Walt,
I differ, antenna wires do have a velocity factor to consider. I built
a 6 meter Moxon beam for my grandson out of insulated wire, and the
resonant frequency was considerably low. Stripping the insulation from
the wire brought its resonance point up to what was expected.
The only reason was because the velocity factor for the insulated wire
was less than that of non-insulated wire.
73,
Don W3FPR
On 12/29/2016 6:33 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)
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Elecraft@mailman.qth.net
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to arch...@mail-archive.com
