Half-Sloper Antennas (1/4 WL wire sloping from tower)
de N4KG
At one time, I used 3 one-quarter wavelength (WL) slopers
on 80M, fed at the 60 ft level on an 80 ft (24M) tall tower, top
loaded by a TH6 tribander (24 ft boom). A good match was
achieved and the pattern favored the direction of the sloping
wires. All feedlines and rotor cables came down inside the
tower all the way to the ground, taped to the legs.
My "intuitive" analysis of this system is that the Yagi and upper
portion of the tower forms the "other half" of the radiating system.
I estimate the top loading effect of a 3L20 (24 ft boom) or large
tribander at about 40 ft. Feeding a 1/4 WL sloper 20-25 ft below
the tribander produces a resonant half-wavelength system,
1/4 WL in the sloping wire and 1/4 WL in the top loaded tower
section between the feedpoint and the Yagi.
My "guess" is that the lower portion of the tower is isolated from
the upper portion. My thinking is that the bottom of the tower is
at ground potential. The impedance at 60 ft up the (wide) tower
should be high since this is approximately 1/4 WL from the low
impedance ground. Therefore, current will flow to the upper
portion that is top loaded and nearly resonant, presenting a
low impedance at the feedpoint, and NOT in the lower portion
of the tower.
This analysis seems to be supported by the reports of
VERY HIGH SWR when slopers are modeled or measured
against towers that have NO top loading.
When I explained this system to a friend, he asked why I didn't
just tie the 3 sloping wires together to make an elevated GP
antenna. That is the origin of the N4KG Reverse-Fed Elevated
Ground Plane Antenna described in QST for June 1994.
With sloping radials, it is a VERY LOW angle radiator which
is not always the best configuration on 80M, especially in the
early evening when the MUF is still fairly high and as are
waveangles.
To remedy that situation, I applied the same concept to a shorter
tower (40 ft) with a TH7 on top. Using 10 elevated radials at the
15 ft level on this tower and a very gentle slope to 10 ft at the ends,
this antenna presented a feedpoint impedance of 17 Ohms which
I matched using a 1/4 WL 36 ohm coaxial transformer. The 36 ohm
transformer was made by paralleling two 1/4 WL 75 ohm lines (RG59).
This antenna has now accounted for 225 countries on 80M since I
started tracking it's performance. In spite of some acknowledged
losses to the ground connection of the tower, I expect it performs
better than inductively loaded short verticals and is a "free" antenna.
Early tests compared to a full size 1/4 WL vertical with 32 ground
based radials on marshy ground reveals NO discernible difference.
If the above analysis is correct, then it should be obvious that a
1/4 WL 160M sloper will NOT resonate in the same manner as an
80M sloper using similar heights, top loading, and feed points, hence
the great variation in SWR and tuning effects reported by several users.
I now have a 160M elevated GP utilizing a 140 ft Rohn 25 tower with
NO other antennas attached. There are 6 elevated 1/4 WL radials,
reverse fed (braid to tower, center of coax to insulated radials) at
the 15 ft level.
An 80 to 100 ft tower top loaded by a large Yagi should be self
resonant near 160M and is a good candidate for an elevated GP
on 160M. Tuning to resonance may be achieved by varying the
feedpoint placement or the radial length slightly. Shorter towers
can be effectively shunt fed against ground radials.
de Tom N4KG
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