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How does one tune a Horizontal Beam's Gamma?

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  #1  
Old 01-24-2011, 07:29 PM
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Default How does one tune a Horizontal Beam's Gamma?


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 by Robb; 01-24-2011 at 10:09 PM.


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Old 01-24-2011, 08:03 PM
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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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Old 01-24-2011, 08:49 PM
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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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Old 01-24-2011, 09:16 PM
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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.)
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Old 01-24-2011, 11:44 PM
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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
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Last edited by Henry HPSD; 01-25-2011 at 12:06 AM.

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Old 01-25-2011, 05:07 AM
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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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Old 01-25-2011, 08:35 AM
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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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Old 01-25-2011, 12:22 PM
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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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