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Re: Topband: Question on common mode chokes

To: topband@contesting.com
Subject: Re: Topband: Question on common mode chokes
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Wed, 03 Jun 2015 10:09:53 -0700
List-post: <topband@contesting.com">mailto:topband@contesting.com>
On Wed,6/3/2015 9:42 AM, James Rodenkirch wrote:
I seem to recall, while reading up on common mode chokes, where I wouldn't need 
one if I employ an autotuner at the base of the antenna...is that true/factual?

The function of a choke on the feedline is to prevent it from becoming part of the radial system. This matters most when the radial system is relatively limited, but is quite important when it is elevated. See N6LF's work on this, published as a 2-part piece in QEX several years ago, and available on his website. Google his call to find it.

The "executive summary" is that ground losses are least when the current is equally divided between many radials, which is affected by their length, their number, soil quality, and proximity to the earth. The loss in a radial is I squared R, where R is loss coupled from the earth. The more radials present, the greater the division of the base current between them, thus the smaller the I. And because power is I squared, lost power falls in proportion to the number of radials.

When there are only a few radials, current distribution will be strongly affected by the nature of the earth under them, which can vary a lot over a radial field. In this case, radials that carry greater current will dissipate more power, and the total power loss will be greater. The significance of the feedline choke is that it prevents the feedline from disturbing that balance.

73, Jim K9YC


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