Topband: Question on common mode chokes

[Jim Brown]

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