My experience matches this precisely. ~35 years ago, I built the huge quad described in "Evolution of a Quad Array" from an article in QST, May 1978, IIRC. It was done with help from experienced antenna builders, measured very meticulously. When we finally got it up on the tower, it was awful, weird SWRs, no F/B, nothing seemed right. I checked all the feedlines and connectors. Nothing amiss.
I had found a roll of No. 12 insulated wire at a surplus place and used that for all the elements. In discussions with the owner of the quad in the article and the designer, we concluded the problem was the insulation, as the design used enameled No. 12, different velocity factor. I spent the next ~2 weeks of evenings, until 10-11 at night, with a pocket knife on my roof scraping insulation from ~1200' of wire in situ. Not only was it no fun, the neighbors never looked at me the same again. Once that was done, the antenna performed wonderfully, as expected. 73 Jim Allen W6OGC > 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) > > > Sent from my iPad ______________________________________________________________ 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
