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GAMMA MATCH LOCATION

Discussion in 'CB Antennas' started by Capt'nCrunch, Mar 10, 2010.

  1. Capt'nCrunch

    Capt'nCrunch KX4R

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    I HAVE BEEN PUTTING TOGETHER ANTENNAS FOR A LOT OF YEARS AND A FRIEND CALLED AND ASKED DOES IT MATTER WHERE THE GAMMA IS LOCATED ON THE DRIVEN ELEMENT. SHOULD IT BE ON THE LEFT OR RIGHT, OR DOES IT MATTER IF IS ON THE TOP OR THE BOTTOM. I DON'T THINK IT MATTERS AT ALL AS LONG AS THE THE ANTENNA CAN BE TUNED AT THE FEED POINT. I WOULD LIKE TO SEE HOW YOU GUYS LOOK AT THIS TOPIC. I HAVE NOTICED THAT DIFFERENT MAKERS OF ANTENNAS PUT THE GAMMA IN DIFFERENT LOCATIONS ON THERE BEAMS.


     
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  2. Marconi

    Marconi Supporting Member

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    IMO, it doesn't matter, but if the matcher has a fixed feed point connector attached, make sure it is located on the element side facing the boom/mast bracket so your coax can exits the driven element correctly.

    Never put a gamma on the top of the horizontal element, this is to prevent heavy birds from roosting on it.

    Vertical elements should be positioned, when possible, so water cannot settle and not drain out of the capacitor section.

    Gammas can be mounted either to the top or bottom side of the vertical element, depending on considerations above.
     
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  3. Bionic Chicken

    Bionic Chicken Active Member

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    You can position the gamma match behind the driven element instead of on the side. This will make the forward pattern much tighter but will also reduce the back rejection. I have never set mine up that way but many have down here and that was the results they obtained.
     
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  4. bob85

    bob85 Supporting Member

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    yes the gamma position does effect the pattern skew, freecell had some info on his site on how to minimise skew with gamma fed yagis.
     
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  5. Henry HPSD

    Henry HPSD Well-Known Member

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    Hi guys,

    Im guessing the gammamatch is still attached to the radiating element ? in that case..
    Is there anywhere we could back-up those claims?, im a bit sceptic..i dont think it will make the forward pattern much tighter..and reduce back rejection..

    If the forward patern becomes much tighter, that would probarbly implicates the gain would enhance too.
    And if the front to back improved..that often means the gain will go down or you will sacrifice in bandwidth.

    If it is true...why wouldnt all the antenna companies use it in that way. Everyone wants a tigther pattern and better FB ?

    Ofcourse i understand that your thougths are that it is true..but is there any change i could get some facts ?

    73 H.
     
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  6. HiDef

    HiDef Active Member

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    One way to compare performance with yagis is to turn them exactly 90 degrees from the signal and look for the depth of the null. Larger ones with a straight guyed boom can do over 25dB received signal strength dip.
     
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  7. bob85

    bob85 Supporting Member

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    the information i have seen said the gamma position effected the skew, i dont recall it saying anything about beamwidth.
     
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  8. W5LZ

    W5LZ Crotchety Old Bastard

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    A gamma match is a 'one-sided' device, meaning it will be found on one side of the element's center or the other. It can't be placed 'behind' the element. If it is, it's no longer a gamma match. You might be confusing a gamma with a delta or beta maybe.
    In any case, placing that match in front of, or behind the element isn't going to change the gain or 'sharpness' of the resulting radiation pattern, or it's F/B ratio.
    A gamma match 'skews' the radiation pattern of a beam? Yes, VERY slightly. And so do a 'ton' of other things near that directional antenna, that's normal. The 'perfectly' formed, 'smooth', pattern representations you see are the results of statistical 'smoothing' done to make the resulting pattern uniform. All the antenna modeling programs do that. And YOU do that when graphic the results of taking measurements and plotting them on paper. You sort of 'average' out the plotted points, make it a 'smooth' curve.
    And then when you come down to actually putting a directional antenna into use, does your rotor really point things that accurately? There's no 'play' in the whole structure that the wind doesn't 'play' with? Those "one degree" marks on that rotor indicator's dial are for filling spaces, not because the rotor is that accurate. :) (They do make handy reference marks though.)
    [Reminds me a lot of the 'jiggle' correction thingy in the newer digital cameras, but you'd better have some extra memory for that thing to work.]
    Why don't you find those matching devices on top of, or below the driven element? Mostly because it increases the 'sail' area of the antenna, which isn't the best idea in the world.
    Think about it, then tell me how I'm wrong.
    - 'Doc
     
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  9. Capt'nCrunch

    Capt'nCrunch KX4R

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    We know the gamma match provides an easy and sure method of matching to the feed point without any loss of bandwidth. There are many ways to feed the Yagi, but they can be condensed into two main categories:The balanced feed and unbalanced feed. The feed point can be center,left or right on the driven element. They all perform the transfer of power from the transmitter to the antenna. Maximum energy transfer of rf at the design frequency occurs when the impedance of the feed point is equal to the impedance of the feedline.

    In most antenna designs, the feedline impedance will be 50 ohms, but usually the feed point impedance of the Yagi is rarely 50 ohms. In most cases it can vary from approximately 40 ohms to around 10 ohms, depending upon the number of elements, their spacing and the antenna's pattern bandwidth. If the feedline impedance does not equal the feed point impedance, the driven element cannot transfer the rf energy effectively from the transmitter, thus reflecting it back to the feedline resulting in a Standing Wave Ratio.

    It seems to me the reasons for the Gamma Match is to match impedance and it doesn't matter if its to the left, right, or center. I like the bottom left side on driven element. A WELL FED YAGI IS A HAPPY YAGI!
     
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  10. Bionic Chicken

    Bionic Chicken Active Member

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    I will tell the folks down here that have done extensive testing with the same antenna and the only changes they have made were changing the position of the gamma. They are the ones that did it and I have witnessed the effect first hand but like I said have never done it myself.
    I like to have the gamma under the boom on the driven element myself. It has always seemed easier to get the desired tuning that way. This is what I have found, your experiences may differ.

    73
     
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  11. Marconi

    Marconi Supporting Member

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    BC, I question how that little bit could result in on-air detectable results too, but you saw what you saw. I'm curious, why don't you describe for us how the testing was done and what you observed?
     
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  12. Capt'nCrunch

    Capt'nCrunch KX4R

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    THE ONLY TIME I WOULD BE CONCERNED ABOUT SKEW IS ON A CIRCULAR POLARIZED ANTENNA SYSTEM. AND THE WAY TO ADJUST THAT IS TO OFFSET THE FED POINT FROM THE REFLECTOR. THAT WOULD HAPPEN IF YOU PLACED THE GAMMA BEHIND OR IN FRONT OFTHE THE DRIVEN ELEMENT. THE CALCULATION OF THE HORIZONTAL RADIATION PATTERN IS DONE USING GEOMETRICAL THEORY OF DIFFRACTION AND COMPARED WITH A MEASURED PATTERN. THE SKEW ON A SATELLITE DISH WOULD BE IMPORTANT IN ORDER TO BE IN ANGLE PATH TO RECIEVE A SIGNAL. THEY ARE CIRCULAR POLARIZED,
     
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