Modeling insulated elements

Hello,
        I need some help understanding how to adjust for the
capacitive effect of insulated antenna elements. The popular
literature suggests the electrical shortening caused by element
insulation is 2-3%. However my recent (trivial) experiments suggest
this might be a too low estimate for my situation. Using "THHN"
single conductor stranded house wire (the type here in the States
with a thin clear plastic sheath over a thicker pvc insulation) I
measured around 12% shortening of the electrical length of the
elements of a dipole (that is 12% shorter than the common 468ft/fmhz
rule of thumb for dipoles made of uninsulated wire. The frequency
involved was 15mhz)
        I want to conclude from this that for my models I would
simply adjust the element lengths by 12% when going between the
model and "real world" elements made with this wire. However the
difference from the shortening effects I've read are bothersome.
        Is the 12% figure reasonable? Are there any hard figures
for actual types of insulated wire I could consult? If I could
determine the capacitance per unit length could I compute the effect
on its electrical length? And are there any subtle factors that would
keep the above "model to real world" translation of element
lengths from being valid?
        Finally, one antenna I'm investigating uses 450 ohm insulated
"ladder line" as one of its driven elements. I assume this balanced
feeder (driven from a short on one end) will be electrically shorter
than it would be if it wasn't insulated. The question is, how much?
Is it simply equivalent to the velocity factor of the line? And if
it is, could I perhaps measure the velocity factor of two pieces of
THHN laying next to each other and then compute the value for a
single conductor somehow?
        Thanks for reading this.

Regards,
Pete

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Pete Soper (psoper_at_encore.com) KS4XG 1+ 919 481 3730, 481 3868/FAX
Encore Computer Corp 901 Kildaire Farm Rd Cary, NC 27511 USA
Received on Thu Feb 01 1996 - 18:43:00 EST