>My apologies for revisiting this thread, which was discussed briefly last
>month.
>
>I am tempted to try a sloper as the first Topband antenna from my new QTH
>here in ZL, as it will be quicker to get going than a shunt feed to my mast,
>but there were a couple of negative comments last month.
// As well there should have been. The intrinsic problem with a 1/4
wave sloper antenna is that two currents of opposite polarity are
traveling pretty much in the same direction -- i.e., toward the ground.
Thus, some H-field cancellation is unavoidable,
>These were mainly
>to the effect that they (slopers) act more like low dipoles.
// Indeed. The E-field produced is much like that of a low inverted V
dipole - i.e., close to the ground. .
>Can anyone elaborate on this? My first intuition is that a sloper should
behave
>in very similar fashion to an inverted L with the feed at the top of the
>vertical section.
>
>My antenna simulation software predicts very little difference in
>performance between a sloper and a shunt fed vertical. Has anyone compared
>them on the air?
>
// I listened to a tape recording made by SM4CAN of Western United
States 80m signals received via longpath into N. Europe. None of the
clearly audible signals came from 1/4 wave slopers. Two of the most
audible signals came from directional verticals.
// My free, unsolicited, advice is to try a Hertz, either full-size or
shortened, supported at mid-point by your tower and a halyard. Typical
feed-Z is 1000 to 5000 ohms, so an L-network is needed at the surface fed
end of the antenna. To maximize the E-field, the far end should be as
high off the ground as possible. To prevent H-field cancellation, the
enclosed angle of the wire at the top of the tower needs to be >90º. On
average, my 160m Hertz beat my 70-foot vertical by about one S-unit.
cheers, Greg, ZL3IX
- R. L. Measures, 805.386.3734, AG6K, www.vcnet.com/measures.
end
|