[Elecraft] Good Low Horizontal Loops - questions

[Ron D'Eau Claire]

If you feed an antenna for use on all bands, those bands it is not
resonant on will have current on the coax feedline shield and the coax
will also radiate -- an undesireable trait.  The feedline will not
radiate if you use a balanced feeder.

The only correct way that I can think of to feed an antenna as an
all-band antenna using a coax feedline is to place the "antenna" tuner
at the feedpoint of the antenna.  A good tuner there can match virtually
any feedpoint impedance to 50 ohms at any frequency, resulting in a 1:1
SWR on the 50-ohm feedline. ==========
73, de Earl, K6SE 
_______________________________________________

Whether or not current flows on the outside of coax is not affected by
the SWR on the line. The flow of antenna currents on the outside of the
shield has nothing to do with the r-f currents INSIDE the coax:. R-F
does NOT penetrate the shield, but flows only along the surface. So
whether or not the impedance is matched has nothing to do with the
presence of currents on the outside of the shield. 

The only way to prevent currents on the outside of the shield are to
ensure that there is no path for currents to move from the inside to the
outside at the ends (use a balun at the antenna and a properly installed
connector at the rig) and to ensure that the feedline runs away from a
dipole at exactly right angles for a good distance (usually stated as at
least 1/4 wavelength). If you don't, the outside of the shield will pick
up more r-f from one side of the dipole than the other and so there will
be a net current flowing on the outside of the coax.  With other types
of antennas, the same issues with keeping the coaxial line symmetrical
to the antenna apply. The shield is a conductor, and it will have
currents flowing on it unless they are either choked off (lots ferrite
beads, etc.) or the line runs so that the net current induced by the
antenna is zero. 

On the plus side, the only effect of having r-f on the outside of the
coax is some slight shift in the radiation pattern of the antenna. This
is ONLY of any consequence with directional antennas like beams. In that
case it is usually very easy to run the coax away at right angles -
that's why most beam installations tape the coax to the mast! 

The shift in the pattern of a dipole or other antenna at HF caused by
r-f on the outside of the coax is not likely to be perceptible. Of
course, if you have some equipment that is highly susceptible of RFI
where the coax runs nearby, it might present enough RF to the device to
cause interference. But any conductor running near the device would do
that. 

Ron AC7AC
K2 # 1289