Roger,
Here's what I am confused abot... Balanced line is balanced because the
current in the two lines (the RF signal) is equal magnitude and of opposite
phase. The field is cancelled when this is true and the line doesn't radiate.
If a dipole connected to a balanced feed line is unbalanced for some reason, it
will generate a common mode current on the balanced feed line, right?
One result is the feedline radiating. I am guessing that another problem is
increased common mode receive noise. Those are the two problems I am trying to
address. That may be impossible in a multiband doublet. I guess that is what I
am trying to understand.
So, if I understand how a common mode choke works, it increases the series
impedance of the common mode current path without adding impedance in the
differential current mode path. If I place a choke at the feedpoint, won't it
behave that way without regard to the type of feedline attached to one of the
ports?
Thanks for helping this slightly dense ham.
73 de bill
From: D Johnson Roger <n1rj@roadrunner.com>
Reply: D Johnson Roger <n1rj@roadrunner.com>
Date: December 11, 2017 at 2:13:01 PM
To: k7wxw <k7wxw@arrl.net>
Subject: Re: [TowerTalk] common mode chokes, baluns and multiband doublets
You cannot use chokes at the feedpoint of an antenna fed with window line! It
works with coax
because the electric field is between the center conductor and shield. It's
contained within the coax.
In window line, much of the field is exterior to the line and will be "choked
off" with anything around
the line. This is the same reason that you couldn't tape the old TV twinlead to
a metal mast.
73, Roger
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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