Wednesday, July 8, 2015
J-Pole: The Magic Dimensions
Last winter we had a post rounding up a bunch of J-pole links. Now Paul Thirst over at Engineering Radio has posted about emergency broadcast FM transmission antennas. Most importantly, he has included a diagram marked out not in inches, cm, etc., but in wavelengths. Even more importantly, the diagram has the elusive "how far from the bottom of the J to tap" dimension.
Do you know how many ham "how to's" do not list this dimension at all but leave finding the correct feed point as an exercise for the student? "Adjust for minimum SWR." GRRRRR. (calm down, calm down, calm down)
Anyway, many thanks to PT and his blog for putting the info out there. (SWR of 1.4 and he's not happy with that? What a perfectionist!) Even though he's more concerned with broadcast band FM frequencies (88-108 MHz), this design can easily be adapted for the 2m ham, marine VHF, MURS, etc. bands. You just have to know the center frequency to plug in. Oh, and the speed of light in a vacuum, but that doesn't seem to change very much these days.
Just in case his blog ever goes away, here's the critical figure: