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Thread: Gamma match question

  1. #1

    Default Gamma match question

    I recently made a 6 element yagi using VK5DJ's yagi calculator program (because I want to pull in an OTA tv station from a ways away). Using all elements fixed to a metallic boom, I took the measurements for the non-folded driven element and attempted to gamma match it. All attempts to do so failed, until I replaced the gamma capacitor with an inductor. The part that confuses me is that if I tell the calculator the driven element is fixed to the metallic boom, I would assume the calculator would know I am going to gamma match it and make it longer to start with so it is inductive. Has anyone tried this using that program with a gamma match? Is there a better free yagi modeling program out there?

  2. #2
    gnuuser's Avatar
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    are your directors and reflector elements isolated from the boom?
    sometimes with a metalic boom it can throw a monkey wrench into the equation when you attempt to gamma match it. especially if the elements arent insulated
    commercial yagis for uhf are sometimes the only element isolated from the beam is the driven one

    the handheld 2 meter yagi i built i isolated the elements from the aluminum beam with 1/4 thick polycarbonate plates and gamma matched it first time out

    heres the one I usehttps://www.changpuak.ch/electronics/yagi_uda_antenna_DL6WU.php
    Im so old dirt was my apprentice

  3. #3

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    Every single element of the beam (driven included) are scrap LPDA TV antenna elements "aluma-welded" to the beam. I used that low temp aluminum welding rod meant for MAPP gas torches to attach the aluminum elements to the aluminum beam. Every piece is repurposed scrap from old TV antennas that didn't work. I ended up using a straight gamma wire with no series capacitor tube (although I did need a shunt capacitor). In the end, everything worked out and it picks up ch12 perfectly (my goal) plus Ch 9. The neighbor has one of those new "digital" antennas and, even with a booster, ch.12 is not without glitches, so I know mine is working great! I get full signal (according to the TV) without a booster, so I must have done something right. The real trick was getting the system tuned for 75ohm coax with a nanoVNA. I had to make a 50ohm coax with a 75ohm type F connector on the end and make 50ohm dummy loads to calibrate to the coax end, then shoot for 75ohm on the smith chart when the coax was connected to the antenna to know it would work with the tv. I assume if I was anything higher than 207MHz, that nanoVNA shenanigan would have failed using 75ohm connectors on 50ohm coax!

    https://drive.google.com/file/d/1G0L...ew?usp=sharing

    I will be testing the beam on 1.25m once I get a radio that can run that band, but for now, it will get me (and dad when he comes up north) sports and the GF her NCIS (from a translator station 60 miles away) lol. I am excited with how well it works!

    I am hoping to buy a real VNA soon. I am tired of this amazon garbage with no dynamic range and horrible software options. There is not much in between the $120 nanoVNA-F and the $6k decent VNA's (with little microwave coverage) but I certainly cannot afford a "professional" VNA. I didn't do bad for the budget tools and scrap metal I had. Very happy with the result, even though it took 2 full days.
    Last edited by brandon lind; Mon 21st Dec 2020 at 06:01.

  4. #4

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    Quote Originally Posted by gnuuser View Post
    heres the one I usehttps://www.changpuak.ch/electronics/yagi_uda_antenna_DL6WU.php
    I'll give that a try next time I build a beam!

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