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J-Pole with switchable feed points.

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KL2UD, Nov 6, 2009.

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  1. KI6J

    KI6J Ham Member QRZ Page

    Another approach?

    How about setting the feed point for a match on the lower frequency and see if you can get a match on the higher by placing a shorting bar up slightly from the bottom on the "J". If you get a match, you can place the switch inside the "J" and switch the short in and out. I think the switch lead lengths were part of the problem and this approach will minimize lead lengths. Congratulations on the attempt.
  2. KL2UD

    KL2UD Ham Member QRZ Page

    Before trying this my first attempt was to add a second feed point above the lower frequency feed point and just switch the UHF connector from one to the other. That didn't work either so I came up with this idea.

    After doing a little more reading I think what I need is a discone. They have a very wide bandwidth so I wouldn't even need any crazy switches and relays. Or maybe a j-pole made out of larger diameter copper?
  3. K7JEM

    K7JEM Ham Member QRZ Page

    See post #2 on this thread.
  4. KL2UD

    KL2UD Ham Member QRZ Page

    Ha! Touche.

    Yeah, when I read it the first time I misunderstood it to mean "get a second antenna". You were right.
  5. K7JEM

    K7JEM Ham Member QRZ Page

    You might construct a folded dipole or something similar. A folded dipole will have at least 10MHz of bandwidth. Cut to 150 MHz it would probably have acceptable SWR from 144 to 156 MHz or more.

  6. WA7PRC

    WA7PRC Ham Member QRZ Page

    Re: J-Pole with switchable feed points

    I don't know what design frequency in the 2m band you used but, at 158.4MHz, you appeared to use a half wavelength in free space (492/f). You would need to consider the Velocity Factor of the particular feedline you're using. Generally, for coax with solid polyethylene dielectric, this is in the neighborhood of 0.66. For foamed polyethylene dielectric, it'll be much higher... as high as 0.88. Check the manufacturer's datasheet for the VF.

    Bryan WA7PRC
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