[Lf] more on Top Loading]

[Andre' Kesteloot]

James Moritz wrote:

> Dear Stewart & LF Group,
>
> > Experience shows that increased top-loading significantly improves the
> > performance of LF antennas.
>
> This is certainly not universally true; I changed my inverted L
> antenna from having two top wires with total length 55m to a single
> top wire 40m long. This reduced the loss resistance by roughly
> 40% to about 35ohms, even though the capacitance is reduced
> from 360pF to around 300pF. Overall, I estimate changing this
> antenna has given a roughly 3dB improvement in efficiency.
>
> I think most of the difference is due to the more favorable
> positioning of the remaining wire - it is further from trees and
> buildings, and the reduced weight means it sags less. I have also
> moved the down lead further away from the house.
>
> I expect there is an optimum trade-off in the amount of top loading
> to use for each location. In general, one would expect better
> performance with increased top loading - but in reality this will only
> work if the additional top load can be sited so it does not introduce
> increased loss.
>
> As far as the vertically stacked top loading idea goes, I think this
> will increase the capacitance of the antenna top section in much
> the same way as having the wires side by side. However, the
> radiation resistance of a top loaded vertical only increases slightly
> with increased top capacitance (provided there was a reasonable
> amount of loading in the first place), while it is proportional to the
> square of the height of the antenna. Since adding parallel loading
> wires will only give a moderately higher capacitance anyway, it
> seems likely that the increase in  radiation resistance due to
> having additional loading wires below the highest part of the
> antenna would be less than the decrease due to the reduced
> average height. So I would keep the highest wire and discard the
> rest.
>
> I think NEC works OK for calculating the radiation resistance of an
> LF antenna, using a perfect ground and zero loss conductors, but
> fails when trying to take losses in the real ground into account. In
> particular, there seems to be no convincing way of making
> connections to a real ground, or of modelling the resistance of
> such a connection.
>
> Cheers, Jim Moritz
> 73 de M0BMU