To Gama or Not to Gama

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KB2WVO, Dec 7, 2017 at 12:26 AM.

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  1. N4OGW

    N4OGW Ham Member QRZ Page

    You can usually come up with a yagi design that has a 50 ohm feedpoint by adding an extra element, often close-spaced to the driven element. Whether this is worthwhile over a more complicated (gamma, etc) matching device depends on the band. For example for VHF/UHF where elements are small, adding one element more is not a big deal. But for the lower HF bands it might add a lot of wind load and weight.

    One advantage of a direct 50-ohm feed antenna is that there is nothing to adjust except maybe slightly the length of the driven element. This gives a check that you have modelled and built the antenna correctly. With an adjustable match like a gamma you can build the antenna slightly off the correct dimensions (giving worse gain and pattern) and adjust the match to still get good a SWR.

    Tor
    N4OGW
     
  2. AI6KX

    AI6KX Ham Member QRZ Page

    IMG_1433.JPG

    I just built the 5-element OWA (3.41 meter boom), using direct feed and a 4-turn balun choke. Testing it 2 meters above the roof now, and the SWR measured in the shack through 70 feet of RG-213 is nuts on.

    upload_2017-12-8_0-44-22.png
     
    Last edited: Dec 7, 2017 at 3:57 PM
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  3. K9STH

    K9STH Ham Member Staff Member QRZ Page

    The Omega variation of the Gamma match is pretty easy to adjust. The gamma rod is 1/2 the length of the "normal" gamma configuration and is fixed in length. There is a second variable capacitor that goes from the connection between the gamma rod and the series gamma capacitor to the feed line to ground.

    The second variable capacitor compensates for the fixed, half as long, gamma rod. The main advantage of the Omega match is that the adjustments are considerably easier to be made when the antenna is installed on the tower since both variable capacitors are installed right at the boom. Of course, since there is a second variable capacitor additional weatherproofing is required and that is a disadvantage.

    Bill Orr, W6SAI, describes this type of matching system in his books on beam antennas.

    Glen, K9STH
     
  4. N4OGW

    N4OGW Ham Member QRZ Page

    Nice antenna. I built his 4.4m boom 5-element design. It works well, but is slightly detuned by being only 6 feet above my 15m yagi.

    Tor
    N4OGW
     
  5. KB2WVO

    KB2WVO Ham Member QRZ Page

    all awsome info... and yes its for all bands.
    keep info comming.. lots to learn for sure..
    reading all. learning tons.
     
  6. WA7PRC

    WA7PRC Ham Member QRZ Page

    In the early 1970s, I found the Gamma Match on my 2-el 15m Gotham yagi easy to adjust. I later added a 3rd element and added a slow speed reversible motor to make the Gamma capacitor remotely adjustable.

    In the 1980s, a friend gave me a 3-el 11m yagi. In a coupla hours (or so), I had 6 el on 2m w/ a Gamma Match.
     
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  7. AI6KX

    AI6KX Ham Member QRZ Page

    The 4.4 meter boom is an LFA, isn't it? Glad to hear it is only slightly detuned, because mine is going to be right below my hexbeam. No choice in the matter!

    73,
    Steve
     
  8. AI3V

    AI3V Ham Member QRZ Page

    I disagree with you about DC grounding of a antenna.

    Since the #1 reason antennas go bad is water in the coax/connectors, a antenna that is open circuit to DC lends itself to simple resistance tests to check for (conductive) water ingress.

    If you have issues with static buildup, a high ohm resistor at the rig end will cure it.

    Rege
     
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  9. KF6A

    KF6A Ham Member QRZ Page

    No, it's a split dipole DE.
     
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  10. KF6A

    KF6A Ham Member QRZ Page

    That may have been true 30+ years ago, but it's not the case today.
     

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