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How to tune a Yagi antenna

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AG6DI, May 4, 2011.

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  1. AG6DI

    AG6DI Guest

    I just setup a Mosley Mini-32A 2 element multiband Yagi and I was wondering if there is a typical routine for tuning yagi's.

    I have adjusted the length of the driving element so that the lowest SWR is right at the spot in the band that I use most (14.070mhz psk31 ).

    But now I'm trying to figure out how to adjust the length of the reflector and space between the reflector and driver.
    Is there an easy way to test Front to Back ratio?

    It seems like the yagi is acting more like a dipole right now, changing direction doesnt effect the signal very much, so I assume my reflector isnt properly adjusted.

    I could use trial and error, but if there are some easy tips or suggestions that
    might make it easier, I'd appreciate it.

    73

    -Aaron AG6DI
     
  2. W8JI

    W8JI Ham Member QRZ Page

    A Yagi has nearly perfect tuning when a reflector is self-resonant right on the tx frequency up to slightly below the tx frequency. So what could be done is feed the reflector like a dipole, tune the reflector for resonance at the lowest expected operating frequency in each band, and then short the reflector feedpoint and add the driven element. I don't think you can do this in the 32A, but all I have owned are TA33 and CL33 from Mosley.

    If there is a way you can remove the driven element outer sections, replace them with the reflector section, and then tune for zero reactance at the feedpoint with ONLY the reflector present and fed like a dipole, you could tune it and test it with almost 100% certainty it will have maximum gain and F/R in the desired frequency range.
     
  3. N0AZZ

    N0AZZ Ham Member QRZ Page

    You might give Mosley a call to see if they might have a copy of that manual that they could email you or an idea from them.

    I'm sure W8JI is correct also thought the manual might help.
     
  4. AG6DI

    AG6DI Guest

    I was looking at the MINI-33A manual (http://www.g6hoq.com/documents/MINI-33-a.pdf)
    which is the 3 Element version of the same Yagi (mine is 2 element), and the boom is the
    same length as the 2 Element MINI-32A. I have the reflector and driver at the very ends of the 6' boom (there are 2 lines on the boom
    which I assumed were the recommended location for the reflector and driver), but I think I was erroneous in my assumption
    and the it should be much closer to each other.

    Looking at the drawing in the picture of the 33a manual (see pdf above), the reflect is 37.5" behind the driver, and the director is is 34.5" in front.
    So I think I should try moving both of my 2 elements in so they are 37.5" apart. I'll give it a shot an see what happens.

    I have a call into Mosley for there recommendations.
     
  5. KC8VWM

    KC8VWM Premium Subscriber QRZ Page

    Any 2 element antenna array is not going to exhibit any formidable gain over a dipole, nor will it have any sharp directional pattern when it's turned in different directions, nor will it have exceptional F/B ratio for that matter. Better performance than a ordinary dipole perhaps, but it's not a night and day difference or anything like that... Will a 2 element array function similar to a 3 or 4 element array.. no, probably not.

    I don't feel adjusting it is going to make much of a difference. I suppose modification will squeeze up the forward gain and make the pattern a tiny bit tighter to achieve maximum results but even then, it's not going to make much noticeable difference.

    I wouldn't assume the antenna is defective or has a problem per se... Its probably doing what it should. Sounds like its functioning as well as it can be expected, considering it's intended design.

    Nothing wrong with experimenting with it although, it's still a 2 element Yagi antenna no matter how you adjust things.
     
  6. AG6DI

    AG6DI Guest

    I heard back from Mosley, and they recommend putting the Reflector and Driver at the ends of the boom.
    So I'll leave it.

    I found out that I was measuring incorrectly when setting the length of the reflector and director.
    After correcting them, the SWR curve looks just like in the manual, so I assume the F/B should be similar too.

    According to Mosley website, I should be getting 3.3dBd forward gain and F/B of 17dB. I cant verify that, but It is does seem more directional now,
    The S meter goes down about 2S when I do a 180 when pointed at someone transmitting, so I think it's working.

    -Aaron AG6DI
     
  7. N3OX

    N3OX Ham Member QRZ Page

    I use G4HFQ's Polar Plot to measure patterns using my radio (you need to be able to turn the AGC off) and my sound card: http://www.g4hfq.co.uk/download.html#PolarPlot

    The trickiest part is setting up or finding a suitable signal source. It at least needs to be polarized the same way as the antenna is, somewhere in the far field, and quite strong. If it's too weak, you won't be able to resolve nulls well. If possible, another ham that's line of sight with a beam they can point at you is probably a good choice.

    Here's a pattern of my flag receiving antenna taken using a local AM broadcast station as a signal source. It's a broadband low band RX antenna and shows a similar pattern on 1500kHz as it would on 160m, 80m etc:

    http://n3ox.net/projects/flag/flagpattern_lg.JPG
     
  8. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I know how to tune an antenna

    But how do you tuna fish?
     
  9. W8JI

    W8JI Ham Member QRZ Page

    That's reasonable. I would expect about 3 dB or so gain and 15 dB or so F/B.
     
  10. KF6ABU

    KF6ABU Ham Member QRZ Page

    That's just silly....it could have 3 or 4dB forward gain and a f/b of 12-20..... thats far superior to a dipole...
     
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