On the original discussion on this subject Giulio IK2DED said
But which may be the best system on this tower for LF, is possible to feed
directly it, may be at the top?
The tower is exatly 80 meters long, with a base of 8x8 meters, and at 70
meters is 1x1 meters, and at 80 meters >is 0.5x0.5 meters, so big antenna
with big capacitance to ground, i think is a smaller version of Decca
insulated >tower, but this one is grounded.
There are practical difficulties in shunt feeding a grounded tower, even one
as high as 80m because it is still only a fraction of a wavelength at
136kHz. I did once use an Omega match to feed a grounded mobile 14MHz whip.
I also used a shunt feed on a mobile 14MHz DDRR antenna which was grounded
at the feed end. Both these antennas are described in 'The Antenna
Experimenter's Guide'.
But as Mike, G3XDV, says, for these matching systems to work the antenna
needs to be resonant.
I think that a tower 80m high is best used for a support for a conventional
long wire antenna with a base loading coil. Make the sloping wire as long as
possible.
Additional top capacity can be achieved using two wires. A boom can be used
to support the end wire insulators . Make the boom as long as practibe to
reduce the end capacity coupling to the grounded mast.
Regards,
Peter, G3LDO
e-mail <[email protected]>
Web <http://web.ukonline.co.uk/g3ldo>
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