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Re: [TowerTalk] common mode chokes, baluns and multiband doublets

To: towertalk@contesting.com
Subject: Re: [TowerTalk] common mode chokes, baluns and multiband doublets
From: Bert <balmemo@sympatico.ca>
Date: Mon, 11 Dec 2017 17:49:17 -0500
List-post: <mailto:towertalk@contesting.com>
Hi Bill,

Choking the window line is not a good idea - you'll choke the signals too. Common mode chokes are mainly used for coax cables and multi-conductor cables. I'm sure you'll get many more detailed replies from the experts.

Good Luck!

Bert, VE3NR



On 12/11/2017 4:33 PM, k7wxw wrote:

Yesterday the wind brought down my doublet, which I mostly see as an upgrade opportunity, at 
least once I finish pulling tangles of wire out of trees.  While a window line fed 
doublet is a well-known beast,  and the one I had up worked okay, I hope the new version 
will perform better by taking into account my particular installation limitations. As I worked 
on the design,  I came up with a couple of questions...

I apologize if this is too much detail, or whatever.

I am putting up a centerfed doublet to cover 80 through 10 meters, focusing on 80, 
40, 30, and 20 meters. I plan on feeding it with a random length of 450 ohm window 
line. I have a Johnson Matchbox and Dentron Supertuner and can use either to match 
the antenna to my rigs.  My antennas come to a common connection panel, which 
is mounted on an ground rod. I rarely run more than 25W and never more than 150W.

For a number of reasons - including the fact that it won't be quite level, it 
is only up about 30 feet and part of it runs over the house - the antenna will 
not be balanced. If I install a 1:1 current choke built using two stacked 
FT-140-43 toroids at the antenna feed point, is it likely to survive given the 
wide variation in feedpoint impedance over the operating frequency range?

I originally planned on using a Morgan balanced line lightning supressor. 
Unfortunately, it is only rated for input and output impedances of 300 to 600 ohms, 
a condition that is impossible to meet in a multiband antenna. My alternative is 
using a 4:1 balun, followed by a polyphase lightning suppressor.  Same question 
about this balun (stacked FT-140-43)... how will it do given the variation in load 
impedance over the operating range?

The 4:1 balun at the base of the feedline has the added advantage that I can 
avoid running window line into the house (I am doing that now) which is a PITA.

Bonus question: I know that expecting baluns and common mode chokes to work 
well over 3 to 30 Mhz is, well, optimistic. Will I get better common mode 
suppression at the feedpoint by using two chokes, material chosen to optimize 
different portions of the operating range?

I have read K9YC's guides, along with a lot of material from W8JI and others 
but I haven't quite figured out how to figure out the answers to my questions.

Thanks for any help and 73 de bill K7WXW



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