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Is there an antenna base that goes through the roof?

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#1
I'm looking for something that is a simple UHF antenna (for GMRS), where the base of it bolts down, and the RF connector is on the inside of the vehicle. I would like to put a GMRS antenna on the fiberglass roof of my van. I have something like this for CB, but I'm not even sure what it would be called, so I've been having a hard time finding it for my GMRS radio.

With my CB, the only thing above the vehicle is the base (with rubber/silicone on the bottom) and the antenna. On the inside, the So239 connector is about an inch and a half long, it has a giant washer on it, a bolt that presses that washer against the roof and keeps it tight, and the PL connector just goes on the end of that. I can look around more for something like that for UHF, but i don't even know what that would be called lol (I'm a bit new to this).
 
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#4
Not a problem.
A lot of early ambulance bodies involved fiberglass rooftops.
There are two things you need to consider:
1. Ground plane. You either need to provide a ground plane under the antenna OR you need to use a ground independent antenna.
2. You need to think of tree branch strikes, parking garages, etc. where if the antenna hit something, it wouldn't damage the fiberglass roof.

Easy solution for this is to use a thick NMO mount with a ground plane underneath. Often the ground plane is a sheet of steel or aluminum on the inside of the roof with the NMO mount coming through the roof and the sheet. This spreads out the forces of a strike and provides the ground plane.

NMO mounts are about as standard as you can get. Once you install one of those, you have a very wide variety of antennas to chose from. If your needs change down the road, all you have to do is unscrew your UHF antenna and thread on a new one set up for the new frequency.

As for simple antenna, it's really hard to beat the low price and simplicity of a 1/4 wave antenna. You can pick them up for less than $10 if you shop around. They are only about 6 inches tall on GMRS frequencies. They are low profile and work very well.

Something like this mount: http://www.theantennafarm.com/catalog/laird-technologies-mabvt8-1153.html
This antenna: http://www.theantennafarm.com/catalog/laird-technologies-qw450-797.html
This ground plane disk: http://www.theantennafarm.com/catalog/pctel-k332-5384.html

Go with the name brand antennas and mounts. I avoid Tram and Browning, they are Chinese knockoffs and while they may save you a dollar or two, you are giving up some quality. A properly installed and maintained quality antenna should outlast your vehicle.

You could use a similar setup for your CB using an NMO mount CB antenna, like this one: http://www.theantennafarm.com/catalog/larsen-nmo27c-699.html
Trouble is, getting enough ground plane under the antenna. You can accomplish that using foil tape with a self adhesive backing. They often sell in in the HVAC isle of a good hardware store. I've used it before to make ground planes on the underside of plastic rooftops on UTV's.
 
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#5
Thank you for your help everyone.

Steve I was going to say I wasn't sure what to do about a ground plane, but I think I'm going with what mmckenna suggested.
 

SteveC0625

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#6
Thank you for your help everyone.

Steve I was going to say I wasn't sure what to do about a ground plane, but I think I'm going with what mmckenna suggested.


If you needed a suggestion, I was going to say that a 12" pizza plate would do the job nicely. But the brass disc is no more expensive and can be ordered when you. Order the mount and antenna.

Just note that the disc has a 3/8" hole so you'd want to use a 3/8" NMO thick wall base. It's easier to drill, too.

I second the simple NMO 1/4 wave antenna suggestion, too. It's what I use in most applications.
 
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#7
...
Easy solution for this is to use a thick NMO mount with a ground plane underneath. Often the ground plane is a sheet of steel or aluminum on the inside of the roof with the NMO mount coming through the roof and the sheet. This spreads out the forces of a strike and provides the ground plane...
i suddenly have a "oh, snap, i should have thought of that" when installing the NMO on the fiberglass camper shell last week.

Guess I know what i'm doing on my day off.
 
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#8
i suddenly have a "oh, snap, i should have thought of that" when installing the NMO on the fiberglass camper shell last week.

Guess I know what i'm doing on my day off.
I used adhesive backed foil tape once on a plastic UTV roof install. Worked well, however after years of hot/cool cycles the tape slowly started to fall apart.
I just put an "X" pattern and mounted the NMO base through the center.
Might work OK on a truck cap if it's protected.

Sorry for ruining a perfectly good day off.
 
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#9
i used a plastic washer under it to protect the fiberglass, but Im thinking of swapping it out with a metal one now.

Its for a booster, 700, 800, 1900, 2100. 1 watt max.

I calculated 1/4 the wavelength at the lowest possible frequency at .55 inches, so i figure a washer with a 1 inch radius will be fine.
 
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#10
At 700MHz, 1/4 wave is about 4 inches. You'd need something a bit bigger than a washer, although a 1/2" washer is better than nothing.

Even some aluminum foil glued down will work. Other ideas:
Coffee can lid,
Paint can lid
Scrap sheet metal
 
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#11
how the heck did i get 10.7cm is half an inch.

this is what i get when i math on the weekend.

I'll just cut the bottom off a coffee can. Thanks
 
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