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. KB2WVO

    KB2WVO Ham Member QRZ Page

    ok iam sure might be a 50/50 thing.
    but.........
    better to gama or direct feed a homebrew yagi ? or diff band one works better then other..
    i did build my 1st 3el 6m yagi and gama feed it.. seems ok ... not sure direct how much improvment would be seen..

    Pros ?
    Cons ?
     
  2. KC8VWM

    KC8VWM Moderator Staff Member Volunteer Moderator QRZ Page

    Pro:
    It makes the antenna SWR adjustable for any area of the 6m band you like.

    Con:
    "Can" limit your antenna power handling capabilities.
     
    Last edited: Dec 7, 2017 at 1:12 AM
    NH7RO likes this.
  3. WA7PRC

    WA7PRC Ham Member QRZ Page

    I used a Gamma Match to feed a 15m yagi.
    Worst-case VSWR was about 1.2:1.
    It handled my SB-220 amplifier no problem.
     
    KC8VWM likes this.
  4. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    If you "direct feed" an optimized yagi for any band (not just 6m) it won't be any kind of decent match for coaxial cable. The whole reason to use a matching system (gamma, beta, delta, anything) is to transform the actual feedpoint Z of an optimized yagi to 50 Ohms, as nearly resistive (non-reactive) as possible.

    To make a "direct feed" yagi with just a split driven element actually match 50 Ohm coax, you have to compromise performance because when performance is optimized for max gain or F/R (or a good compromise for both), the feedpoint Z won't be 50 Ohms. The native feedpoint impedance is based on not just driven element length, but spacing to the reflector and first director, and maybe even the second director.

    As such, I'd be tempted to stick with the gamma match, if it's an optimized design. A good gamma match that is well made and well protected from rain, snow, etc. doesn't really have any loss and one advantage to the design is the driven element can be grounded at the center directly to the boom: So, if the boom/mast/tower are well grounded, that provides a degree of lightning protection.
     
    WB5YUZ, KM4FVI, WA7PRC and 2 others like this.
  5. K9STH

    K9STH Ham Member Staff Member QRZ Page

    I have had very good luck with omega matches, for HF, which is a variation on the gamma match, and is easier to tune because one does not have to vary the length of the gamma rod. However, a 2nd variable capacitor is involved. There are situations where I have also used a gamma match and, although those took longer to adjust, worked well.

    For VHF, I have had problems with certain Yagi designs in that the gamma was either very "touchy" or I just could not get a good match. As such, I generally use a folded dipole arrangement with a 2:1 ratio of the diameter of each half wave section. I ground the center of the larger, closed section, to the boom and then use a 4:1 coaxial balun from the "split" feed side. The impedance of this type of folded dipole is very close to 200-ohms (not 300-ohms when each side is the same diameter) and then a 4:1 balun matches 50-ohm coaxial cable very well.

    Look closely at the Yagi antennas on my "short tower" in the photograph on the 2nd page at:

    http://nebula.wsimg.com/58d5f90d22e...50C433DB440D6B60D&disposition=0&alloworigin=1

    The top antenna (6-meters) and the bottom antenna (2-meters) have this folded dipole arrangement. The center Yagi (1.25-meter) came with a gamma match and it works "OK". However, I am seriously considering changing the driven element to a folded dipole with the 2:1 ratio of tubing diameter the next time I have the antennas "on the ground" for preventative maintenance.

    Glen, K9STH
     
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  6. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    My VHF-UHF antennas all have "T" matches with coaxial baluns.

    6m beam is from M2, a 6M7 and it does also. Not touchy at all.
     
    WA7PRC likes this.
  7. K6CLS

    K6CLS Ham Member QRZ Page

    If it works, don't fix it! Get on the air and enjoy your new antenna.
     
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  8. K6CLS

    K6CLS Ham Member QRZ Page

    Well, one design claims direct coax connection

    http://www.g0ksc.co.uk/intro-to-owa.html

    I don't have one so no direct experience, but I'd enjoy trying one.
     
    NL7W likes this.
  9. W1VT

    W1VT Ham Member QRZ Page

    I've used quarter wave matching sections coiled into choke baluns to "direct feed" a split driven element. 35 ohm RG-83 coax will match a 25 ohm feedpoint impedance. Two runs of 50 ohm coax in parallel will provide a 25 ohm line for matching a 12.5 ohm feedpoint impedance.

    Zack W1VT
     
    NL7W likes this.
  10. WB3BEL

    WB3BEL Ham Member QRZ Page

    A Yagi can be designed to use a match at the antenna feedpoint or not. There is no clear-cut universal answer to whether using a match is better or not.
    It depends on the design and the design objectives.

    But I will say this, and am prepared to take a lot of heat for it:

    The Gamma match is an inferior match choice among the many other options.
    It is something some old timers have an affection for because of nostalgic feelings from their early days. I count myself in this group in many ways.

    Here are some of the cons:
    1. Unbalanced match at a balanced point of the antenna. This is a potential source of common mode or feedline radiation and/or asymmetry in the antenna pattern and increased sidelobes. Most amateurs don't measure any of these things so they are blissfully ignorant.
    2. Hard to implement without requiring tweaking and adjustment during construction, installation or testing. The gamma match does not lend itself well to calculating the required dimension and building to print. You get in the ballpark and adjust it until you are satisfied with the VSWR. Other matches can be implemented with no required adjustment if desired. But some people like adjustments even if they make them incorrectly. The Gamma has multiple degrees of freedom due to the multiple adjustments. So these may be harder for beginners to adjust.
    3. The gamma capacitor is hard to weatherproof correctly, and may be a source of antenna performance degradation over time. Water or insects get into the capacitor and cause shifts in the VSWR or additional losses in the match.
    4. Non DC grounded matching system. This may not be that big of an issue, but other matches like the Beta and some Tee matches can be implemented with a DC ground at the antenna which can discharge static on the antenna. In some dry environments, for example, blowing sand and snow or ice can cause charges to build up on the antenna and may lead to equipment damage if not properly protected.

    There are a few pros as well:
    1. This kind of match allows the driven element to be a continuous conductor. In some cases this is a simpler construction and may be stronger than implementing an insulator at the driven element center. However, there are other matches that also allow this kind of construction that are superior. The Tee match is one example.
    2. There are lots of older existing designs that work reasonably well that use this kind of match. They have been built by generations of amateur antenna experimenters and work well enough that they are satisfied.

    I am not sure that changing your current antenna to get rid of the Gamma match would result in any obvious electrical performance improvement if you are not suffering from one of the issues above. But if you investigate other antenna options in the future, I would seriously recommend that you study the alternatives and draw your own conclusions.

    Good luck with your antenna,
    Harry WB3BEL
     
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