[Towertalk] Re: Insulated Wire

[corneliuspaul@gmx.net]

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