You wrote onTuesday, March 21, 2006 1:19 AM
Cebik had me thinking about a three-armed center-fed dipole array,
but that seems really ambitious for my first attempt. I'm having
enough trouble getting my act together to put up anything there.
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For a multiband antenna there is not much wrong with a doublet fed with open
wire line if the right type of 'balanced' tuner is used at the shack end. I
would never use a balun between a doublet's centre feedpoint and a coax
feeder. Reason- to avoid loss and possible fireworks. The leg lengths are
not critical, but should be equal, and I would suggest that the overall
length should not be less than 65% of a halfwave at the lowest frequency
that you want to use, although you will still get out with a shorter
antenna. The overall length used will of course determine the radiation
pattern for each band, and gain, but in my experience the nulls shown by the
textbooks are usually filled in to some extent unless the antenna is really
high and there are no trees, houses etc within many wavelengths. Although
the legs are made equal in length for reasons of 'balance', in real life
objects like trees, houses will 'unbalance' the antenna and unequal currents
will flow in the feeder wires - unless you are lucky! With the old-fashioned
and probably the best type of ATU, you can fiddle with coil taps to correct
this problem to a large extent.
Verticals can be great, or they can be miserable beasts, I have some here.
They are usually very fussy about their environment, ground included, which
can be explained. My personal choice with verticals is to use a dipole
bottom fed rather than a quarter-wave plus radials, but here a 40m vertical
element has to be about 40ft away from a large Chestnut tree before it works
properly. Fortunately the ground here is 'good'.
If I can help anyway, please call off-list.
73,
Geoff
GM4ESD
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