[amsat-bb] 70cm antenna SWR problem solved?
mat_62 at netcommander.com
Thu Jun 5 16:30:14 PDT 2008
After several unsuccessful attempts at solving my 70cm antenna problem while it was still on the mast, I resolved today to just go ahead and get it down off there. Not that big a deal to do really and besides I've been thinking of rebuilding the whole shootin match anyway.
I was able to use my 2 meter SWR meter after a fashion once I got the antenna inside. I could see fluctuations in the amount of power the antenna was taking as I tried various things. The problem lies in the area of the matching and feedpoint of the driven element. Again those of you familiar with the WA5VJB " cheap yagi" concept will know that the driven element is bent back upon itself forming a U shaped section on one side. Those less familiar with the design might not understand this but there's something going on right there. Either with the spacing between the legs of the U or the braid and center conductor being too close together at that point where they are soldered. Changing the spacing of the U legs didn't seem to help but spreading the center conductor and shield out from each other right where they are connected to the DE made a marked difference. I suspect it's because at this point they are very close to the "matching area" of the antenna. Hope that makes sense.
At any rate I have decided to rebuild both my 70cm and 2 meter antenna. I will keep 11 elements on 70cm but I'm going from 4 to 6 elements on 2 meters. The other change will be to replace the boom material. I used PVC pipe before and the antennas have developed a bit of a sag along their booms. I will be using wooden booms coated with spar varnish this go around and possibly add reinforcement if any sag appears evident. BTW, both of these are linearly polarized as I don't have an SWR analyzer to figure out velocity factor on cable while making phasing harnesses. Maybe someday!
Thanks much for everyones help and 73,
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