If a J-Pole Antenna is Good for 2m, Would it be 3X as Good for 6m?

Nick, KD8IPE, one of the guys in my latest Tech class got interested in working 6m and asked me about 6m antenna. Well, having never worked 6m, I didn’t have a very good answer for him. Then, I thought about the J-pole antenna. I have built many 2m J-poles. They’re simple to build, and using my favorite set of plans, the SWR in the repeater portion of the band has always been 1.5:1 or less. So, I suggested he build a 6m J-pole.

Nick then turned the tables on me and suggested that we each build one. Then, he reasoned, he’d at least have someone to talk to. I took him up on that, and last night, we built one using plans developed by DK7ZB. This antenna differs from the 2m J-pole in a couple of ways. First, the radiator is made from antenna wire, not the 450-ohm ladder line, as in the 2m J-pole.

Second, the feed point seems to be at a different point. In the first set of plans, the feed point is approximately 20% up from the shorted end of the matching stub. In DK7ZB’s plans, the feedpoint is 5-10% from the shorted end. Since I’d had such good success with the first set of plans, we decided to go with the 20% figure and placed the feedpoint about 13.5-in. from the shorted end.

Third, the DK7ZB design uses a choke balun at the feedpoint. You rarely see this on 2m J-poles, although this is debated now and then.

After some discussion about whether to solder the coax directly to the ladder line or to install an SO-239 (we opted for the SO-239), we soldered it all together and then took it outside to hang it from a tree branch. We ran the coax inside and then hooked it up to my Icom IC-746PRO. I don’t have an SWR meter for 6m, but the 746PRO’s internal antenna tuner was able to find a match, so the SWR mustn’t be too bad.

Of course, 6m didn’t appear to be open at the time, so we didn’t work anyone. Heck, we didn’t even hear anyone or any of the beacons. Even so, it’s kind of cool to be able to say that I’m now capable of working 6m.

I do have some questions, though:

  • What’s up with the feedpoint? Why do the two designs differ as to where to place the feed point? I wish I had an antenna analyzer to make some measurements.
  • Is the balun necessary and/or useful? Does using a balun affect the feedpoint placement?
  • Does polarization matter? I’d guess if I were trying to operate a 6m FM repeater it would, but that it wouldn’t matter so much when working CW or SSB DX.

Comments

  1. Hmm… given that beam antennas are relatively small at VHF and above, I’ve never bothered with a J-pole. As a very new ham I built a 6m 4-ele quad out of wood and was very pleased with it (dimensions are available all over the ‘net). It was ugly as sin and I had to turn it by hand (on a pole, a mere 10′ above my back deck), but it worked wonderfully. I’ve moved up to a commercial quad now.

    There are times on 6m when I’m sure a J-pole will work for DX contacts, but a small beam is easy to make and well worth it.

  2. Here’s a 6 meter J-pole built from 450 ladder line. I hope to have one in my trees in the near future.

    http://www.astromag.co.uk/j-pole/

    73
    Bill

  3. Eric WY7USA says:

    What’s up with the feedpoint? Why do the two designs differ as to where to place the feed point?

    Basically a j-pole is an endfed antenna. the radiator is the 1/2 wavelength piece that extends over the 1/4 wave matching stub (the two parallel sections).

    You can find any impedance you like on the matching stub. At the bottom you will find a low impedance (zero ohm short circuit) and at the top a high impedance (in theory infinite ohms, in reality about 4000-5000 ohms). The location where you will find a 50 ohm match depends on the impedance of the radiator section. And that impedance depends on the dimensions of it.

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