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Message-ID: <D28F995908304730B09CC347757C8BFB@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <4BAF2B2D.1010300@dsl.pipex.com>
In-Reply-To: <4BAF2B2D.1010300@dsl.pipex.com>
Date: Sun, 28 Mar 2010 12:14:18 +0100
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Subject: LF: Re: Antenna Matching
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Dear Tim, LF Group,

Since your T is much less than a quarter wave long, you will need series 
inductance (a few hundred uH) to resonate it at 500k. The antenna will have 
a capacitive reactance of the order of 1kohm, and series loss resistance 
probably between several ohms and a few 10s of ohms. With 100W, this will 
result in no more than about 5A antenna current, so about 5kV maximum on the 
antenna, not very high voltage as these things go.

The two most common methods of matching a 500k antenna are to use a series 
loading coil to resonate the antenna, and a multi-tapped ferrite cored 
transformer to transform the resulting resistance to 50R, or a tapped 
loading coil with the "cold" end grounded, and the tapping point a few turns 
from the grounded end connected to the transmitter adjusted to match to the 
TX. In both cases, the loading coil usually has several taps to make coarse 
inductance adjustments, with usually a variometer for fine tuning to 
resonance. It is possible instead to use a high voltage tuning capacitor for 
fine adjustment of resonance, but this will generally increase losses in the 
coil.

Good luck on 500k,

Cheers, Jim Moritz
73 de M0BMU