Sunday, January 17, 2016

Antenna Work, Part II

OK, so I dropped the antenna down today, and swapped out my homebrew PVC adapter for the OEM metal version.

I took a bunch of pictures....BUT.....I had my camera set wrong, and the pix came out so overexposed that I couldn't even save them with GIMP.

So, all I have to show are some before and after plots, and those didn't covert very well, either.

Here's the 2 Meter VSWR graph with the plastic mounting:



And here it is with the OEM metal mounting:




Here's the 70cm plot wth plastic:



And here's the plot with the OEM metal mount:



Sorry for the terrible size disparity with the plots. I had to "print to file" on the Windoze machine, then convert them to png's on the Linux machine, and the "print to file" utility that creates the pdf dows whacky stuff.

I have the Linux version of the capture/display software for my AA-520 analyzer, and I think it's high time I install it, and learn to use it!

ANYWAY......the plastic vs metal plots don't look much different, so I'm not expecting much difference in performance. I called "CQ" on our "club-comm" frequency of 145.510, but couldn't raise anybody on either radio, so I assume they're all busy tonight.

Now one thing kind of jumps out at me, and that's how FLAT the VSWR curve is on 2 Meters. That kind of "flatness" over a wider bandwidth than the antenna is rated for can be indicative of an excessively lossy feedline.

I'm going to run some plots on the little GP-1 antenna, and see if it looks more like the published curves. I physically inspected the feedline on the GP-3 today, and there's no external evidence of damage to it, the PL-259's are properly soldered and the one at the antenna was still nice and shiny and clean after I took the tape off of it.

Yeah, I know, I should have made some loss measurements, but I got clobbered with some Honey Dews right in the middle of this today.

It's an easy 10-minute job to lower the antenna and disconnect the coax, so if the plots on the GP-1 look like the ones Comet publishes, I have a feeling I'll be dropping the GP-3 down again on Monday......

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