[Elecraft] Balun loss

[Earl W Cunningham]

Ron, AC7AC wrote:

"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'."
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Such currents on the coax shield are called "common mode" currents and
are more noticeable to the user when in the receive mode as an apparant
degradation in directivity of the the array the feedline is attached to.

The usual cure for the problem is to use a "choke" balun at the feedpoint
of the antenna.  The most common methods are:

1) Coil up the last few feet of coax into a tight coil at the feedpoint
end of the coax.  A dozen turns in the coil is sufficient at 160 meters
and fewer turns are needed at higher frequencies.

2) Use a string of high permeability ferrite beads over the coax at the
feedpoint end.  12 inches of beads is sufficient at 160 meters, less are
needed at higher frequencies.

3) When feeding a 50-ohm balanced load, use a 1:1 balun at the feedpoint.
 Broadband baluns of this type are available from a number of sources.

73, de Earl, K6SE