After a few hours of frustrating adjustments, I still could not match a (recently shortened) Decibel Products PD-210 antenna I was modifying for use on our 52.88 repeater. Initially, I used an HP- 8920 and SWR bridge and could not adjust the "trombones" any better than -8dB return loss. I am using a 6 foot pigtail of 50 Ohm, VB-8 coax. Best resonance was found at about an equal 52 inch extension on either side of the antenna's mechanical center.
This morning I took a fresh start and pulled out the old SWR meter and an old 6 meter mobile radio. Sure enough, the forward power was about 40 watts and the reflected power was nearly 20 watts. "OK, what's wrong here," I mumbled as I stared at the antenna's upper and lower folded elements pointing skyward on its mounting pipe. It passed through my mind that the antenna looked a lot like a folded monopole ground plane with two of its radials pointed straight down. So, I yanked out the lower trombone for some reason and the VSWR went to nearly zero! Next, I shoved the trombone all the way in (noted the same VSWR) and started making measurements. Now, it is obvious to me that the antenna is not really symmetrical. The hot end feed is 2.5 inches off mechanical center. By changing my measurement point to the point where the feed line connects and measuring to the tip of the hot end is 51 inches. Similarly, by measuring from the other side of the exposed insulator to the cold end of the dipole is now also 51 inches. Anyone out there have this happen to them? I wonder if this is something I can measure on the 145 and 440 antennas I have in the back yard? Jim WA6VPL Lompoc, CA
