When I read the recent questions in "Tower Talk" about Beverage antennas I
couldn´t help thinking of a very interesting antenna - "The Super Sloper" -
that was described in QST Dec 1995 by Roger Sparks, W7WKB.
A feature of that antennais that the resulting current distribution on this
antenna, seen from the far field, resembles the current distribution on a
(slightly mismatched) Beverage antenna. The currents in the two wires of the
"Super Sloper" antenna are approximately 90 degrees out of phase and about
90 degrees displaced physically (for example), which means that vector sum
of the currents will resemble a spiral around the antenna structure (if you
use a current display like the one used in EZNEC or AO). The current around
a Beverage wire has a similar look.
I believe this reasoning is OK because the spacing between the Supersloper
wires is a very small fraction of a wavelength. An "observer" in the far
field could not know whether the field he "sees" is created by the currents
in two wires or by the sum of those two currents in a single wire!
Anyway - the "Super Sloper" radiates better in one direction along (or at a
small angle to) the wire direction than in the other direction, even if the
antenna is horisontal. Tilting it will improve the F/B ratio, but I believe
that even a 1.5 wavelength horisontal supersloper type of antenna can be
constructed to give a respctable F/B ratio. An advantage of a Super Sloper
antenna relative a Beverage antenna will be that it can be used for
transmitting and has more gain than a Beverage.
For those folks having EZNEC the W7WKB simulation files are located at
<http://www.arrl.org/files/qst-binaries/supslope.exe>
W7WKB-s plane supersloper model has the filename F46A-FLT.EZ and an
interesting exercise could be to rescale that model to 3.5 MHz, put it at a
moderate height and and finally compare it with a Beverage of the same
length.
I am a bit surprised that Roger Sparks´ideas have not led to more
experimental work - they seem to have development potential, and the antenna
is very simple. One idea would be to use active phasing of two adjacent and
longitudinally displaced longwires in order to achieve a good F/B along the
wire direction! What would the optimization criteria be?
73 Jan/SM0AQW
________________________________________________________________________
Where do you get ICE bandpass filters & beverage matching boxes? The
same place that pays for the hosting of this list: The eHam Store.
Order online at http://store.eham.net.
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