[TowerTalk] Help with Shutfeed model
w3ahl at att.net
Mon Jan 1 14:42:39 EST 2018
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
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:
(mail me if link is scrambled by TowerTalk or just Google : Ham radio may
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?
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
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?
Kelly - N0VD
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