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 > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to [email protected] ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
