If you analyze one of these slopers in EZNEC, you can see why some
people have good luck with them and others don't. The configuration
that works the best is one in which most of the currents are confined to
the top part of the tower above the attachment point and the sloping
wire, and very little current flows in the bottom part of the tower.
You want the currents in the bottom part of the tower to be low because
the tower is connected to the dirt, and the sloping wire makes a sharp
angle with the tower which causes field cancellation between these two
(if the tower has significant current), reducing the radiation
resistance and causing more ground loss.
Of course people don't change their tower configuration just to make a
sloper work. They just put it up and take what they get. As a mental
exercise, if you did reconfigure your tower, you would want the top part
of the tower above the attachment point to be close to resonance (low
impedance) on the band of interest, and the sloping wire also resonant
on this band, and the bottom part of the tower NOT resonant close to the
band of interest. Trying to get this on multiple bands with a common
feed point is very difficult.
One configuration that should work very well on 160 is a 150 ft tower
with a full size 3 element 40 meter Yagi on top 5 ft above the top of
the tower, and no other wire antennas hanging off the tower. A 140.5 ft
wire attached at the 120 ft point on the tower and the far end pulled to
30 ft off the ground. All the cables exiting the tower have their
shields tied to the tower at the bottom. Assuming Phyllistran or
polyrod guys. You should have at least a minimal radial system to
decouple any cables exiting the tower. This system doesn't depend on a
good radial system, a poor one will do OK since the major currents in
the system are mainly confined to the sloping wire and the Yagi at the
top. Ground dissipation should be minimal even with a poor radial
system. EZNEC says that this system should perform as well as a full
size 1/4 wave 160 vertical which has a good radial system, except in the
direction opposite the sloping wire. However it will have a narrower
bandwidth.
Jerry, K4SAV
Edward Sylvester wrote:
>Been trying to get up to speed on these...Learned that height is not
>important, as it's the tower that will radiate, primarily. My tower is 70'
>with a Steppir 3 ele and other assorted antennas for VHF, all on a 10' mast.
>Looking at the Alpha Delta DX-B, which would give me 30/40/80/160m. Has
>anybody installed one of these on a 70' tower? If so, what height was ideal?
>Would like to do this right the first time....Thanks.
>
>Ed NI6S
>
>
>
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