[Elecraft] Good Low Horizontal Loops - questions

[k6se@juno.com]

Stuart, K5KVH wrote:

"One of the great myths of ham radio is that parallel lines radiate or
couple and coax does not.  Misapplied, either can radiate.  The field
from a balanced line, is just that, balanced, and should be minimal a few
diameters away.  The coax should not have current on its outside shield,
but it WILL if it is connected to an antenna that is nowhere near its
impedance and if the line is for example, a quarter wave long between rig
and antenna or an odd multiple of a quarter wave."
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Kudos to Stuart for pointing this out.  Some statements by others on this
reflector were erroneous on this subject and they seemed to ignore my
previously post about the subject.  Stuart's statement is indeed correct
and is much more eloquent than mine was.
==========

"A good rule of antennas is to feed balanced antennas with balanced line.
 An unbalanced antenna such as mobile whip, quarter wave ground mounted
vertical, and gamma matched beams are unbalanced feeds where coax is
useful, if you can confine all fields inside it."
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As Stuart said, it's "a good rule", but not absolutely necessary. 
Balanced antennas can be fed with coax and work fine, but to make sure
there are no common-mode currents on the line you should use a choke
balun at the feedpoint.  Most often, a choke balun is made from winding
the last few feet of coax near the feedpoint into a coil of several turns
about 6" in diameter.  Another way is to use a number of ferrite cores
made for use on the outside  of the coax.  Of course, a commercial 1:1
balun will do the job also.

Feeding loops with coax and bringing the 100-ohm feedpoint impedance down
to 50 ohms by use of a quarter-wave length of 75-ohm (which acts like a
transformer) is quite common practice and a choke balun would improve the
common-mode rejection on this feed method as well (although it's not
absolutely necessary).

Consider any antenna fed with coax as a single band antenna (except when
using the antenna on its 3rd harmonic, i.e., a 40-meter dipole of
vertical cut for the CW portion of the band is also resonant as a
3/2-wave dipole or 3/4-wave vertical on the high end of 15 meter with a
reasonable SWR on the feedline.

Stuart also mentioned that an antenna need not be resonant to radiate
efficiently.  This is very true, but such antennas *must* be fed with
Hi-Z balance line via a tuner (remenber, the feedline does NOT radiate if
care is taken in its installation so as not to disrupt its balance).

The convenient thing about resonant antennas is that they can be fed with
coax (the way I prefer to do things, i.e., conveniently).

73, de Earl, K6SE