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Discussion Starter · #1 ·
I am an amateur radio operator. I was on a ride in Ouray (see this link) when I had opportunity to play a small part in a mountain rescue (see some photos and brief description on the Ouray Mountain Rescue Team's website here) using my 2m handheld radio. My comms were spotty using that handheld--the repeater I was hitting was in a saddle over 10 miles away, blocked by some peaks in the way.

So I decided when I got home to put my Kenwood TM-261A into my KRX (more power and hopefully better antenna). I mounted it out of the way under the glove box on the passenger's side. I used one of the tabs on the roll cage in the back to mount my antenna, and that's where this DIY info comes in. I don't know if it will be helpful or not, but I thought I'd post it for the group.

I bought this antenna on Amazon. It has a brass plate that helps seal it down onto a hole in the roof of a vehicle. In fact, and this is where the info here is important, it's designed to work over a "ground plane"--that is, a large flat metal sheet. Inasmuch as I have the KRX plastic roof, I don't have a ground plane.

After I installed the antenna, I used my SWR meter to measure the Standing Wave Ratio. This is a number between 1 and "infinity" that measures how good your antenna impedance matching is. One way to think of it is the amount of power the antenna reflects back to the radio if it's not well matched. A perfect SWR is 1.0. Anything in the low 1.x range (e.g 1.2, etc.) is considered "good."

Anyway, I measured the SWR after installing the antenna and it was 1.6. That's probably acceptable, but not ideal. "Tuning" the antenna (by changing its length) didn't help because the amount of adjustment was small. I had read somewhere that an antenna like this shouldn't be installed over a painted or powder-coated surface as that will impact the impedance matching (raw metal is best). A "ham" friend suggested grounding the antenna to the chassis to see if that would help. So I did, and, bingo: the SWR went down to 1.3.

So I decided to sand away the paint in the immediate vicinity of the tab I was using on the back of the roll cage, reinstalled the antenna, and got the SWR down to 1.25. I'll accept that for a cheap antenna I bought off Amazon.

Here are some photos:

Radio in the cab (before I tidied up the cables in the back)

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The metal tab on the back before sanding off the paint:

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After sanding off the paint (quick job, kinda sloppy; I did it on the underside as well, but you can't see that in this photo):

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Brass fitting installed:

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Note here: don't install this brass fitting too tight--if you squish the o-ring on the bottom too much, you'll mess up the impedance. When I did that, the SWR went up above 3. Just a finger-tight gentle snug is enough. Just engage the o-ring.

Antenna installed:

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Cables snaking up to the cab:

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Note that I used zip-ties to secure the cable. Generally, you don't want to tighten them too much as you might crush the coax. They make special cable wraps for this purpose, which I didn't have. However, I noticed that if I put the cable at the "joint" of the zip tie, I could secure the cable quite well without crushing the cable. See the detail of the nearest zip tie in the photo above.

This info may or may not be useful to some of you.
 

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Discussion Starter · #4 ·
Why not just use a NGP antenna?
Based on what I see online, the antenna I installed is probably intended to be a no-ground plane (NGP) antenna, though it's not advertised as such. The out-of-the-box SWR of 1.6 indicates a reflected power of just over 5 percent, which is perfectly acceptable.

However, I wanted to know if I could make it better, and "baring the metal" under the antenna helped, so, mission accomplished.

Another advantage of this antenna is that it is an "NMO" antenna, which allows you to replace the antenna itself with other antennas that have the same mount.
 

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I get that, I just figured that running a NGP in a more desirable location is better than trying find a ground in less desirable spot. I run 1/2 wave NMO on the rear of my roof. We run UHF (GMRS) in the east so it's only about 14 inches overall. To stay with a 1/2 wave for NGP and be dual band I would the 3ft range and not be able to keep high and centered mounting without killing antennas every 50 ft. Of course you be in the same boat running a NGP 2 meter whip since all NGP are 1/2 wave.

You can see it here rear center of the roof.
 
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