Dear Martin, LF Group,
For relatively low powers like 100W, a loading coil wound on about
100mm dia PVC tube using about 0.8 or 1mm diameter enamelled
wire is fairly satisfactory. It is not the most efficient loading coil
possible, but the difference between it and a more elaborate coil
with litz wire, etc is probably not going to be more than 1dB or so
with this type of antenna, and heating will not be a problem with
100W. The brown PVC main drain pipe sold by DIY shops is OK; I
have yet to find any type of plastic pipe with significant loss at LF.
As a very rough guess, you will need about 4 - 500 turns for the
G5RV. Put in taps every 10 - 20 turns for one half of the coil, for
coarse tuning. A variometer is ideal for fine tuning, but if you want
to avoid the work involved, a simple fine tuning method that works
quite well is to use a large ferrite core (one from an old SMPSU
with the windings stripped off is OK) which can be slid in and out of
one end of the loading coil. You can find the resonance point
roughly by peaking up a received signal on the band ( or DCF39 on
138.83kHz).
You may well get a reasonable impedance match simply by
connecting the loading coil in series with the antenna and TX
output, and tuning for resonance. The resistance of the resonant
antenna will probably be in the range about 20 to 100 or so ohms,
depending on all sorts of factors. To improve the matching, either a
seperate transformer wound on a ferrite core can be used
(probably better on the whole), or the loading coil can be used as
an auto-transformer by grounding the "cold" end and having several
taps every few turns from the ground end. The tap can be selected
for best matching, although there will be some interaction between
tuning and loading adjustments.
The best way to check performance is with an RF ammeter in the
antenna down lead - either thermocouple or one of the current
transformer/rectifier designs around. I guess about 1-2 A will be
quite good going. However, although max antenna current means
max radiated signal, it does not neccesarily mean good matching
for the TX, so one of the LF SWR bridge designs will help a lot for
setting up.
If possible, have the loading coil directly at the bottom of the
antenna feeder. Lifting the coil a few feet off the ground will reduce
losses. Some kind of shelter to keep the water off is advisable.
Hope this info is helpful - hope to see you soon on 136k!
Cheers, Jim Moritz
73 de M0BMU
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