[TowerTalk] Shunt-feed caps

[Jim Lux]

At 09:11 AM 9/18/2005, Barrie Smith wrote:
>I recently aquired a hipot tester and tested the vacuum-capacitors I had 
>planned to use for matching my shunt-fed tower.
>
>The high-voltage caps flunked.

what was the failure mode?  Testing some devices with a Hi-pot tester can 
give spurious answers.


>I have two 5 KV caps of suitable capacitance that test well beyond their 
>rated voltage.
>
>The several plans I have are all over the map as to what the voltage 
>rating of the matching caps should be.
>
>My thought is that if I tune-up (the caps are to be motorized, and 
>controlled from the shack), using low power, the 5 KV caps should then be 
>sufficient at higher power.

Use some sort of modeling program (XLZIZL Excel spreadsheet, for instance, 
or one of the free SPICE versions around) to simulate the system. (Or, do 
the calculations by hand, but it's a heck of a lot easier to let the 
computer do the complex math)


>Anyone have any practical knowledge on this subject?

For a non-resonant system (that's important!): Worst case overvoltage (into 
a infinite mismatch) is going to be twice the matched voltage.

For a resonant system (like if you use a LC network for matching), then it 
gets a bit tricky.  You need to look at the circulating power in the 
network (which is Q times the power flowing through), and then look at the 
impedance of your component.  You can get pretty darn high voltages this 
way: compact loop transmitting antennas are notorious for this, as are 
Tesla Coils.  A small tesla coil might have a C of 30 pF, and a Q of around 
10-15, and develop 100kV potentials with a few hundred watts input power.

The calculations depend a lot on the exact circuit topology, and the best 
way to do it is to get a program that lets you model it.  Something like 
Spice or Microsim will let you rearrange the components in seconds with a 
few mouse clicks and crank out the numbers in a few more seconds.

There's a downloadable old Eval version of Microsim 8 at
  http://hot-streamer.com/TeslaCoils/Programs/Programs.htm

Intusoft has demo versions of pSpice on its website.
There's even some ancient DOS SPICE programs around for downloading which 
would work.
Sure, it's limited in the number of components, etc., but just how complex 
is your matching network anyway.