions yet.

My biggest trepidation comes from having to drill the composite. I'm concerned about delamination, mostly. About to go do some reading on composite airplanes and aerial mounting.
 

Cognomen

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I have no experience with CFRP, but try to find out how thick the material is. You may need an NMO mount made for thicker material than sheet-metal, ie. a Larsen NMO-THK, especially if you're going to add additional material for use as a ground plane, which you'll have to do if you're going to use a dual-band antenna.
 

mmckenna

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^^^ yes, good point @Cognomen I was thinking that as I was posting early, but forgot to add it.

They make a number of different mounts for different ranges of thickness. Shouldn't be an issue to find one that will work.

As for drilling, I get the concern, but I think if you use a fine tooth blade (like the ones on the good NMO mounts) and go slowly, you'll be OK. I can imagine it's similar to drilling fiberglass.

As for the foil tape, the copper tape will work fine. You can also get wide tape for HVAC use that works really well. I've got a big roll of that I've used. I think I got it at Home Depot.

You only need an "X" or "+" shape under the roof for it to work.
 

RFI-EMI-GUY

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I've seen relatively heavy gauge (as tape goes anyway) copper foil tape...I expect an asterisk pattern of that material nearly the width of the roof would be useable on VHF. There is no sunroof to contend with, though I have not dropped the headliner to see if there are any other obstructions yet.

My biggest trepidation comes from having to drill the composite. I'm concerned about delamination, mostly. About to go do some reading on composite airplanes and aerial mounting.
You might talk to a body shop that does work for BMW. There may be an adhesive to line the hole to keep moisture from penetrating layers. Or it may be no problem at all.
 

mmckenna

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You might talk to a body shop that does work for BMW. There may be an adhesive to line the hole to keep moisture from penetrating layers. Or it may be no problem at all.
I'd be interested to hear what they say. Only a matter of time before we see more composite installs.
 

ergbert

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Did a bit of looking, and it appears the roof panel is a separate CFRP panel laid in over the actual bodyshell, with a support beam halfway, so I can either center the antenna in the front half or rear half of the roof. I had anticipated increased thickness, being a composite, so planned on a thick NMO.

I know a guy who does composite boat stuff, I'm gonna pick his brain about drilling through it, and if any treatment is necessary following, to prevent any delamination concerns.
 

ergbert

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Well, boat guy wasn't sure if his composite knowledge would translate...definitely wasn't willing to attempt, but he said that a very fine tooth count was best. His concern was that the teeth would tear out the fibers, especially exiting the hole on completion (punch-through, he called it). The best he could tell me was go really slowly, slow rpm as well as feed, and stop to clean the chips out frequently.

We shall see. At present I am preoccupied with designing a mast to mount antennae on, to be mounted to a trailer hitch (same vehicle, for stationary vehicular *OTA use, likely with a rotator and yagi on top, but that's a future build thread). So drilling the roof is back-burnered for now. Thanks so much for the insight, folks!
 

mmckenna

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The best he could tell me was go really slowly, slow rpm as well as feed, and stop to clean the chips out frequently.
One of the better ways to prevent the tearing on break through is to start drilling on one side and make sure the pilot bit makes it all the way through, but to stop before the hole saw does.
Then drill from the underside and complete the hole.

Fine tooth hole saws are the way to go. The commercially made NMO holes saws I use are all fine tooth and do an excellent job. Where people usually run into issues is when the go to their local hardware store and purchase a 3/4" hole saw designed for wood and start drilling away. It usually leads to disappointment.
 

radioman2001

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If you have the room underneath I have had good results with a piece of sheet metal or even cookie sheet for VHF. For UHF I use a 1900 electrical box cover with the prepunched hole. I have used a lot of them on commercial busses, and ambulances. I am not knocking 1/2 wave antenna's but never got them to be stable when used for repeater and simplex channels in the same radio.
Stick on foil always has a tendency to peel from the heat.
 
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