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
Cheers, Jim Moritz
73 de M0BMU