[Fwd: LF: Advice on Antenna.]

[Andre' Kesteloot]

vernall wrote:

> Rik Strobbe wrote:
> >
> > Hi Des,
> >
> > In his mail Geri, DK8KW, has given a very good description of what to do.
> > I would like just to add 2 options :
> >
> > 1. If it is mechanically possible you might gain some dB's by placing the
> > loading coil (or at least a part of it) at the top of the vertical section.
> > Especially with a rather small top-hat as you described this will increase
> > the effeciency of the antenna. I believe that G3XDV has some experience
> > with it.
>
> True in theory but often difficult mechanically.  The lower the loss in
> a given loading coil, the bigger and heavier it generally becomes.  Some
> of the dBs can be "won back" by having a very well designed loading coil
> in a conventional ground mounted enclosure.  Observations of
> aeronautical non-directional beacon (NDB) T antennas generally shows
> that none use loading coils "up in the air".
>
> > 2. Having several parallel wires in the top-hat will increase the antenna
> > capacitance and is in some cases mechanicaly simple to implement. I use an
> > inverted-L antenna with a 13m high vertical section and a 26m long
> > horizontal section that consists of 4 parallel wires each 0.8m separated.
>
> Spacing at 0.8 metres is a good choice.  Proximity effect progressively
> reduces effectiveness of parallel spaced wires as they are brought
> closer together.  When wires are very close, they are similar in
> capacitance to ground as one fatter wire!  For top loading, the "first
> wire in" is always the most effective, but suitably spaced parallel top
> loading wires would probably improve every T or inverted-L antenna at
> LF.
>
> I have intended to try a further variant, using a "second droopy wire"
> below each existing top loading wire, and terminating on the same
> insulators.  This seems to be a "cheap and easy" method.  If a second
> wire is a few centimetres longer, it will naturally droop below the
> first one.  This would reduce the effective height above ground, so
> there could be a trade-off between moderately higher capacitance but at
> moderately lower effective height.  As the first wire takes most of the
> tension, the second wire could be a thinner one.  It could also tangle
> with the first wire in windy conditions!  But as I said, I have not yet
> trialed this idea, so I can not say it is proved to be worth doing.
>
> Regards,
>
> Bob ZL2CA