[TowerTalk] Need Capacitors for Matching Network

[Jim Brown]

Interesting design problem. In addition to voltage breakdown, an 
important issue with capacitors for this application is ESR. The cheap 
stuff you found don't satisfy that. I did something like this on a 160 
Tee vertical, for which I needed series C in the range of 800 pF.

I'm lucky enough to live an hour from one of the few remaining 
electronics surplus warehouses that used to be so plentiful in our major 
cities. http://www.halted.com/  I was able to browse the aisles and find 
a wide range of suitably rated capacitors, and bought a bunch to stock 
my junkbox. Without access to doc on their specs, I performed the simple 
test of putting them inline with the antenna feed, transmitting for a 
while at full power, then giving them the "touch" test to observe 
whether or not they were heating up. Enough passed that test that I was 
able to use various series/parallel combinations to get what I needed. 
Unfortunately, I doubt that the contests of these bins are listed 
online. :)

Another suggestion. Have you considered designing a network using coax 
stubs?  Or suitable lengths of coax as a capacitor? SimSmith is a great 
tool for doing that.  Export the Z file from your antenna model to 
SimSmith and see what you come up with. I don't pretend to be an expert 
on voltage ratings of coax, but I do remember hearing that published 
voltage ratings are often for the outer jacket, not the dielectric. 
Perhaps voltage ratings could be inferred from power ratings. I'll bet 
that someone on the reflector knows more about this. :)

SimSmith will compute dissipation in circuit elements, including coax, 
if loss data for the components are known and entered, and it includes 
data for lots of commonly used coax. Loss in coax below UHF is all I 
squared R from conductor resistance (center plus braid), and thanks to 
skin effect, loss is pretty low below 10 MHz.

73, Jim K9YC

  On 8/14/2017 5:52 AM, Art Greenberg wrote:
> I want to build a matching network for a 53-foot vertical for 80/75 and
> 40. I should be able to cover 80/75 with three LC pairs and 40 with a
> single pair. So a set of fixed inductors and capacitors with a few
> relays would work.