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How does one tune a Horizontal Beam's Gamma?

Discussion in 'CB Antennas' started by Robb, Jan 25, 2011.

  1. Robb

    Robb Yup

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    Someone told me recently, that the BEST way to tune the gamma match on a beam antenna is to do this operation on the ground before you put it up. Resting the beam on the reflector and pointing it upward.

    I tried using the dimensions on the gamma match as prescribed by Sirio; but the match had a best of 5.4.

    The antenna in question is the Sirio SY27-4 4 element horizontal beam.


     
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    Last edited: Jan 25, 2011
  2. ill be looking forward to the replys you get robb .
    can you post some close up pics of the sirio ? having a quarter next to the edge of the aluminum so we can get a idea of its thickness and diameter . and the gamma top , middle and bottom . its mounting plate and element brackets would be nice too . ;)

    thanks .
     
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  3. HomerBB

    HomerBB Well-Known Member

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    As you describe is the method I've used with success. However, trying to get one to tune for vertical has been difficult compared to horizontal due to the attenuation from the mast in line with the elements. That was one of the reasons I built my 2el Quad.

    Someone else may know some shortcut to getting it right.

    What I've done before is I ditched the gamma, and used a delta match with a 4:1 coax transformer. I've also heard you can pull the first director in close enough to the driven for a slight sacrifice of gain, but a direct feed to 50 Ohm coax.

    Maybe some dedicated beam users have some good answers for you.
     
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  4. W5LZ

    W5LZ Crotchety Old Bastard

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    I think that 'butt down' method of ground tuning a beam is just about the most practical way of doing it. It'll never be exactly 'right', but should get you in the ball-park. Unless you are just really lucky, it almost always means at least one 'up/down' (you or the antenna) before it's really 'close'.
    Following the suggested methods of tuning of the manufacturer is good, but it's only a starting point. You have to make adjustments from there to end up where you want. It's also a very good idea to go back and check all of the measurements again... at least once (or twice?). You know how those @#$ 'gremlins' are, always messing with what you've done 'right' the first time! (Paint it pink! 'Gremlins' hate pink thingys.)
    Good luck.
    - 'Doc
     
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  5. Henry HPSD

    Henry HPSD Well-Known Member

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    Robb,
    To answer you question, no that is not the best way.
    The best way would be to place the antenna where it needs to be and then set the antenna.
    (though im sure you are aware of that lol)

    The reasons for that is:.
    A antenna at different heigth from the ground will (sometimes sligthly) have a difference impedance. Therefor the SWR will change.
    For example a horizontal dipole almost on the ground will have a impedance of almost 0 Ohms, but up in the air in can be as much as close to 100 ohms. Those excesses are present below 0,5wl in heigth. Beyond that heigth the impedance will vary between 60..80 Ohms.

    As W5LZ says it is for practical reason (well said), you can try the described methode.
    IT is the recommended menthode. In most cases will produce the best stetting.

    It worries me that your SWR is far off by using this methode.
    In case of a horizontal antenna and if i interpertate your reading "sky high" correct?
    If the antenna is "free" of obstacles (beside the earth in the back ground) you should be able to get a lower SWR reading. In that case, please verify if nothing else is wrong.

    @ Homer BB,
    The one wich suggested to you the two elements quad can be 50 ohms is rigth (But!)_
    There is a very very big BUT, there will be a heavy sacrifice in almost all other aspects (Gain/FB/bandwidth etc) in such a way i wouldnt recommend it to anyone.
    I am not aware of a 2el Quad wich still has a good performance and is 50 ohms.

    Your decision to use a 2el Quad for vertical work is probarbly the best.
    Not only did the mast have a negative influence on that tuning proces for that yagi, but keep in mind that also goes for the rest of the values of the antenna.
    Where SWR is something you can adjust, that doesnt goes for FB/Gain etc...

    I still am a bit puzzeld why most manufacturers claim that the yagi can be set vertical.
    As it often has a very negative influence on gain/fb etc.

    Regards,

    Henry
    11 meter Dx antenna systemx
     
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    Last edited: Jan 25, 2011
  6. linearone

    linearone King of NY

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    I have tuned a bunch of maco beams and here is what I have learned.

    Single polarity antennas always seem the easiest to tune.
    Horizontal antennas are much easier (read as quicker) to tune.
    With the MACO gamma's it was pretty simple to get it right down to 1.2:1 on center with 1.5:1 on each end of the band. This is good enough for most folks.

    If you fiddle with driven and reflector element length you will get 1.0:1 at the expense of bandwidth. I do not know what effects this has on its gain etc.
    I consider doing the above worthless.

    I tune them mounted to a fence pole hammered int he ground about 8 ft off the ground with a short piece of coax hooked up to an mfj. I follow the instructions with the antenna and the gammas (at least on macos) tend to be fully closed and only slight adjustment of the strap is needed for tuning.

    When it goes up it is typically a hair lower on swr.
     
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  7. Marconi

    Marconi Supporting Member

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    Robb, I would do just like Henry HPSD says, tune where installed. But that is me, and I never try something I know from the get-go I can't do by myself with no help, so I keep my beams within a good safe working range for adjustment even if I have to stay below 30' feet high installed.

    Have you tried to tune just one driven element on the boom, without the influence of the other elements? If it will work like that, then it should work with the elements in place and a little more tuning.

    I can't use a dual polarity beam here, so I use only flat side setups. But, when I've tuned dual polarity beams in the past, I always found tuning the vertical side in the horizontal position the best way and I recall most of the time I could get close on a 10' pole if I was well into the clear.

