Topband: Sloper Antenna

[Augie Hansen]

On 2/17/05 1:19 PM, "wa4fki" <wa4fki at nc.rr.com> wrote:

> I thought I found a good antenna that would work in a small city lot. The
> "Sloper" seemed to be the answer. Three were built with no success. I followed
> to the letter slopers that were described in QST, they would not work.
> 
> Is it required to have a tower behind the sloper? The last one I build was
> from QST. It was fed at the top with RG-8. There was a loading coil at the
> top, as I recall about 90 turns that connected to the sloper wire and the
> other end of the coil connected to the center part of the coax..

Hi Bill,

I built the antenna described in the March 1998 QST called "A Reduced-Size
Half Sloper For 160 Meters" (and reproduced in the "ARRL's Wire Antenna
Classics" publication, chapter 7). Mine is installed on a 60-foot tower with
a fair amount of antenna top-loading from an LPDA, a 6m Yagi, and a 6m
J-Pole at the top (tip at about 90 feet).

Without a tower you would need some wire or pipe structure to form the
vertical element. From my experience and some computer modeling I believe
that the half-sloper only serves the purpose of exciting the tower or other
metallic vertical support. It took a bit of "fiddling" with the wire angle
and length to get a reasonable match at 1830 KHz. The quality of the radial
system at the tower or support base has a significant influence on the
tuning and overall performance.

The shortened half-sloper has very narrow 2:1 SWR bandwidth and the 90-turn
coil tends to heat up at power levels above about 200 watts causing the
resonant frequency to shift unacceptably. This past Winter I replaced the
sloper with a shunt feed on the tower. The SWR bandwidth is now close to 75
KHz (vs about 25 KHz) and it can take full legal power without any drift in
the resonant frequency.

Good luck getting something going on 160m,
Gus Hansen / KB0YH