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.

The post Velocity factor? No, capacitive loading. appeared first on KB6NUs
Ham Radio Blog.

On 5/7/2014 11:26 AM, KB6NU via rec.radio.info Admin wrote:

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.

Well, actually it is. You'll find that a piece of bare wire will
be somehwat longer than a piece of insulated wire. For example,
using THHN electrical house wire for an antenna.

Jeff


--
Jeff-1.0
wa6fwi
http://www.foxsmercantile.com

On Wednesday, May 7, 2014 9:18:34 PM UTC-4, Foxs Mercantile wrote:
On 5/7/2014 11:26 AM, KB6NU via rec.radio.info Admin wrote:

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.

Well, actually it is. You'll find that a piece of bare wire will
be somehwat longer than a piece of insulated wire. For example,
using THHN electrical house wire for an antenna.

I have to agree with Jeff.

What happens is that there are quite a few factors into what makes the correct length for a wire antenna. Height above ground, loads of course, capacity hats, ground type, wire size. And the insulation on the wire has an effect.

If you look at an antenna design program, say EZ-Nec, you'll see that both the insulation thickness and the dieletric have an effect upon how fast the RF travels through the wire. Capacitive effects? Yup.

http://en.wikipedia.org/wiki/Velocity_factor

Explains VF

Some antennas have been constructed that use the VF to a relative extreme. It's no free lunch, but can make for interesting antennas.