How does one tune a Horizontal Beam's Gamma?

[Robb]

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). Mark

As per instructions for the dimensions of the gamma rod, I did that by the book. Then I tested it with the analyzer. It was waaaaay off. I just used the analyzer and adjusted it to the best reading possible - which turned out to be not even close to the instruction sheet dimensions for the gamma rod AND the clip between the gamma rod and the driven element.

 

[Marconi]

As per instructions for the dimensions of the gamma rod, i did that by the book. Then I tested it with the analyzer. it was waaaaay off. I just used the analyzer and adjusted it to the best reading possible - which turned out to be not even close to the instruction sheet dimensions for the gamma rod AND the clip between the gamma rod and the driven element.

Robb, if we listen to those "in the Know" the longer boom is always best, and that is sorta' true with the Eznec models I have for these two antennas set very close to the manuals. But, as usual...the difference is not much.

I was surprised when I compared these two, because I thought the Maco would be noticeably better.

I noted the element dimensions and spacing in inches so you can see how long the Sirio is compared to the Maco. I also used 1/2" diameter elements on both models.

The only difference is I did not add a boom for the Sirio. The Maco is modeled using two 1/4 wave wire elements, and the Sirio uses only 1 wire at 1/2 wavelength, same difference. I find this makes some difference, but again, it is a very small difference.


I think I recall you commenting that your Sirio tuned best at <>27.500 mhz, but as I noted...my model turns out to be pretty long compared to the Maco. I find the Sirio model resonant best at about <26.500 as setup by the manual and with no matching device. Being as these antennas are made in Europe, where the frequency is higher than US CB, it would seem your results would tend to be correct. However, my model shows resonance to be well below channel <26.965.

I also added an overlay to compare the patterns, and I don't see much difference.

View attachment Robb's Sirio SY27-4 .pdf

 

[Marconi]

Robb, I had to redo the pdf file, it was only partly there. Check the models again.

 

[Marconi]

Robb, how does you bandwidth compare to Sirio's specs.

 

[Marconi]

I also have some models that test the idea of tuning a 4 element yagi standing straight-up, on its reflector, a few inches above the ground.

Eznec will not work with elements on the ground.

 

[vkrules]

Marconi. I got similar results when I modeled with yagi cad. applied boom correction and used 12 m for element size( I figured that would be about right allowing for element taper) resonance came out lower than ch 1 and swr min about 300 kcs higher. Siro say 27.205 is center frequency. Looking at other models the driven and reflector are to long. Could they have done that to increase bandwidth!, are we missing something? As for boom length ,have a look at a lawson long boom model with a 8 meter boom( twice that of a siro) it's very impressive .

 

[Marconi]

Marconi. I got similar results when I modeled with yagi cad. applied boom correction and used 12 m for element size( I figured that would be about right allowing for element taper) resonance came out lower than ch 1 and swr min about 300 kcs higher. Siro say 27.205 is center frequency. Looking at other models the driven and reflector are to long. Could they have done that to increase bandwidth!, are we missing something? As for boom length ,have a look at a lawson long boom model with a 8 meter boom( twice that of a siro) it's very impressive .

Well VK as I noted, my model is not perfect either. I did not consider tapper and as you'll note there is no boom on the Sirio. However, in other yagi models I've compared, I've noted that the boom does have some effect, but again...the difference is very small with a horizontal setup in particular.

I also agree that some things related to matching could be used to effect the bandwidth maybe, so if Robb's bandwidth is very wide compared to the specs that Sirio published...then maybe that might help us explain the dichotomy that Robb has reported in this regard.

Sirio shows us a very narrow bandwidth, probably at the feed point, for the SY27-4, <>600khz, and that is why I asked Robb about his actual <2.00:1, bandwidth.

 

[Marconi]

As per instructions for the dimensions of the gamma rod, I did that by the book. Then I tested it with the analyzer. It was waaaaay off. I just used the analyzer and adjusted it to the best reading possible - which turned out to be not even close to the instruction sheet dimensions for the gamma rod AND the clip between the gamma rod and the driven element.

Robb, just to call your attention to this fact, did you notice the Sirio SY27-4, setup is old style, balanced across the boom. It does not show the modern optimized design we see most often today?

This is more like the 4 element I use to run that had a Moonraker boom. IMO it worked very well, better than the others, even larger, I've tried over time.

 

[Robb]

Robb, just to call your attention to this fact, did you notice the Sirio SY27-4, setup is old style, balanced across the boom. It does not show the modern optimized design we see most often today?

This is more like the 4 element I use to run that had a Moonraker boom. IMO it worked very well, better than the others, even larger, I've tried over time.

Are you referring to the fact that the Sirio and Maco elements are directly attached to the boom - rather than being isolated with insulators?

It would be interesting in discussing how a beam is behaving with either method to achieve the same end.

Regardless of being 'new' or 'old' method; they still work. All of Henry HPSD's Yagis use the insulator method . . .

 

[Marconi]

Are you referring to the fact that the Sirio and Maco elements are directly attached to the boom - rather than being isolated with insulators?

It would be interesting in discussing how a beam is behaving with either method to achieve the same end.

Regardless of being 'new' or 'old' method; they still work. All of Henry HPSD's Yagis use the insulator method . . .

It really doesn't matter much Robb, but you missed my point entirely. I should have posted pictures. You and someone were talking about the differences in spacing. In my real world experiences with beams I never was satisfied with any optimized spacing design I ever tried. That was my only point.

I think you told us about how Sirio designed their beam with high tech ideas and tools. I see that Sirio decided, thru testing, that the more traditional design for their 4 element with a more balanced design across the boom must be better. You can see the difference in the numbers I added for the spacing of each.

BTW, Maco use to credit their improved M104c design to optimization.

Here is an example of what I was trying to describe.

View attachment Robb here is balanced spacing vs. optimized..pdf

 

[KC1PNR]

Hello all! I have a Maco m105c not sure why but swr is 5 and the antenna is perfectly measured. I set it for 28.4 since I'm a tech and all I can use is limited.
Radio is. Yaseu ft 840
Tuner ldg z100 plus
50 ft coax rg 58
Antenna is m105c maco 26 ft

 

[jtrouter]

Hello all! I have a Maco m105c not sure why but swr is 5 and the antenna is perfectly measured. I set it for 28.4 since I'm a tech and all I can use is limited.
Radio is. Yaseu ft 840
Tuner ldg z100 plus
50 ft coax rg 58
Antenna is m105c maco 26 ft

Just to let you know the last post on this subject is 10 years old.
Not that you may not get some responses just a FYI.

 

[nomadradio]

First thing I want to know when an antenna behaves this differently from what I expect is to 'sweep' the frequency of the test signal from way below to way above the desired frequency. The frequency where the SWR is lowest will point to what's causing the problem.

73