While the gain you report is totally crazy, indicating some sort of gross
error, the current distribution is not unusual at all. What I found when I
played with slopers (1/4 wave) on 160/80/40 was the following:
On 160 m, the sloper actually acted more like an omega feed for the tower.
The current was always way higher in the tower than in the sloper itself.
On 80m the effect was very similar, but not quite as pronounced as on 160.
The tower was still doing the majority of the work.
On 40m, the sloper actually showed more current than the tower, so it became
the radiator. This was confirmed by putting up another sloper on the
opposite side of the tower and switching between them. A front to back was
readily apparent, about 6 dB or so. Doing the same experiment on 160 and 80
showed no front to back.
This little experiment confirmed what the model was telling me:
On 160 and 80 the majority of the current was in the tower, giving omni
coverage, and a simple vertical fed against ground, excited by the sloper as
a feed.
On 40m, the sloper actually emerged as the antenna, and the tower was no
longer carrying significant current. (But was helping provide some f/b)
I was using EZnec for the modelling and W9INN multi-band 1/4 wave slopers
for the antenna, mounted at about 68', fed against the tower.
73,
...hasan, N0AN
----- Original Message -----
From: "Gene Bigham" <jbigham2@kc.rr.com>
To: <TowerTalk@contesting.com>
Sent: Wednesday, December 24, 2003 9:13 AM
Subject: [TowerTalk] Sloper & Tower
I actually have a dip in the swr on 160 of 2:1 with this arrangement. The
tower has 10 radials on its base fanned out in a semicircle toward the back
fence line, laying just on top the ground, which I am not able to model due
to the 20 segment limitation on the demo EZNEC.
Looking at the currents on the model shows the entire tower as a vertical
with current highest at the base and tapering up the entire tower structure
just like you would expect on a vertical, there is also some current
distributed on the wire.
I would have expected the opposite results.
Am I doing something wrong?
Thanks
Gene Bigham
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