A bit from my own on-the-air experience with slopers:
When I returned to the air in 1996 after a long hiatus (I QRT'd in 1984
after achieving 160m DXCC), I needed an antenna for 80m to go after
5BDXCC. I installed an 80m sloper on one of the towers and used it for
about a month. Results were only mediocre at best. Also, it screwed up
that 160m shunt fed tower, so to QSY to 160m I had to bring the 80m
sloper straight down and short its far end to the tower base so that it
did not affect the 160m shunt feed system.
After about a month of this hassle, I decided to try shunt feeding the
same tower on 80m. It worked quite well, so I 80m shunt fed the other
tower and use them as a 2-el phased array (spacing 1/2 wavelength on 80m,
so two I have bi-directional patterns with gain over a single vertical).
This array is red hot on 80m and seldom do I have to call the DX more
than once or twice in a pileup. So, in my case, the shunt-fed tower is
head and shoulders above the sloper on 80m. 80m DXCC was achieved that
same winter season to complete my 5BDXCC.
With the top-loading Yagis on my towers (TH6DXX on one, 402BA on the
other), they are both self-resonant at about 2100 kHz. Therefore they
are approximately 3/8 wave on 80m and high angle radiation is not a
problem. The only top loading in addition to his TH6DXX on SP5EWY's
tower is a17/12 meter dualband beam and his tower is still slightly
shorter than 1/2 wave electrically, so I doubt that high-angle radiation
on 80m is his problem. I suggest that he re-try using a single-capacitor
method (rather than his Omega match). Being close to 1/2 wave, the shunt
wire tap point will be quite high above ground for 80m -- perhaps near
the top of the tower.
I also have shunt feeds on both of my towers for 160m which works quite
well as a phased array on that band. Addition of the 80m shunt feeds
does not appear to have degraded the 160m signal at all.
73, de Earl, K6SE
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