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Author Topic: Problem tuning UHF J-pole  (Read 1508 times)
JS6TMW
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Posts: 1172




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« on: February 01, 2017, 04:03:54 AM »

Has anyone else had problems adjusting the center frequency of 70-cm J-pole type antennas? I built a single-band portable Slim Jim with ladderline and it had minimum VSWR above my target frequency. No reasonable amount of lengthening the elements or feedpoint would bring it down.

Today I made a dual-band "Ed Fong" portable from ladderline. The 2-meter part tuned up easily, but again I can't budge the 70-cm part. It's 1.0:1 at 425.0 and I want to bring it to 435 or so, but snipping the 1/4-wave section or the coax trap do not move it up as expected. A 4-turn choke and ferrite bead helped bring the SWR down on 433 but I would like to actually move the resonant frequency.

Steve in Okinawa

(also posted in Antennas)

« Last Edit: February 01, 2017, 04:06:16 AM by JS6TMW » Logged
KC9NVP
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Posts: 200




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« Reply #1 on: February 01, 2017, 08:47:24 AM »

Have you try emailing Ed?  A search of the Internet resulted in a couple of sites with his email address.  I built one several years ago and as I recall, tuned up without any issues.  I should drag it out and recheck it to see where it actually is tuned at.

Good luck,

David
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KC9NVP
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Posts: 200




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« Reply #2 on: February 01, 2017, 03:32:57 PM »

Steve, just checked my 2m 70cm j pole mounted inside of pvc pipe for watherproofing as indicated in Ed's article. The resuls using my Coment caa500 MKII shows 2 meter to be bekow 1,5 SWR from 143 to 148 MHz.  fo 70cm, I could not get the reading below 4 for an SWR.  Looks like I will need to do some more investigation.

My 2m 70cm roll up j pole did a lot better, a 438 SWR is 1.3, 2.9 at 450.
The rolled up j pole was also build per plans found on the internet.

David
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JS6TMW
Member

Posts: 1172




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« Reply #3 on: February 01, 2017, 03:46:08 PM »

Steve, just checked my 2m 70cm j pole mounted inside of pvc pipe for watherproofing as indicated in Ed's article. The resuls using my Coment caa500 MKII shows 2 meter to be bekow 1,5 SWR from 143 to 148 MHz.  fo 70cm, I could not get the reading below 4 for an SWR.  Looks like I will need to do some more investigation.

My 2m 70cm roll up j pole did a lot better, a 438 SWR is 1.3, 2.9 at 450.
The rolled up j pole was also build per plans found on the internet.

David
Thanks David - that's pretty much the same as I'm looking at now. I'll do some tinkering today.
Steve
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JS6TMW
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Posts: 1172




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« Reply #4 on: February 02, 2017, 06:57:43 PM »

I've been posting the results in the Antennas forum.

73
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