You aren't being overly pedantic ... you're simply being wrong. Velocity
factor for electromagnetic emmissions is the ratio of any signal in any
environment compared to its value in free space. The velocity factor of
a signal in a transmission line is a function of geometry and
surrounding materials, and the same is true of a signal in a single wire.
https://en.wikipedia.org/wiki/Velocity_factor if you aren't convinced.
Dave AB7E
On 12/29/2016 4:33 PM, Walter Underwood wrote:
This is quite likely overly pedantic, but “velocity factor” is a characteristic
of transmission lines. Interestingly, it is independent of frequency (up to the
limit of the dielectric). It depends on the geometry of the line and the
dielectric material.
Antennas don’t have a velocity factor. The shortened elements are caused by
capacitive loading against (RF) ground. There is a percentage of the free-space
electrical length due to capacitive loading, but it is not a velocity factor. I
don’t think this has a snappy shorthand other than “electrical lengthening due
to capacitive loading”.
For example, dipoles with capacity caps on the ends, like the N6BT designs,
don’t change the velocity of propagation along the elements. They use massive
capacity loading on the ends (the high-voltage part of the dipole) to shorten
the elements.
http://www.force12inc.com/products/sigma-20-hd-20-meter-heavy-duty-vertical-dipole.html
<http://www.force12inc.com/products/sigma-20-hd-20-meter-heavy-duty-vertical-dipole.html>
OK, overly pedantic mode off, plus I’ve nearly hit the limit of what I remember
from my fields and waves class decades ago. I was pretty happy to get a B- in
that class.
wunder
K6WRU
Walter Underwood
CM87wj
http://observer.wunderwood.org/ (my blog)
On Dec 29, 2016, at 2:08 PM, Jim Brown <[email protected]> wrote:
On Thu,12/29/2016 1:53 PM, Bill Frantz wrote:
If this is really what is going on, then a bit longer than 1/8 wave should be
about right for radials on/under the ground.
I did an NEC study several years ago placing a half wave dipole at heights
above ground from several feet down to an inch, and varied the length of the
dipole so that it remained resonant at each height. From that I plotted VF. As
close to the earth as I could model it, VF was about 0.7. At heights of 3 ft or
more, VF on 160M was close to 1.
N6LF published a detailed study of radial lengths and recommended an optimum
length for elevated radials slightly less than a quarter wave. His basis was
that making them slightly shorter caused current distribution between them to
be more equal, which reduces ground losses. Rudy's website is a wealth of great
info about radials.
73, Jim K9YC
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