> -----Original Message-----
> From: email@example.com
> [mailto:firstname.lastname@example.org] On Behalf Of Richard (Rick)
> Sent: Tuesday, December 29, 2009 9:47 AM
> To: Jim Thomson
> Cc: email@example.com
> Subject: Re: [TowerTalk] Motor Driven vac used in 40M DE?
Unless I'm missing something, it shouldn't require an expensive vac cap to
tune the DE match of a parasitic beam. RF peak voltage across the gamma cap
shouldn't normally exceed perhaps a few hundred volts even at maximum legal
power. I did something similar with modest air variables.
About 15 years ago I built a dual-band antenna for 20m and 40m... 20M had a
65' dual boom carrying 5 wide-spaced elements with DE fed via a gamma match.
Maybe it was 6 elements - actually tried both at various times. The boom was
extended by 20' of 3" tubing on each end, then a loading coil of #12
enameled Cu wire on a 3" well-sealed oak mandrel on each end, and then 12'
end sections of ~1" tubing. This entire extended "boom & 20m beam" assembly
resonated near 3.75 MHz...no coincidence - I'd built an ~1/12 scale model
using 1/4 Cu tube representing the 3" boom sections & #12 bare Cu for the
20m elements & 80/75m end sections & gamma.
Don't recall whether I was using YO and ElNec yet, but in any case estimated
gamma match dimensions for the 20m conventional gamma and the longer (~20'?)
wire gamma to feed the 100' long boom assembly on 75/80m. The 20m feedpoint
was ~22' away from the tower, so the only practical way to adjust it was
Used relatively small 1500V-series EF Johnson air variables as gamma caps,
each tuned by a small Dayton gearhead 1 RPM reversible DC motor. Just 3
wires to shack: one common and one "hot" wire from each motor. Maybe I was
lucky, but climbed down from the 80' guyed Tri-Ex tower (squeezed in among
~55-foot lodgepole pines at 10,700' elevation in the Colorado Rockies), went
into the shack.
Fired up the rig on 20, watched inline SWR meter and used 12VDC via a simple
DPDT polarity-reversing, center off toggle switch to tune 20m gamma for
perfect 1:1 indicated VSWR.. anywhere from 14.0 to 14.35 MHz! Same results
from 3.5 to 4.0 MHz on the "centerfed boom". Thereafter, W4ETO/0 (later
W0ID) ran numerous contests and expedition pileups running maximum legal
power on both bands and never had a problem ... u n t i l ...
...lightning scored a direct hit on a lodgepole about 150 from the tower.
Flash and bang arrived at the shack virtually simultaneously. THAT will get
your attention. Checking antennas after the storm passed, SWRs on both 20
and 40 were somewhere in the vicinity of 2:1 in-band, both improving to 1:1
above their upper band edges. Amazingly (to me, anyhow), both antennas were
easily dipped back to 1:1 VSWR by a few seconds of gamma cap rotation from
the shack. Everything was back to normal & worked perfectly until Halloween
night 1997, when a 100-year windstorm peaking 97 mph buckled that 100' long
boom horizontally, and messed up the entire 15-20-40-75/80m stack. The only
reasonable conclusion: lightning-induced DC in the control cables rotated
both gamma caps perhaps ~10 degrees without damaging the motors or caps.
Telephones, stereo, other electronics in the house fared worse. The only
failure I ever experienced with an ALPHA 87A running at that location from
1991 to 2007 (when we moved to the Blue Ridge country of south-central
Virginia) happened that day ... apparently the lightning stroke induced
enough firepower into our underground AC mains to take out the phones, a TV,
a stereo... and the main control board in the 87A. Too late, I installed a
husky surge suppressor in the 240V distribution box feeding the shack ....
Dick W0ID and W4ETO, Call of the Blue Ridge Alpha Club
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