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[TowerTalk] Half slopers using a tower

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Subject: [TowerTalk] Half slopers using a tower
From: (
Date: Tue, 1 Jan 2002 09:04:37 -0600
My view of Half Slopers, better described as Quarter Wave Slopers,
is that the best way to get them to perform and provide a reasonable
match to 50 ohm coax is to attach them BELOW a fairly large TOP
Load (Yagi) such that the TOP Load plus the vertical separation
between the load and the feed point is an electrical 1/4 wavelength.

I had a set of 3 Quarter Wave 80M slopers attached at the 60 ft level
of an 80 ft tower topped with a TH6 tribander on a 24 ft boom.  These
performed well, giving me my first 80M LP Asians, and provided a
good match.  I estimate the equivalent top loading effect of the TH6
to be around 40 ft, combined with 20 ft of tower to the feed point.
Since my feedpoint was at 60 ft on a grounded tower which is close
to 1/4 WL on 80M, I expect the impedance to ground was fairly high,
thus isolating the lower portion of the tower.

Using a 1/4 WL sloper for 160M on an electrically SHORT tower 
s an entirely different animal.  The BEST solution would be to 
shunt feed the tower  and Yagi.  I have matched a 50 ft tower
and small TH3 tribander on 160M with reasonable results.  A
taller tower with a larger top load should work even better.

The simplest approach would be to use an inverted L against
several ground radials (preferably 30 or more).  The down side
is that there WILL be some interaction with the tower.  To 
avoid tower interactions, shunt feed the tower.

Tom  N4KG

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