Modeling insulated elements (summary)

Recently I asked a bunch of questions related to use of THHN
insulated wire and ladder line for antenna elements and how to model
them. I got three replies. They both suggested I do a
careful A/B experiment with identical dipoles (identical height,
watch out for metal close by, etc), with one dipole made with bare
wire and one with THHN. I did this, and measuring the two
resonant frequencies I found the THHN antenna had elements 2.7%
electrically shorter than the bare elements and will use this
for "adjusting" the elements of my models. Only had time and patience
to do a single comparison, so don't cast this number in stone
(ice storm here made a single experiment quite difficult).

A key concept that I was gently reminded of is that resonance
changes with the height of an antenna above ground, so the
"468ft/Fmhz" rule of thumb that gave me an apparent 12% electrical
shortening was completely bogus.

Nobody commented about any other means of determining the capacitance
of wire except by the two dipole experiment, and there were also
no estimates of the electrical length of ladder line when it is
used as a driven element in an antenna (e.g. "T" for 160 meters).

Regards,
Pete
KS4XG
Received on Mon Feb 05 1996 - 16:25:00 EST