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LF: 73kHz beacon/antenna problems

To: [email protected]
Subject: LF: 73kHz beacon/antenna problems
From: "James Moritz" <[email protected]>
Date: Tue, 19 Jun 2001 18:16:13 +0000
Organization: University of Hertfordshire
Reply-to: [email protected]
Sender: <[email protected]>
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



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