[TowerTalk] Help with Shutfeed model

[Steve, W3AHL]

Kelly,

I suggest you buy or borrow a copy of ON4UN’s Low-Band DXing, 5th edition 
book from ARRL and look at Chapter 9, section 6.9, starting on page 9-62. 
At the end of that section they discuss modeling and recommend not to model 
the top load capacitance.  There is also a section on insulated elements, 
arcing and how the driven element may act as if it isn’t insulated on 160M 
due to coupling of the feed line to the tower.  Which doesn’t quite make 
sense for the side fed by the center conductor, so modeling it has more 
uncertainty.

What they suggest instead is to attach a drop wire to the top of the tower 
down to the ground level and use a dip meter to determine the resonant 
frequency of the tower+top loading.  Then calculate the effective electrical 
length, then design the matching wire spacing and tap height.  Add a 
matching network as needed at the feed point.  There are tables and graphs 
that help with that.

Based on that same book, in section 8, the effective diameter (reference in 
Chapter 9) for Rohn 45 is about 10.8 inches or 27.4 cm.

There’s other good info and tables that should make the design simple once 
you know the electrical length by grid dipping the tower system.

Another useful article referenced in ON4UN’s book is in the May 1989 Ham 
Radio magazine, page 32, The Fold Wire Fed Top-Loaded Grounded Vertical 
Schulz, Jr, Walter J., K3OQF, downloadable from: 
http://cahabatechnology.com/aa4pg/hrmag/hamradiomag/Ham%20Radio%20Magazine%201989%20%20Issues%201-12/05%20May%201989.pdf 
(mail me if link is scrambled by TowerTalk or just Google :  Ham radio may 
1989 ).

If I get some spare time this week I may try to model your tower with the 
XM-240.  How much mast is between the tower top and the antenna?

Steve, W3AHL
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Date: Sat, 30 Dec 2017 00:23:01 +0000
From: Kelly Jones <kjones at virtualcohesion.com>
To: "TowerTalk at contesting.com" <TowerTalk at contesting.com>
Subject: [TowerTalk] Help with Shutfeed model

Hi all,

I'm considering shut feeding one of my newly installed towers for 80/160.  I 
had good luck with this when I lived in Colorado and would like to try this 
again in AZ.

The tower is 70' of Rohn 45 with an XM-240 (reflector grounded) on the top. 
In my previous installation, I used 1/2" copper tubing spaced 12" off the 
face of the tower with the tap around 40'.  The match was a simple Omega 
match using just capacitance in the matching network.

My question - would someone much smarter than me be willing to model my 
tower and provide direction on the matching network?

73
Kelly - N0VD