[Fwd: LF: Antennas]
Sun, 27 Jun 1999 10:55:28 -0400
> 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