[Lf] LF antenna]

[Andre' Kesteloot]

Bob Vernall ZL2CA wrote:

> Most of the modelling software available to amateurs does not model real
> ground, and neither is there a way of accounting for trees and shrubs in
> the antenna environment, but there are suggestions that "greenery"
> losses occur (I think it was Finbar who obtained higher current after
> avoiding clutter).
>
> Commercial beacon stations do not employ loading coils at the top of the
> "up wire", but they likely have very good ground systems along with a
> clear site, so dielectric loss in the soil or in trees is avoided by
> engineering their site conditions.  Amateur operation from home stations
> is where a difference occurs, with clutter and greenery losses waitng to
> soak up LF power :(   Thus there could be advantages in having much of
> the loading coil further up the antenna system, so that losses near the
> up wire are reduced (lower voltage means lower field strength gradients
> around the up wire, thus lower losses in greenery, etc).
>
> The practical disadvantages of an elevated loading coil are mechanical
> support needed, and it surviving high voltages on transmit, during wet
> or humid weather.  I am not a supporter of using ferrite cores in
> loading coils, as it could become nonlinear under transmit current.
> High Q air cored coils for LF loading are rather large, and heavy, hence
> the practicality of using them at the bottom of the up wire ...
>
> I also agree that practical evidence is what counts for assessing the
> possible improvement with having loading inductance at the top of the up
> wire.  The evidence so far is that it can improve the efficiency of an
> amateur LF antenna system.
>
> While I have not implemented this myself, I had considered how to obtain
> MULTIBAND use of the LF antenna system, for the likes of 160 and 80
> metres.  My thinking is to instead of having only a loading inductor at
> the top of the up wire, additionally use a parallel capacitor so the LC
> "trap" is resonant at say 500 kHz, making it usefully inductive at LF,
> but at higher ham bands the LC network has a low enough capacitive
> reactance to make use of the top loading wires to carry useful currents
> on 160 and 80 metres.  I have not done the calculations as to what L/C
> ratio is suitable, nor what voltage rating the capacitor would need to
> be for not breaking down on LF transmit.  The idea is floated for
> consideration.
>
> Bob ZL2CA