[Antennas] coax 'sweet lenght'

[David Robbins K1TTT]

> I am looking at the 1974 version of the ARRL Antenna Book wherein they
> show
> feedline lengths to detune the line for avoiding feedline-plus-antenna
> resonance, when the coupling apparatus is not grounded, and grounded
is
> more
> complicated.  This not just for coax.  They are talking about currents
> induced in the feeder by the antenna due to not being brought from the
> antenna at right angles, or other assymetry.  They also note that
induced
> currents are entirely independent of  normal operation as a
transmission
> line.  I am thinking this is not a serious problem, as they also say
tuned
> lines can be any length.  Maybe the Gurus can enlighten our ignorance.
> 

this brings to mind an odd case I had here that took about 2 years to
track down.  I had a feedline for a 6m antenna that was hardline that
had a plastic jacket so that it was insulated from the tower.  When
transmitting on 20m the shield was excited enough to arc over at some
point (probably where the insulation rubbed on a bolt or something
else).  This by itself was not a problem, but the insulated length was
such that it was resonant on 40m.  combine the arcing with the resonance
and it would pump it into oscillation on 40m, a true 1/2 frequency sub
harmonic... mix that with the 20m fundamental and it generated a 15m 3/2
frequency harmonic, something that 'just shouldn't happen'.  So yes,
currents induced on transmission lines can cause problems, and sometimes
very strange ones.