Greg, One property you should look at when deciding the material to use for your insulator (especially if exposed to water) is a property called water absorption and normally it's listed as a weight percent. Some nylons are better than others, but nylon in general absorbs a lot of water and therefore probably not your best choice of material.
73, Don (wd8dsb) On Fri, Dec 13, 2013 at 9:19 PM, Greg - ZL3IX <zl...@inet.net.nz> wrote: > The Tx antenna I have been using very successfully for the last 8+ years, > is a top-fed vertical. This feed arrangement requires the yagi used for > top loading to be insulated from the mast. I have been using a nylon > insert between the two halves of the stub mast as my insulator. This > morning I noticed high SWR, firstly only on high power, but then at any > power level. > > Today I brought the mast down for inspection, and the only sign of trouble > I can see is in the insulator, which has bubbled visibly. This may (or may > not) be the problem, but I propose to change the insulator even if only to > eliminate it as the culprit. I have a couple of questions for this group. > > 1) Does anyone know if I can upload a jpg file to contesting.com, so > that guys can see what I am talking about? Tree, I guess I can't attach a > photo to a post to the group? > 2) I always thought that nylon was a pretty good dielectric, and did not > expect problems, especially at 1.8 MHz. The gap in the insulator is 7.5mm, > or about 0.3". I estimate that there will be around 2 kV across this gap. > Is nylon perhaps not as good as I thought it was? > 3) If I replace the nylon with Teflon, will I lose anything in mechanical > strength? > > Unfortunately this problem means that I will not be able to enter the > Stew. We are going away for a week for the festive season, next weekend, > so won't have time to fix the issue. > > Comments welcome. > > 73, Greg, ZL3IX > _________________ > Topband Reflector Archives - http://www.contesting.com/_topband > _________________ Topband Reflector Archives - http://www.contesting.com/_topband