> 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
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