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Old 01-13-2016, 07:00 PM   #7
NN5I
Carl, nn5i
 
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Join Date: Sep 2011
Location: Tallahassee, FL
Posts: 1,441
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I'm planning a 20-meter J-pole to hoist into a tree. The design procedure is the same as a twin-lead J-pole (or any other J-pole), but for the matching section I'm gonna use RG58 instead of twin-lead (it's a bit shorter that way because the velocity factor is 0.66 for RG-58 and about 0.80 for twin-lead). The antenna part, instead of being twin-lead, will just be wire.

It'll be easier to build, too. Instead of soldering the end of the feedline into the appropriate matching point in the matching section, I'll make the matching section from two pieces of RG-58 with PL-259s and a UHF tee between. Then the feedline connects at the tee.

Also it'll be easier to deploy. The matching section, being coax instead of twin-lead, can be left rolled up -- the only part that has to be unrolled and hoisted (or dumped off a bridge or balcony) is the radiator wire (about 34'4" long).

I've got the lengths all calculated, and I've got some RG-58 coming from an eBay vendor. I've also calculated the sensitivity of the antenna to errors in the lengths of the three parts.

It turns out that the antenna is very insensitive to an error in the top part of the two-piece matching section. Errors in the bottom part of the matching section will affect the minimum SWR, and errors in the radiating section (the long wire on top) will affect the resonant frequency but not the minimum SWR.

The radiator impedance (at the point where the top part of the matching section meets the radiator wire) is very uncertain, so some experimentation will be required to get the optimum length of the bottom part of the matching section.

If the radiator impedance is, for example, 2000 ohms (a good guess), then the top part of the matching section should be 9 times the length of the bottom part. The total length of the matching section should be λ/4, which is 90 electrical degrees, so 1/10 of that is 9 electrical degrees, and the square of (sin 9 degrees) is about 1/40, giving an impedance ratio of about 40 between the bottom of the radiator (2kΩ) and the tap (the tee, where we attach the feedline and want 50Ω). 2000Ω/40=50Ω, so that's about right.

I'll build the top part of the matching section as calculated, and the bottom section a bit long, and the radiator a bit long too. Then I'll hang it up and start testing and cutting.

First, of course, I'll measure the velocity factor of that RG-58. That's easy. It's Belden 9907, and Belden says it's 0.66, but I'll measure it anyway.

Who knows, it may actually end up working.
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