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[TowerTalk] 40 M DE adjustment.. via vac cap.

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Subject: [TowerTalk] 40 M DE adjustment.. via vac cap.
From: "Jim Thomson" <>
Date: Thu, 31 Dec 2009 05:54:19 -0800
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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.
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 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 
variable motor
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 
relay contacts
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 
variable caps
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. 

 The only 
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 

Jim   VE7RF

~{ ;-((

Dick    W0ID and W4ETO, Call of the Blue Ridge Alpha Club
            Forest, VA 


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