From: "James Moritz" <j.r.moritz@herts.ac.uk>
Organization: University of Hertfordshire
To: rsgb_lf_group@blacksheep.org
Date: Tue, 19 Jun 2001 18:16:13 +0000
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Subject: LF: 73kHz beacon/antenna problems
Message-ID: <E15CP5g-0000L5-00@gemini.herts.ac.uk>
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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