[Lf] [Fwd: LF: Drooping top loading]

[Andre Kesteloot]

Vernall wrote:

> Bill,
>
> > My experience with many shapes of top hats is that it DOES reduce the
> > radiation measured in the far field. The theoretical explanation I have
> seen
> > is that the effective height is reduced due to the current foldback.
>
> I have an LF antenna that uses a telescopic mast for one part of multiple
> top loading (three masts in all).  The mast is parked in the down position,
> and cranked up for LF transmitting.  The far field is definitely lower (from
> S meter reports of several others) when the wire is below the "full" height.
>
> The situation may well depend on how good the ground conductivity is, as a
> "drooping" part of top loading means higher field strength to ground over
> the droopy part, and higher field strength means higher ground current and
> consequent I squared R losses.  In other words, RF warm spots can arise
> under drooping wires.  If the antenna is built over an immaculate ground
> system with hundreds of ground radials (as per a text book design), then the
> likes of an umbrella antenna would not be as lossy as one built over average
> soil with perhaps a few radials.
>
> 73, Bob ZL2CA