[TowerTalk] Testing a vacuum variable Cap for an Inv-L

[Jim Thomson]

Date: Sun, 29 Aug 2010 16:24:14 -0700
From: "Jim Brown" <jim at audiosystemsgroup.com>
Subject: Re: [TowerTalk] Testing a vacuum variable Cap for an Inv-L

A question to ask is whether you really need a vacuum variable at 
all. Unless the antenna is close to a half wave and fed from the end, 
the voltage at the feedpoint is pretty low, even running very high 
power.

### IF you make the ant  aprx 3/8 Wave long... it ends up being 50 ohms..
but with a bunch of XL.     You need to make the ant long enough to it ends up at
50 ohms.  Now you have to cancel out the XL  with an equal amount of XC. 

## You cam easily end up with some pretty wicked peak voltages  across the
cap used.   




 Rather, you need a capacitor that can carry relatively high 
current, because the antenna is probably being fed at a low impedance 
point. If that's true, all you really need is a capacitor of stable 
value and with a low equivalent series resistance (so that its 
dissipation is low when carrying high current).

### 1500w into 50 ohms = 5.5A  CCS. 





 A length of good 
quality RG8 would do that job!

### Be real careful when using coax as a capacitor.  It's lossy
at best... and doesn't make for the best of capacitors... but will
work in a pinch. 




 I've done it that way, but now I use a 
parallel combination of fixed disc capacitors. I tweaked the value  
by adding and subtracting smaller caps in parallel until I got it 
where I wanted it. Several years ago, I stocked up on a selection of 
HV caps at our local electronic surplus house, HSC, also known as 
Halted. They have an internet presence.

##  disc ceramics are not noted for handling high RF currents,
esp on 160m... and ditto with HEC/Centralab/ITT/Jennings TX doorknobs. 
Any cap will have to be either NPO.. or N470.  You don't want it to drift about.
Or use a parallel combo that will easily handle the current involved..abt 3-4 x over. 




The other question is why it needs to be variable. I tune my vertical 
with a fixed capacitor whose value I tweaked to get a match. As 
initially installed, the SWR was under 2:1 over half the band. That's 
good enough for most uses, but I wanted the antenna to be a bit more 
broadband to make it easier to QSY during contests. My antenna is a 
Tee vertical, which is a close relative of yours. So I simply added a 
second conductor in parallel with the vertical section, using 8-inch 
pieces of 1/2-in PVC conduit as spacers. That made the SWR under 2:1 
over the entire band! 

73, Jim Brown K9YC

##  a buddy built a 4 x wire cage  for 160m.  With the 2500 pf motorized vac
cap the swr is dead flat across the entire 1.8 to 2.0 mhz range.   Motor driven vac
caps are ideal for this application.  They don't drift, they handle loads of peak V,
and they handle loads of current.   They are not affected by WX either..just mount it
inside a box.  You won't blow it up with YC-156 power levels either,esp in bad WX. 
 Another friend used an air variable... which has had problems with spider's,
bugs, high humidity, water ingress, and  now loads of corrosion on the sliding surfaces. 

##  If u can procure a motor driven vac cap..go for it.   IF it spins too fast, like some of the units
Allen Bond was selling a while back... the fix is to use a small PWM board... which will slow it right
down, and still maintain torque.   A simple  DPDT- CENTER OFF   [ spring loaded  toggle]  is used. 
Just hit the switch, while watching the  swr meter.. or use the swr bar graph on the Array solutions meter.
Most [ but not all] motor driven vac caps can be slowwed down by juts reducing the vdc on em. 

## another easy method.... If the Z is < 50 ohms..is to just  stick a helical hairpin right across the feedpoint.
Just compress/expand the coil  for flat swr..just like is done on a F-12   80/40m  yagi.  Now this scheme works
very well.. and also DC grnds the  center conductor.   It's also very broad..... but if you want to operate the 
entire band with a flat swr.... the motor driven vac cap is the ultimate solution.  The un loaded Q of a typ vac cap
is well  > 5000.   They are typ 50 ppm temp coef..and are better than NPO caps.   The ceramic ones will
handle more current than the glass ones... but that's a moot point. 

later...... Jim   VE7RF