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
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