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Problem's tuning 2 meter beam with gamma-match

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KE5HWE, Sep 1, 2013.

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  1. KE5HWE

    KE5HWE Ham Member QRZ Page

    built a 7 element yagi on 3/4 pvc and can't get it tune to a decent swr. made gamma-match with 7in piece of 213 and 7.25" 3/8 copper tube..Not sure if 11in rg 58 would've been better..driven element is no.6 copper with 1/8 in alum. for rest..
  2. WA8ZYT

    WA8ZYT Ham Member QRZ Page

  3. VU2NAN

    VU2NAN Ham Member QRZ Page

    Hi OM Terry,

    What's the height of the antenna while testing?

    Is it out in the open?

    To keep the antenna from interacting with the ground and surrounding structures, it should located at

    a height of about 12', pointing upwards.


    Last edited: Sep 1, 2013
  4. KB4QAA

    KB4QAA XML Subscriber QRZ Page

    If all else fails, put the gamma match in a middle position, and try adjusting the Reflector element spacing.

    I don't know it this is your design or someone else's, but in my experience when using others' they should be built exactly to spec with same materials, sizes, dimensions (at least to start with) since any variation typical causes problems. Best to start with a working known design before experimenting.

    In any case, good luck! bill
  5. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Do you have an antenna analyzer?

    If so, what are the actual readings for R and X?

    I assume based on the descriptions of the materials used this beam is for indoor use. Are you testing it indoors also?
  6. KE5HWE

    KE5HWE Ham Member QRZ Page

    Thanks, not high enough during test, will try your suggestion..73
  7. KE5HWE

    KE5HWE Ham Member QRZ Page

    This is not worth the hassle!! Ordered hy-gain VB214FM for 89.95...Wasted too much time on this.
  8. KE5HWE

    KE5HWE Ham Member QRZ Page

    Cheaper to buy vhf yagi than build one!

    Gave up on 2M yagi, wasted too much time and hassle ordered hy-gain yagi, should have done this in the first place!!
  9. K8MHZ

    K8MHZ Ham Member QRZ Page

    That conjures up an interesting mental picture.....

  10. WA7PRC

    WA7PRC Ham Member QRZ Page

    Years ago, a friend gave me a 3-el 11m yagi. In a couple of hours, including tuning the Gamma Match, I had 6-el on 2m. My tuning indicator was the relative output meter of my Icom IC22S (better match = more output). For tuning purposes, I had it on a short mast, lashed to a 6' step ladder. That antenna worked VERY well.
  11. K9STH

    K9STH Ham Member QRZ Page

    You don't have to get the antenna like 12-feet in the air to point it straight up! Even HF yagis tune very well with the reflector only a couple of feet off the ground pointing straight up.

    Having the antenna like 2 or 3-feet off the ground while pointing straight up works fine.

    Now, the length of the gamma rod, the diameter of the gamma rod in relation to the driven element, the spacing of the gamma rod from the driven element, and, finally, the gamma capacitor all play a very important part in getting the gamma match to work properly.

    I have, on HF, used the coax cable inside the gamma rod for capacitance. However, I have found it much easier on VHF to use an actual air variable capacitor. Even then, the shorting bar between the end of the gamma rod and the driven element often has to be adjusted along with the gamma capacitor.

    There is a program, as part of the yagi-max antenna suite, that is very accurate in giving the length of the gamma rod and the capacitance needed to complete the gamma match. However, one does have to know everything about the feed point including the "j" factor. When yagi-max is used to design the yagi, then those figures are given. I have built a number of yagis using the yagi-max program and getting a good match is fairly easy.

    A variation on the gamma match, called the omega match, is easier to adjust. The omega match uses a gamma rod 1/2 the length of a normal gamma match. However, there is an additional variable capacitor involved. The second variable capacitor goes from the junction of the gamma capacitor / gamma rod and the enter of the driven element. The late Bill Orr, W6SAI, was very fond of the omega match. From Orr's Beam Antenna Handbook, the value of the gamma capacitor is around 1 pf per the wavelength of the antenna.

    All that aside, I have found using a folded dipole driven element, with a 2:1 ratio of the diameter of one section to that of the other, and then using a coaxial 4:1 balun, gives a very good match to 50-ohm coaxial cable. The is the arrangement that I use on my 6-meter yagi and horizontally polarized 2-meter yagi.

    Glen, K9STH
  12. KM1H

    KM1H Ham Member QRZ Page

    A VHF yagi can be tuned with the reflector only a few feet off the ground and pointing up at about 45 deg and at no obstructions.

    Try and get back to it some time just for your own edumecation as we used to say. BTW for 6M and up I use only an all metal T match.

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