Mike Dennison wrote:
I have done very many antenna experiments over the past 2-3
years to get a better signal at 73 and 136kHz from a small garden.
snip snip
I have found by using a telescopic mast at the outer part of my LF top
loading that there is a significant benefit in being "all wires high"
and certainly avoid a low wire such as in an "inverted Vee" or "downhill
L". Readings were made by a local amateur using a calibrated selective
level meter, and done several times, with consistent results. A lower
wire likely causes an electric "hot spot" (extra ground loss) or reduces
effective height (efficiency is height squared), or a combination of
both. As always on LF, sky hooks are very welcome :)
One a similar topic, what influences the capacitance of a multi-wire
top section? Is it simply wire diameter so a thick wire has more
capacitance than a thin wire? What about multiple parallel wires -
these are often suggested to be a metre or so apart - why? If the
distance makes a difference, do I assume that the further apart
they are the better the effect, and if so why is this?
The formula for capacitance of a wire over ground (which I do not have
on hand at present) involves a logarithmic relationship with wire
diameter. The practical implication is that the capacitance of one wire
does not vary much with wire size, but of course capacitance still
varies directly with wire length. Using more than one wire needs to
have some spacing between wires, else they are similar to one slightly
fatter wire, so the increase in capacitance is small. In the other
extreme, if two wires are far apart, that can double the net capacitance
to ground. However, most practical amateur antennas have a limit in
support points, so running wires in parallel is worth trying. Proximity
effect takes its toll as wires are placed closer together, so spacers of
the order of half to one metre likely give a good compromise.
Some digital multimeters (DMM) on the capacitance range give a meaninful
reading of antenna capacitance at very low frequencies (typically in the
audio range, depends on the type of DMM). This can be used to assess
"before" and "after" capacitance when playing around with changes to top
loading. Generally the higher capacitance obtained the better (so long
as it does not include drooping outer ends!).
I used to run three top wires about 300mm apart but replaced these
with a single wire of the same diameter of each of the three (for
other reasons) and it seemed to make little difference. Was I doing
something wrong?
The three wires should have been better, but probably by only a dB or
so. Unless careful before and after tests were done then it could be
difficult to pick any improvement. But on LF transmit, every dB helps !
Also, if I run three top wires in parallel, should I join them at the far
end? Or perhaps I could join them so they make a single zig-zag
wire down the garden, back again and down again.
Probably little difference, but it is practical to join them at the
central node (top of the "up wire") and doing that avoids any voltage
differences at top loading spacers.
Bob ZL2CA
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