    I have also found when I got the tune close with the gamma, I stopped my tweaking of the gamma and then tried at least a couple of adjustments to the element length. My recollection is that I most often saw a nice improvement in response and bandwidth when I made the driven element a bit longer than self-resonance indicated. Of course such adjustments were very small and on both ends, like 1/16" to maybe 1/4". I was watching for a dip or the bandwidth curve to get better.

    So, I guess I disagree that a narrow bandwidth makes for better all around performance. My old records for my favorite beam of all time for me was my Wilson 4 element horizontal Yagi on a stock moonraker boom set a few inches longer @ 16'5" which was due to it being frozen up in the center support section. My old records show this beam with a 2 mhz bandwidth or better.

    Have you recorded your adjustments using an analyzer? If so, post a few iterations and maybe I can see a trend. Hopefully your X value for reactance shows the sign + -, with values for R, SWR, and frequency noted as well. That might be helpful if you kept careful notes as you made adjustments.

    If you have an HF rig, scan up and down frequency and see if the SWR improves, then try and determine resonance. If your using an analyzer, describe your testing coax line length and VF, and the frequency desired.
     
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  8. TonyV225

    TonyV225 Supporting Member

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    Yupp you have to remember that as soon as the antenna is installed where its permanent resting place is you will see changes and depending how big the change is you may need to readjust a bit.
     
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  9. Captain Kilowatt

    Captain Kilowatt Professional Amateur Staff Member

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    Obviously the best place to tune an antenna is the place that it will be installed in. HOWEVER if that is not practical then yes, the best way is to point the antenna straight up but do not simply stand he antenna on it's boom. Get the antenna a few feet at least in the air and this will help minimize coupling and detuning of the reflector element. I have tuned several antennas this way and had good results with minimal detuning when placed on the tower after initial tuning.
     
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  10. Al n3izr

    Al n3izr Member

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    antenna tuning

    I try to put it 18' up for cb / 10m you can tune by pointing at
    the sky but it can not set on ground. use some pvc pipe and
    mount it with tap so your antenna is off the ground 2' or so.
    I have a pipe in the ground just for this tuning.
     
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  11. vkrules

    vkrules Well-Known Member

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    All good info from the earlier posts. Sit the reflector off the ground a few feet (you could use some pvc pipe or plastic milk crate ) Adjust for a dip a few hundred kcs lower than you intend, it will go up in frequency when you raise the beam . Check your connections.are the grub screws tight? You shouldn't have to move the gamma match bar more that 2 inchs. The siro is usually pretty good straight out of the box.
     
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  12. jazzsinger

    jazzsinger Bullshit Buster

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    i do all antennas one of two ways,

    if roof mounted i use a 1/2 wave repeater to get me away from antenna to minimise my effect on it.adjust it till satisfied then introduce longer high quality half wave multiple to radio, if anything very slight lowering of swr from extra loss in longer cable.

    if pole mounted using premade 1/2 wave multiple to go to radio plus a bit extra to allow for moving radio (but still 1/2 wave multiple), i lower mast adjust, put it back and test, lower it adjust, put it back and test till i'm happy, i will do it all day if have too.

    most antenna problems above 4:1 suggest bad connection in coax plugs or faulty connection on antenna or corrosion, or water damage in coax,or seriously wrong dimensions of antenna (extremely unlikely with off the shelf),possibly iffy gamma match,or connection between gamma and so239 centre pin.

    tune it on the ground by all means, but test it in situ, time spent here reaps rewards, check coax in my experience 90+% of antenna problems lie in poor soldering of plugs,especially nickel plated ones.

    haven't done towers but i'd think a tip over one would be far superior for maintenance
     
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  13. hotrod

    hotrod Well-Known Member

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    hey rob double check yor spacing. i looked at the instructions from sirio vs maco
    se.ems the sirio elements are slightly different in lenth same for the boom. im not
    sure try to setup to maco specs. also spacing on the sirio looks different from what ive read
    in the arrl antenna it says the refectlor and driven element get the most spacing .and the
    refectlor is the longest element and getting slightly shorter to the director. im not impresses
    with the sirio instructions ive seen so thats why l looked at macos and seen the differences
    im not sure if thats why but i thought id mention it at least
     
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  14. Robb

    Robb Yup

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    The spacings on the Sirio are all done at the factory; the elements are mounted along the boom with small fixtures that are locktite/allen screw in their factory location. These spacings were done using a computer program. One would be hard-pressed to get them in a different location, as these fixtures were all well secured. I only loosened the reflector fixture to get it parallel with the other elements as it was just slightly off. That wasn't easy to do; but I got it done.

    I did write a quick review of the Sirio beam on this forum. It is very difficult to assemble this beam wrong unless you really try to do it wrong. I have been more than pleased with it when cost and performance are considered.

    I set the gamma with the beam pointed skyward first; then re-mounted the beam on the push-up pole and final location. The settings that were made while on the ground did not budge at all when it was installed. It did NOT need to be re-tuned at all. Got a reading of X=2 @ 27.560mhz and the SWR was 1.2:1 with the MFJ-259B antenna analyzer. The suggested factory tuning/measured lengths for the gamma wasn't even close; so I went by the readings of the analyzer and got it dead nuts on that way.
     
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    Last edited: Jul 2, 2013
  15. vkrules

    vkrules Well-Known Member

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    Don't change the spacing unless you know what you are doing.! The element length will change if you change the spacing. Rob did you measure the gamma bar length by any chance ? might be a good starting point. ( mine is 30 ft in the air and I didn't measure it:oops:). Mark
     
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