> Radiation will occur regardless of where the 1/4 WL
> sloper is attached. IF the attachment point results
> in the upper portion of the tower plus mast plus
> TopLoad representing a (resonant) 1/4 WL, THEN
> you will have ZERO reactance at the feedpoint,
> resulting in a reasonable match to 50 Ohm Coax
> (i.e. Low SWR).
Even if the tower and yagi combination, at the feedpoint of the
slopper, is not resonant the antenna can have a good SWR.
All you do is change the length of the slopper, until the system is
The problem is mainly the resistive part of the impedances. What
messes the system up the most is when the tower system is
resonant, and the slopper connects some distance away from a
current maxima in the tower.
For example, a 30 foot tower with enough top load to be resonant
on 80 meters would be useless (so far as direct 50-ohm feed is
concerned) with a slopper attached almost anywhere more than a
dozen feet away from earth end of the tower.
If you model or analyze what everyone does who "lucks into" a
working slopper, you will see that somehow they either have a
tower and yagi system NOT resonant on the operating band, added
a counterpoise, or attached at a current maxima if
the tower system is resonant or near-resonant.
In example, my 318 foot tower would make a dandy counterpoise
for a 160 slopper if I attached a ~139 foot wire at 190 feet. Virtually
no current would flow in the tower below the attachment point,
impedance would be about sixty-ohms or so and J 0.
According to the model the only problem is it has less field
strength than a 1/4 wl vertical over a good radial system, as most
of these systems do. The exception is a system with working
reflectors and everything optimized, than it does better than a 1/4
wl vertical with a good ground system and not nearly as good as a
four-dipole array or a 4-square.
It's pretty easy to model this stuff and watch how it works. It is
systems that are not planned (work only by accident) or fully
understood that drag in long discussions.
73, Tom W8JI
This list is sponsored by the new eHam.net Store. When you buy products
like ICE filters, Array Solutions StackMatch, or M-Squared Antennas from
the eHam Store for the same price you pay direct, a portion of the sale
price goes to support this list. Check it out at http://www.eham.net.
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Administrative requests: towertalk-REQUEST@contesting.com