On Wed, 26 Oct 2005 00:15:11 +0000 (UTC), "Reg Edwards"
wrote:

do you have theoretical or experimental support for the
inference that insulated vs uninsulated wire will be different in
length by around 3.5%? What thickness wire / insulation?

Owen

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To calculate change in velocity due to plastic PVC insulation we need
to know :-

Wire diameter = d
Diameter over insulation = D
Height of wire above ground = H
Permittivity of insulating material = K

First calculate capacitance of bare wire to ground.
Then calculate capacitance of insulated wire to ground.
Velocity Factor = SquareRoot of their ratio.
Neither Terman's nor Kraus' Bibles will mention the following
formulae. So you'll just have to take my word for it.


Velocity Factor = SquareRoot( ( A + B ) / C )

Where -

A = K * Ln( 4 * H / D )

B = Ln( D / d )

C = K * Ln( 4 * H / d )

Example :
Bare wire diameter = 1.6 mm = 14 awg.
Diameter over insulation = 2.6 mm.
Height above ground = 10 metres = 33 feet.
Permittivity of insulation = 3.5

Velocity factor = 0.983
Or a decrease in resonant frequency of 1.7 percent which can nearly
always be forgotten about.

The electrical effect of the enamel on magnet wire is entirely
negligible. But it does protect the wire from atmospheric pollution.

Thanks Reg. I had not doubt there was an effect, but it was the
arbitrary percentage figure that was implied irrespective of physical
parameters which could have a large range (such as your examples).

Clearly, the effect is a minor one in practical HF wire antenna cases
compared to the influence of nearby structures, ground conditions etc,
and one that is adequately dealt with by normal expectations of
trimming length of an antenna for resonance (where that is the
objective) rather than "designing" for the insulation factor.

Owen
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