From PA0SE to All
Bob, ZL2CA. wrote:
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".
Henk, PA0HVN, who used to be responsible for the NDB's in the Netherlands,
once told me the antenna circuit of these beacons includes a resistor to
decrease the Q. Without the resistor detuning of the antenna due to snow or
glazed frost would cause unacceptable signal strength variations.
It is obvious that in this case there is no point in putting a loading coil
high up the antenna.
Henk also said that these beacons (in the Netherlands) radiate about 100
milliwatts. As I think the transmitter output is of the order of 50...100 W
it shows that in spite of the loss in the resistor these beacons do better
than most of us, amateurs. No doubt due to the rather long vertical antenna
and a very good earthing system.
73, Dick, PA0SE
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