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.
### This is NOT a gamma match set up. The yagi in question is a F-12 340N
which is a 3-el 40m 85% of full size yagi, using 3/16" solid AL rod for
The LL section is aprx 4' long, then across, then doubles back on it self.
The feed system is simply a split DE [ fiberglass center insulator] and a
hairpin. The hairpin is just a few turns of wire to make up the uh. The DE
has XC at the flat swr freq. The XC of the De ..plus the XL of the helical
hairpin, make up the LC network.. and step the Z UP to 50 ohms.
All as I'm doing here is inserting the vac cap in series with the hot leg of
coax.... and electrically shortening /lengthening the resonant freq of the DE.
F-12 does the same thing.. but uses coils in both legs.. and DPDT relays to
out the coils. IE: F-12 will resonate their DE way up high in the band...
then add various combinations of coils, to lower the resonant freq of the DE...
and do it in small increments.
## my scheme amounts to the same concept... except in my case, the DE is cut
the low end of the band, [very bottom].. and the variable XC from the cap,
electrically shortens the DE. It amounts to the same thing. IMO, one
driven cap is simpler than a mess of coils and latching vac relays.
## whether say 75 ohms of XC is used.... [cap]... or 75 ohms of XL is used
the peak v across the caps or coils is the same...but only in the case where
loading is done in one leg only. With coils in both legs, peak V is split
them. Worse case for myself would be running 750W of cxr on AM.. and 125%
pos mod. 3750 w pep into a 30 ohm load. In the case of the variable cap,
with low values of C... XC is high.. and peak V across the cap can get pretty
[top end of the band] V = I x Xc At the low end of the band, XC is
low.. and peak V
is also low. Voltage end to end across the coil is not a big deal, it's the
that have to stand off the peak V.
## Both the PWM board and 27 vdc @ 12 A CCS switching supply arrived today,
I will try some experiments to slow down the cap, and maintain torque.
Used relatively small 1500V-series EF Johnson air variables as gamma caps,
each tuned by a small Dayton gearhead 1 RPM reversible DC motor.
### I have had nothing but grief, when using small air variables for gamma
caps in the past. Too much corrosion on the rotor portion . [ 1980's]
Back in the 80's.. I saw a scheme whereby the gamma cap was tossed, and the
required XC was obtained by shortening the DE. I won't even use air
in any of my HB linears. The latest fiasco was the air variable load cap on a
HB GS35B, 6m amp overheated. Amp was rebuilt, using vac caps for the tune
and load. 2 x tubes
used the 2nd time around. 100% reliable, no heating... and globs of power out.
reasonable conclusion: lightning-induced DC in the control cables rotated
both gamma caps perhaps ~10 degrees without damaging the motors or caps.
## I would have installed MOV's on all the control cables [ each conductor to
at the top of the tower.... and again inside each gamma box, and again in
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 ....
### apparently, in Florida, the insurance co's will make you install huge
at the 240 vac 200 A main panel in each house. At all the Telco's and cell
I worked at, we used Josyln lightning suppressors, which consist of huge 3"
square 140v MOV's....[fused].... from each hot leg to neutral. 2 used for
single phase.. and 3 used for 3 phase. You can even get em with 'event
Dick W0ID and W4ETO, Call of the Blue Ridge Alpha Club
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