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[TowerTalk] Half slopers using a tower

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Subject: [TowerTalk] Half slopers using a tower
From: (Tom Rauch)
Date: Tue, 1 Jan 2002 07:50:19 -0500
> My FT-817 does not have a built in tuner....I use a MFJ tuner and it
> does not like the antenna on 160 meter at all !

Remember when you use a "T" tuner, always use the maximum 
possible capacitance and minimum inductance that provides a 
match. On lower bands, at least one of the capacitors should be 
fully meshed.

Someone else pointed out the very important fact that the tower is 
NOT an actual ground, and the tower acts as another leg of a 
dipole. That is a very good analogy. Would you think that the 
tower, stuck in the dirt 50-80 feet away from the attachment point, 
makes a god "other leg"? I sure wouldn't!

There are many problems when antenna or radial systems are 
worked against a grounded tower. Problems include efficiency 
issues and impedance (SWR) issues.

With a typical large tribander antenna at the top of a 50-80 foot 
tower, and with insulated guylines, the top area of the tower can be 
a very high impedance when "pushed against". That means the 
feed impedance of a sloper would be the sum of the wire 
impedance and tower impedances at that point. Since a 1/4 wl wire 
worked against a "perfect" ground is perhaps 35-40 ohms, you 
need a VERY low impedance at the attachment point or the sloper 
will have a high SWR.

With slopers and **ALL** other feed systems exciting the tower, 
considerable current can flow from the tower into lossy earth. The 
only way to decrease losses is to add a large radial system at 
earth level.

There are few ways to handle the impedance problems, other than 
adding elevated counterpoises. 

Since you basically have a bent inverted V dipole anyway, like an 
inverted V dipole with one leg dropped vertical and stuck in the mud 
with a tribander attached near the feedpoint, why not get rid of the 
"mess" and just install an Inverted Vee dipole? At least then you 
can predict how it behaves, and even if radiation isn't like a vertical 
at least it will have a good SWR and not be a dummy load.

If you want a vertical, install a normal vertical like an Inverted L fed 
at earth level and a good radial system, or shunt feed the tower and 
use a good conventional radial system with it. it might be a little 
work, but you will be 5 or more above any of the "free lunch" 
systems that rarely if ever work as expected.

73, Tom W8JI 

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