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Re: Topband: alternative to vacuum variables

To: "Pete Smith N4ZR" <n4zr@contesting.com>, <topband@contesting.com>
Subject: Re: Topband: alternative to vacuum variables
From: "Tom W8JI" <w8ji@w8ji.com>
Reply-to: Tom W8JI <w8ji@w8ji.com>
Date: Thu, 24 Jan 2013 21:59:23 -0500
List-post: <topband@contesting.com">mailto:topband@contesting.com>
One of the reasons I have mot gone to high power on 160 is the cost of capacitors for my omega match - at least $300. Anyone know of any workable alternative? I remember someone writing about using coiled-up RG-8, RG-213 or maybe Teflon coax. Where can I find more information?

Coax isn't the best for two reasons:

1.) Transmission line effects increase voltage at the open end. This aggravates arcing issues
2.) They are lossy

Here's something to think about, Pete. The voltage across a capacitor is I^2 * Xc

So if you have a shunt feed and use a long, thin, wire....you not only decrease bandwidth and efficiency, you also increase the voltage. A thin shunt feed wire requires higher Xc (lower capacitance), and that makes voltage increase and BW narrow.

Use a wire cage and tap the shunt wire down at a 50 ohm point, and voltage goes way down. Do it the lazy way and use an omega match, and you give up bandwidth and need higher voltage parts.

73 Tom




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