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[TowerTalk] Half slopers using a tower

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Subject: [TowerTalk] Half slopers using a tower
From: (hasan schiers)
Date: Tue, 1 Jan 2002 08:36:01 -0600
On Monday 31 December 2001 03:59 pm, Stephanie WX3K wrote:
> My tower is 65 feet with a Mosley Pro95 beam sitting just above the thrust
> bearing. The "Half-Sloper" is hung at a 45 degree down at the 50 foot
> level.....about 128 feet of wire...fed at the 50 foot level with 50 ohm
> coax and the shield ground right there on the tower. Anyone ever try this
> ??? The match I have is still unacceptable...2.5 to 1.....Any suggestions ?
> Stephanie R. Koles WX3K

Changing the length is one option. Changing the included angle (the slope of 
the wire), also affects vswr. 

Assuming yours is top fed, you are likely to find the following:

1. The tower and wire do not really act like a dipole as some claim.
2. The wire portion tends to act like a slant feed for the tower.

If you look at a model of the 1/4 wave sloper fed against the tower up high, 
you will see the current distribution in the wire is MUCH lower than that in 
the tower itself! The tower is the acting as a vertical radiator, and the 1/4 
wave wire is acting as a "feed". There is very little radiation from the wire 
itself, compared to the tower. 

I've used the w9inn 160/80/40 sloper on a 72' tower with hf antennas above as 
a capacitive top load. On 160 the vast majority of the current was in the 
tower. On 80, the tower still dominated, on 40 the wire portion started to 
share the current distribution more equally.

I would model your situation wit EZnec and see what is really going on. It 
might also help you adjust your feed height, included angle, and put quite a 
bit more attention to your radial system below the tower, if you haven't 
already done so. 

Good luck, that antenna can be a real fine performer, but it is picky to get 
set up, unless you're lucky. I never got my vswr on 160 below 2.5 : 1 , but 
it worked very well. On 80 and 40 it was < 2:1 at resonance. 73

hasan schiers, N0AN

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