[Elecraft] Balun loss

Tue Dec 16 19:12:00 2003

Ron, that makes a good argument for another balun at the coax where it
enters the shack.

73, George W5YR
[email protected]

----- Original Message ----- 
From: "Ron D'Eau Claire" <[email protected]>
To: <[email protected]>
Sent: Tuesday, December 16, 2003 5:32 PM
Subject: RE: [Elecraft] Balun loss

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).

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".

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.

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.

Ron AC7AC

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