It does, doesn't it? But I've seen RF do strange things, so it doesn't surprise me at all (USAF avionics tech for 27-1/2 years, and ham operator for many years before that). If you have ever seen a TV antenna aerial (the kind you put outside your house on a pole, not an indoor one), they have various lengths of rods mounted on a pole. there might be a few to maybe ten or more. In general, the longer rods are behind the actual element that receives the signal (the one the coax/twin lead is attached to). The shorter ones are in front. The longer ones in back reflect the signal forwards, and the shorter ones in front continue it on its way (if you think in terms of transmitting, which you're not, of course, but that doesn't matter). The folded top is probably working crudely like an array of elements, giving you the unexpected SWR readings. Back in the day we used to just ascribe such goings on to FM: Fracking Magic (but we didn't BSG the phrase).

 

Your vehicle is the ground plane so a lower swr with the top up than down is correct. If you installed a metal hardtop with good ground to the frame you'd get better results. A tractor trailer is a big solid ground plane that why the cb works so well with them.
Here's what I did. If you skim through the pics there should be a couple of the antenna. I had installed a 2' and swapped to the 3'. It is a K-40 3' super whip for $18.95, made in USA, adjustable.
https://www.jk-forum.com/forums/jk-cb-ham-radios-128/3rd-brake-light-antenna-install-96042/

I was getting a 1 on 19 and 1.4 on 1 and 40.

 

Nice setup, its really similar to mine but I mounted the antenna a little higher. I am starting to think I am just going to need a new antenna. I just didn't think the canvas top would act like a ground plane. I figured the antenna wouldn't even know it was there. I might try a heavier gauge wire on the ground strap I ran, I only used an 18 gauge and have since thought I should have gone heavier.

 

You got that right, I am a Mechanical Engineer and we had to test RF on a rotary joint on a gimbal we built for satellite antenna and we ended up bringing in a consultant to work his voodoo.

 

Good grounds will help for sure.

Funny about the top. Something to do with the metal frame in it, I didn't think it would make a big difference either.

 

Yes.....it's not the "canvas" top that is doing anything...the rf wouldn't be affected. It's the metal frame of the soft top that is throwing things off. ANYTHING (metal) that is changed around when it comes to rf can, and will, do all kinds of different things.

 

So then is the reading I get with the top up real or not (i.e. Is the metal frame of the top giving me false readings)? What can I do to lower the SWR with the top down? Do I need a NGP antenna?

 

I thought I'd posted a good long reply, but I see that it's not here....

No.....no false readings, that is actually what's happening to the RF with both the top up and down.

Anyway......."If it was me".....lol, If you want to get the best TX and RX that you can, I'd abandon the 2'er, and get a good "tunable" / "longer" antenna and go from there.

If you're worried about hitting the garage door/ceiling, there are a number of quick disconnects as well as pivot mounts that allow you to swing your antenna down and out of the way.

The choice is yours, but when it comes to antennas, as well as radios and gear as a whole.......you get what you pay for.

 

You should get better(lower swr) readings with the top up. More ground plane, like a truck.

 

Thanks guys I appreciate all your help. For now what I have will get me through the trail ride I am doing this weekend and I can upgrade before I go out again. I am leaving the top up this weekend anyway because it’s a dusty trail so my SWR won't be that bad.

Thanks again for all the input, next time I will know not to skip corners when it comes to the antenna.