[Elecraft] 9:1 Balun

[Joe Subich, W4TV]

On 1/31/2017 2:16 PM, Fred Jensen wrote:
 > A bal-un is a transformer.  Technically, it has two windings on a
 > ferromagnetic core material which for RF is typically some flavor of
 > ferrite, usually in the form of a toroid.

Most common baluns are *not transformers* as the energy does not pass
solely from input to output by magnetic coupling.  In fact, I would
hazard a guess that *none* of the devices advertised/sold as baluns
are transformers.  Yes, many of the inexpensive 4:1 "baluns" - the
voltage type (auto-transformer) baluns - may qualify due to the
magnetic coupling between windings but they are *not* baluns in that
they do not provide a balanced to unbalanced transformation (they are,
in your terms an "un-un").

> In the classic case, a balanced load [e.g. center of a half-wave
> wire] becomes unbalanced [coax, shield grounded] by the bal-un.

Again, NO!  The balanced load is not "unbalanced" by the balun.  A
properly designed balun *keeps the system balanced* by preventing
current from flowing on the "third wire" (the *outside* of the coax)
which would otherwise "unbalance" the system.

Due to skin effect, a properly terminated coaxial cable is a three
wire transmission line.  The center conductor and *inside* of the
shield form one circuit (which is "balanced" due to the laws of
physics) and the *outside* of the shield carries "unbalanced" (or
common mode) current due to any difference in potential between
the ends of the cable or induced currents from external fields.

73,

    ... Joe, W4TV