[TowerTalk] 4 square for 80

[Rik van Riel]

On Mon, 2016-09-26 at 08:47 -0600, Steve London wrote:
> Interesting observation.
> 
> Where does all that common mode energy go, if not into heat ? Does
> it 
> get converted to useful RF power, or is it dissipated someplace else
> in 
> the system ?

It is not just a question of "where does it go?", but
also "where does it come from?"

The current on the outside of the coax happens because
that is one of the many paths the antenna current can
take. A lot of the power will go onto the radials, and
some of it goes onto the outside of the coax.

If you have 64 radials and 1 coax, 1/65th of your power
may go onto the outside of the coax.

If you have 4 raised radials and 1 coax, 1/5th of your
power might go onto the outside of the coax.

This assumes equal impedance for every current path.

Now assume each radial has a 40 ohm impedance, and the
coax has a 1000 ohm common mode choke on it.

Kirchoff's law then makes it so that almost all the power
goes onto the radials, and very little onto the outside
of the coax.

The higher the common mode choking impedance, the less
common mode current will flow on the outside of your coax.

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