[Elecraft] Balun loss

Tue Dec 16 21:01:00 2003

Ron,

You seem to be 'singing my song' too ...

I would go a bit further and say that if one cannot run the coax away from
the antenna at 90 degrees to the radiator (for at least a quarter
wavelength), a balun at the antenna is a futile addition.  Besides, it is
really only important if one is striving for a particular antenna
directional pattern (as would be true with a beam).  If omnidirectional
coverage is the desire, dispense with the balun at the feedpoint and let the
line radiate a bit to help fill in the deep nulls off the end of the
dipole. -- but some kind of choking device (like an 'in-line'  1:1 current
balun) would be appropriate - locate it either at the shack entry, or if
possible 1/4 wavelength from the antenna (do not include the feedline
velocity factor when determining a quarter wavelength - the outer surface of
the shield is just another conductor to the RF, just like an additional
antenna wire).

Of course, the quarter wave really doesn't apply with a multiband antenna
(unless it is a trap arrangement), but then I don't understand why anyone
would want to feed a multiband non-resonant wire with coax anyway - it is
just too lossy except for short runs at low frequencies.

73,
Don W3FPR

----- Original Message -----

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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