Message-ID: <3B300E3F.4070BAEB@ns.sympatico.ca>
Date: Tue, 19 Jun 2001 22:45:19 -0400
From: "John Currie" <john.currie@ns.sympatico.ca>
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To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: 73kHz beacon/antenna problems
References: <E15CP5g-0000L5-00@gemini.herts.ac.uk>
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Hi Jim , I'll be watching   73 de John VE1ZJ

James Moritz wrote:

> Dear LF group,
>
> Thanks to those who have been looking for my 73kHz beacon
> signal, and to Ko, NL9222, for the screen shot. I have been unable
> to run the 73kHz beacon for the last couple of days, due to
> problems with the antenna, or rather the loading coil. The
> increased power level I have been using has made the coil run
> much hotter (it is probably dissipating 200 - 300W with the key
> down), and this together with the tension in the wire has caused
> the PVC tube former to shrink. This in turn caused the windings to
> come loose, and made it impossible to maintain the antenna in tune
> due to changing inductance and arcing between turns. I have fixed
> the problem with the aid of tape and a cooling fan, but this can only
> be a temporary solution - a more efficient 73kHz loading coil is now
> on the list of things to do.
>
> Assuming there are no further problems, I hope to run the beacon
> tonight, from 0000 - 0300 utc, on 71.924kHz again.
>
> I can confirm some of G3XDV's observations about trimming
> antennas - I went through a similar excercise about a year ago.
> Having a larger top load on an antenna is an advantage in that
> voltage is reduced, effective height is slightly increased, and
> smaller loading coils are required, which have less critical tuning.
> G3AQC has shown that under his circumstances, increasing the
> top loading can significantly reduce losses. However, if the top
> loading wires run close to trees and buildings, increased losses
> may outweigh the benefits. Also, the physical height of the antenna
> may be reduced by the sag in the longer wires. After lots of
> experimentation I ended up with only 75% of the original amount of
> wire I had in the air, but with about twice the original ERP for the
> same TX power. Obviously, there is a best compromise for each
> individual situation.
>
> Cheers, Jim Moritz
> 73 de M0BMU