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Re: [TowerTalk] Topband: T-loaded vertical

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Topband: T-loaded vertical
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Tue, 18 Feb 2020 09:58:02 -0800
List-post: <mailto:towertalk@contesting.com>
I agree with all of this advice, and it's how I matched my Tee. I did not use the ladder snaps, but I did make the vertical section two wires spaced with PVC conduit to increase the SWR bandwidth, and I did model it in NEC.

With respect to current distribution with the lowered resonance -- N6BV modeled it in his NEC-4 licensed software, and found no increase in the far field compared to a resonant antenna.

73, Jim K9YC

On 2/18/2020 9:29 AM, Paul Christensen wrote:
Pete,

You'll pretty much be required to model it to get any semblance of accuracy.  
The T will produce a more symmetrical azimuth pattern than an inverted L and 
will also do a better job of canceling radiation from the horizonal wire 
section.

You can experiment with different T-Top length values in 4Nec2 or EZNec.   When 
space is available, my preference is to over-resonate the T down to about 1400 
kHz.  Doing so allows the maximum current point to shift up off the ground to 
the middle of the T's vertical radiator, makes the vertical less dependent on 
the extent of the radial field, and raises the R portion of the feed-point Z to 
50 ohms such that a simple capacitor can be used to cancel the inductive 
reactance.

The 160m T that N4CC and I are currently using has a 100 ft. vertical section composed of 
"LadderSnaps."  The LadderSnap wires are shorted together at the top and at the 
feed.  The symmetrical T section is supported by 140 ft and 100 ft towers separated about 
180 ft apart.  The overall horizontal T top length is 80 ft, but I want to extend it to 
100 ft to produce a nearly linear current distribution on the vertical section.  This 
also increases radiation resistance.

Of course, you can always find a T length value that results in resonance at 
some point on the 160m band.  Many folks do that to ensure they have a resonant 
antenna, either not having space for a long T top or they simply don't 
understand the over-resonating benefits.

Paul, W9AC
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