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Re: [TowerTalk] Need Capacitors for Matching Network

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Need Capacitors for Matching Network
From: Art Greenberg <art@artg.tv>
Date: Mon, 14 Aug 2017 14:19:23 -0400
List-post: <mailto:towertalk@contesting.com>
Glad you rang in, Jim.

I haven't looked at anything other than lumped LC at this point. I
should look at the alternatives. I'd probably learn something. 

In the meantime, thanks to other responses I located and purchased a
number of affordable NOS doorknob capacitors from eastern Europe which
ought to work just fine.

-- 
Art Greenberg
WA2LLN
art@artg.tv


On Mon, Aug 14, 2017, at 13:46, Jim Brown wrote:
> 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.
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