[Elecraft] Balun loss

[Ron D'Eau Claire]

Keep in mind also that ANY RF flowing on the outside of the coax will
produce radiation from the "feedline". Of course, currents flowing from =
the
transmitter cannot radiate from good coaxial line because the RF field =
is
completely contained within the dielectric inside the shield. Currents =
are
induced in the shield, but RF only flows on the SURFACE, so it is =
completely
confined to the inner surface of the shield and doesn't produce ANY
radiation under any conditions (as long as the shield is good).=20

However, such a coax line will radiate a great deal IF it is anywhere =
near
the radiating portion of the antenna! Since the transmission line runs =
TO
the antenna, it's got to get close to the antenna. The shield radiates
because RF current will be induced on the outside of the shield by the =
field
around the radiating portion of the antenna. This current produces its =
own
electromagnetic field and so the shield "radiates".=20

Carefully-installed dipoles under ideal conditions minimize this =
radiation
by running the feedline away from the dipole at exactly right angles.
Assuming the currents in the dipole are balanced, the current induced in =
the
shield by one half of the dipole will be equal and opposite the current
induced in the shield by the OTHER side of the dipole. If the feedline =
is
running away at exactly right angles from such an antenna, these current
will be equal and opposite, resulting in a zero net current and no
radiation. A common "rule of thumb" is to make sure the feed line runs =
away
at right angles for at least 1/2 wavelength.

Many Ham installations can't meet this criteria, but they aren't
particularly bothered by having the outer surface of the shield =
radiating
some RF either.=20

With the antenna end of the coax line "open", RF currents can flow =
directly
from the end to the outside of the coax as well. That's why a lot of ops
advocate a balun at the dipole feedpoint. A balun will stop the flow of =
RF
directly from the inside to the outside of the shield, but it's only =
useful
for that purpose IF you can keep currents from the antenna from being
induced in the shield as well. Since few Hams have the feedline running =
away
at exactly right angles for a half wave or more, it's not much of an
advantage for most ops.=20

Ron AC7AC