Hi all, whoops sorry I am becoming a bore...that's two messages to the
reflector at once!!
Well never mind....this query by Jan-Martin is interesting.....my answer is
that you must NOT believe all the figures that the computer programs tell
you!! Go and measure it !
Even the much vaunted NEC does not model short amateur type LF aerials
properly. It can only calculate the parameters that are built in to it. Its
dealing with things like elevated inductive loading and ground loss are
adequate for HF, or for commercial type LF station configurations but are
way out on an amateur scale. I do not know of anyone, even Finbar who lives
on the sea shore, who would get a 18 ohm real part impedance for a 30m high
L with 50m top. The measured figure is more likey to be of the order of 80
to 100 ohms even with extensive "radials" and ground spikes. This is not
just a result of my measurement but the reported results of others with real
aerials. Reg's equations are an interesting guide but should not be regarded
as completely accurate at LF. I found his Topload program to be greatly
different to measured examples.
Extra components for matching can give extra loss, and its difficult to
calculate or even measure this. The single coil works in most cases, it does
not however give a dc path to ground, which can be a problem on reception,
and where high induced voltages are present (nearby thunder clouds). From
the simplicity point of view it wins for most stations. Variable capacitors
to withstand the kind of voltages generated at 400w to 1kW are not cheap. It
looks like a practical solution to a problem but I am interested to hear
other opinions.
The dc path and the matching has been solved to some extent by some stations
grounding the coil and tapping the feed up from the earthy end. This can
still avoid expensive capacitors, but require a lot of experimenting and a
more complex coil build.
Cheers de Alan G3NYK
[email protected]
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