Hi,
I very much liked W1HIS's detailed explanation as of what causes the
velocity factor.
However I would not say it does not matter much.
As others have said, the rough value for the velocity factor is 0.95,
that means, you have to cut the wires approx. 5% shorter.
The real value can be between, say 3-7%, you have to find out individually
for each type of wire/insulation.
So, while these 5% will surely not hurt much when putting up
dipoles or 1ele loops, they will definitely hurt when you put up
wire yagis or quads.
A 5% change in resonance frequency of your director and reflector
elements will totally offset the whole design!
Remember, e.g. at 28.5 MHz, 5% means 1.5MHz!
So, instead of being tuned to 28.5 MHz, your antenna is now tuned
to 27 MHz. Not good!
73 Con DF4SA
At 21:49 03.10.2002 -0400, W1HIS wrote:
>At 7:53 PM -0400 10/3/02, K5RC wrote:
>>One problem.... 468/f does not work with insulated wire. You have to
>>consider the velocity factor of the jacket.
>
>Yes and no; but mostly no. Coaxial cable has a velocity factor
>significantly less than one, and equal to one divided by the square
>root of the dielectric constant of the insulating material that fills
>the space between the center conductor and the shield, because 100%
>of the electric stored energy of the wave(s) is stored in that space,
>in that insulating material. In other words, the capacitance per
>unit length of the transmission line, which together with the
>inductance per unit length determines the velocity of a wave
>traveling along the line, is proportional to that dielectric constant.
>
>
>... detailed explanation following
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