Comet GP-3 analysis

[Firekite]

I finally got a NanoVNA-H and got it set up to sweep my Comet GP-3 in the attic connected via about 25 feet of LMR-240. It’s a 6-foot base station antenna advertised for 2m/70cm, claiming gain figures of 4.5dBi for 2m and for 70cm 7.2dBi. Since MURS and GMRS are closely adjacent, I tested those, too. These measurements are of my setup, but the results I thought were pretty good across the board. MURS was tolerable-ish, but GMRS was shockingly good. 2m and 70cm unsurprisingly should work well on this antenna. I don’t know that I would call it particularly wide banded, but it works for my needs.

I really do want to get it up on a mast on the roof, but I need a lot of hands-on help figuring out "the right way" to get it done. Right now it's hanging upside down in the attic via some high-test braid fishing line. It does pretty well, considering, but not as well as I'd like, and I'm sure adding another 7 to 12 feet of elevation up and out of the attic will help. Here's a pic for reference. Not ideal, but generally working.



Here’s the wide analysis first, 130 MHz to 480 Mhz:





2m:





70cm:





MURS and GMRS:





All in all, it seems like it gets the job done. Flattening out the radiation pattern is helpful in my neck of the woods, as I have no mountaintop repeaters to hit. There's a rural repeater that's exactly 33 miles from me in a straight line that I can pick up kind of choppy with my Larsen 2/70SH on a mag mount on my truck's roof. I'm hoping I'll be able to hit it fairly cleanly from the base station once I get it on the roof.

 

[prcguy]

VSWR is only part of the story, your radiation pattern out of band can loose a lot of gain at the horizon. On a colinear type antenna I believe when you go higher in frequency the radiation pattern tilts downward. GMRS is about 20MHz away from the UHF center of band on that antenna and that is enough to skew the pattern 5 degrees or more on some antennas.

 

[K6GBW]

I'm wondering if hanging it upside down like that skews the radiation pattern. I put up an antenna similar to this at my friends place. Same situation, but what I did is create a small loop out of parachute cord and taped it to the top of the antenna. I put a small nail in a rafter and simply hung it up. His works fantastic, but then we have repeaters all over out here in Los Angeles. On simplex it works decently.

 

[dwhit29689]

Technically I've no idea. My GP-3 works well at 40'.
Setup moves easily. KN4YBP