Dear Richard, LF Group,
I would suggest using an "umbrella" top-loaded vertical using a telescopic
fibreglass mast. From experience, antennas with horizontal sections like
inverted Ls or Ts have the disadvantage of requiring at least two masts
(unless there is a suitable support already on your hilltop), and are a real
handful when the wind starts blowing due to the masts flexing, requiring
several guys to keep the antenna in shape. The umbrella is somewhat less
efficient for the same overall height, but only needs a single mast, and the
sloping top loading wires double as guys to keep the mast straight. The
height is the most important dimension. A 10m mast would make a reasonable
500kHz antenna.
For a reasonable estimate of antenna capacitance, assume 6pF per meter of
wire. For antennas with a number of wires running close together, such as an
umbrella, the actual capacitance will be a bit lower due to proximity
effects, but this is a good first estimate. You can use NEC antenna
simulators to calculate the capacitance quite accurately, although the
resistance will probably be way off. Small antennas will have very sharp
resonance, so you will want to add a variometer to your loading coil, or
some other means of fine tuning. G4FGQ's DOS software runs fine in a DOS box
under Windows 98 and Vista here.
For insulated ground radials to work well, their capacitance to ground needs
to be much larger than that of the antenna, so you probably want a few
hundred metres of wire in total. If the ground is soft, a few 1m ground rods
connected together works well as a ground.
There is a lot of handy info on LF antennas at
http://www.wireless.org.uk/on7yd/index.htm
Hope to work you /P soon...
Cheers, Jim Moritz
73 de M0BMU
Dear all
I am hoping to be granted an NoV for portable operating on 500kHz. I
have parts on order for the GW3UEP Economy transmitter which will be run
on 12 Volts. Suggestions for portable aerials that can be set-up by one
person on remote and windy hilltops will be most welcome.
73
Richard
G3CWI
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