A two conductor transmission line, with Earth or other conductors
providing a potential third conductor, has two defined impedances. One
convenient and intuitive way of defining these two is as common-mode and
differential-mode (or even and odd mode) impedances. Our measurement
equipment invariably measures the differential (odd) mode impedance, so
when we speak of impedance "matching" or "mismatch" we virtually always
mean matching of differential mode impedance.

Differential mode currents don't contribute to radiation. Differential
mode impedance match or mismatch has no effect at all on antenna
pattern. It can have an indirect effect on antenna *system* performance
due to potential losses in impedance matching components. But otherwise
the quality of impedance match can be ignored when considering antenna
performance.

Common mode current radiates, so a transmission line carrying common
mode current (a twinlead line with imbalanced currents, or a coax line
with current on the outside of the shield) is actually part of the
antenna. Anything which alters the current flow or distribution of this
current (for example, a current balun) will alter the overall radiation
pattern. If a transmission line is perfectly balanced, the common mode
current is zero, and the term "match" or "mismatch" can't be
meaningfully applied to the common mode impedance.

Roy Lewallen, W7EL

Cecil Moore wrote:
wrote:

No. Feeder radiation has *nothing* to do with impedance
matching.


Note Webster's has a different definition of mismatch
than the IEEE Dictionary. That's why I asked for a
definition. For instance, going from a black coax to
an orange coax could be called a color "mismatch".