[Elecraft] Good Low Horizontal Loops - questions

[Stuart Rohre]

Correction, operating near resonance does not necessarily have anything to
do with feedline radiation. It is a matter of balance in the two sides of
the line for parallel lines, -equality of current.

Also, for coax, you could have a critical line length that results in
current flow and imbalance on the outside of the shield.  (Normally, coax
currents flow on the center wire and the inside of the shield.)

Resonance is only a condition of the antenna not having a reactance term at
the feedpoint at a frequency.  It is NOT required for the antenna to be
efficient, and most of the time you do not worry about having the loop
resonant, you just adjust the transmatch and that takes care of optimum
operation.

A loop is a balanced antenna, and as such SHOULD be fed with balanced line,
such as twin lead, 450 ohm ladder line, 300 ohm ladder line, or open wire
feeders, NOT unbalanced coax.  Coax will be much more lossy, as well as far
from the impedance match at the antenna.  The antenna looks like 100 ohms or
higher, and not 50 ohms of coax commonly used.

Balanced lines are lower loss since they are higher impedance, (and
therefore carry lower currents),  even if there are reflections in the line,
(SWR).  The balanced lines that are manufactured are all cheaper than coax,
so I see no reason to hinder your performance with higher losses of coax,
when several parallel feeder solutions are as available as Radio Shack, The
Wireman, and W7FG and other suppliers of parallel lines.

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.

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.
72, Stuart K5KVH