KB6NU's Ham Radio Blog

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Velocity factor? No, capacitive loading.

Posted: 06 May 2014 11:00 AM PDT
http://www.kb6nu.com/velocity-factor...itive-loading/


Most hams who build dipoles know that 468 is the magic number when it comes
to building dipoles. That is to say, the length, in feet, of a half-wave
dipole antenna is:

L (ft) = 468 / f (MHz)

That number is about 5% shorter than what youd expect if you used the
formula:

wavelength (in meters) = 300 / frequency (MHz)

Now, I was always told that the reason for this is that the speed of a
radio wave in a wire is slower than a radio wave in free space. I bought
into this explanation and even included it in my No-Nonsense Technician
Class License Study Guide. Walt, K6WRU, disagrees with this.

In his critique of the latest edition of the study guide, he pointed out
that a dipole antenna is not shorter because of the velocity factor of the
wire, but instead, It is caused by capacitive loading due to the proximity
to ground. It varies with height, which is a big clue about the capacitive
loading. You can find a more complete explanation by reading this
StackExchange item.

This is not to say that velocity factor isnt a real phenomenon, just that
it isnt a factor when cutting a length of wire for use in a dipole. A
single conductor doesnt have this distributed inductance and capacitance-at
least not to the extent that it would cause a 5% change in the velocity
factor. Coaxial cable does have a velocity factor less than 1, and that
velocity factor is dependent the cables distributed inductance and
capacitance, and that, in turn, is dependent on the cables dielectric.

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