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

[Jim Brown]

I said:

>>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. 

That's an old wives' tale. There is VERY little dielectric loss in decent coax at 2 
MHz. Heck -- I used 10 ft of Commscope 3227 to resonate a transformer at the high Z 
feed point of an 80M half wave vertical. No problem at all, and that was a VERY HIGH 
VOLTAGE POINT.  

>##  disc ceramics are not noted for handling high RF currents, esp on 160m... 

Another old wives tale. Some do, some don't. You have to study the specs. OR, you can 
try them in an antenna. That's what I did -- I bought a bunch of HV caps (3kV, 6kV) at 
the local electronics surplus store, stuck them in series with the antenna, transmitted 
at 1.5kW for a while, then turned off the TX and went out to feel the caps. Some ran 
hot, some ran cool as a cucumber. 

>Any cap will have to be either NPO.. or N470.  You don't want it to drift about. 

Another old wives tale. Temperature coefficient only matters if it gets hot. :) The 
good caps don't get hot, so TC doesn't matter. 

>Or use a parallel combo that will easily handle the current involved..abt 3-4 x over. 

Yes, caps in parallel are a good, inexpensive solution. 

> 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 sexy, high tech, and VERY expensive. I have lots of other things to do with my 
money. Low cost surplus disc caps work just fine. Some of us are obsessed with VSWR. I 
am not. As long as your VSWR is less than 2:1, the additional loss due to SWR is less 
than 0.1dB. It is EASY to get a simple vertical to be flat over 160 M -- simply double 
up on the wire, with spacers between the two wires. Cost is under $10 for the wire and 
spacers. 

Lots of hams waste lots of money on the basis of old wives tales. There ARE good uses 
for vacuum capacitors, but the feedpoint of a low impedance antenna near resonance is 
not one of them. 

73, Jim K9YC