Topband: Core for K9AY loop transformer

[Tom Rauch]

> As the line reaches an odd quarter wave the reactive part 
> of
> the impedance will move through 0 ohms and the real 
> impedance
> will be relatively low - at this point lower impedance 
> chokes
> (1 K or so) will be relatively more effective.  As the 
> line
> reaches an EVEN quarter wave the reactive part of the 
> impedance
> will also move through 0 ohms but the real part of the 
> impedance
> will be quite high - at this point common mode chokes may 
> be
> ineffective because they lack sufficient impedance.

Not quite.

Common mode chokes are most effective when the line is even 
multiples of 1/2 wave, not 1/4 wave. As a matter of fact if 
the line is 1/4 wl long and suspended away from ground or 
large objects a balun isn't even needed!

This trips a lot of people up.

There is a very popular reference book on baluns and un-uns 
that has that very error in the start. The author compared 
two dipoles, one with a balun and one without, and wrongly 
concluded a balun isn't needed on a dipole because current 
wasn't high without the balun. He then wrongly attributed it 
to the fact the gap at the feedpoint was small. The real 
reason current was low is a 1/4 wl line length was selected 
for the feedline. With a 1/4 wl long line hanging free of 
other objects common mode current was minimal and any 
current balun placed at the antenna has no effect.

We have to be very careful when looking at systems.

73 Tom