Feeding and Matching my Beam

[OneTwentySeven]

I've pretty much decided on the Maco five element specs for my next project. I have two issues however that I need to resolve with respect to the design.



1 - Matching, Should I go with a gamma matching system or a Beta?. I've heard pro's and cons on both; any advice??



2- Feeding, Should I just bolt the coax directly; use the RF choke (coax looping); or go with a balun???? I kinda like the idea of using a balun and insulating the driven element from the boom but I can't seem to find one to my liking. I'd like to build the beam so if I upgrade to higher power I won't need to do anything to my antenna system. The highest rated balun I've seen is for 10KW. What do ya'll thank???


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[Moleculo]

1- Jay has done some specific testing of gamma vs beta matches on beams. His testing shows better results with the beta / hairpin match. You can look at some the details on his site: www.a1antennas.com. I'm sure he'll chime in here for more details. I believe he is making a "bolt on" matching system that will be available soon.



2 - The "RF choke" as you put it is a type of balun. It's purpose is to eliminate common mode current along the coax which will screw up the beam's signal pattern.



Moleculo


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[Moleculo]

Remember, coax is an unbalanced feedline. An antenna is balanced. Any current that flows back along the shield of the coax from the feedpoint is caused by the imbalance that occurs when you attach the two together (unbalanced feeline to a balanced antenna). The goal of putting a balun or choke at the feedpoint is to correct this situation which also prevents the common mode current.



You see baluns most often used with wire antennas that have no matching system. Whether or not you need one when a matching system is employed is a different story. I certainly wouldn't buy one if i'm using a matching system, but I might make a coax balun just because it's easy to do and won't hurt anything.





Moleculo


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[Jay in the Mojave]

Hello One Twenty Seven:



I have made a few Prototype Bolt on upgrade Hair Pin / Beta Matching and Insulated Driven Element Assemblies. And seen a significant increase in the field strength after the Hair Pin and Insulated assembly was bolted on and tuned.



Maybe thats why Hy-Gain, M-Squared, Force 12, Cal-AV, and all the other Big Boys use them, and NOT Gamma Matches!



Without doing any formal testing, the few Prototype users are reporting back what I have seen the first time we tried replacing the easy and cheap to manufacture Gamma Match and Uninsulated Driven Element. With the Hair Pin / Beta Match and Insulated Driven Element.



Here's what we did:



www.a1antennas.com/crusader.html



Plans are for a bolt on Aftermarket Hair Pin / Beta with Insulated Driven Element, for around 100 to 120 dollars. Fitting a Boom size diameter size of 1 1/2, 2, and 3 inches. Again after formal testing is done.



Jay in the Great Mojave Desert, .... just down the road ah ways from the fillin station.






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[Jay in the Mojave]

Hello One Twenty Seven:



Yes I have sent out a couple of Prototypes for the Jo-Gunn Beam antennas.



They use a insulated Rod and Aluminum Tubing going thru the Boom. Then they have the elements mounted over the rod.



The Hair Pin Tuning Element is then adjusted to length, as it is on a round Boom. Just a little different mounting sceem.



A SO-239 is also added, this helps keep out moisture.



Jay in the Mojave


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[Shadetree Mechanic]

Good info.I am interested in the Maco 5 element and will probably get it going with the gamma match. Then try the hair pin match at a later date.

 

[Shockwave]

While I've tried many different matching networks on Yagi antenna's, I can honestly say I've never been able to measure even a db of change from one working design to the next. That's simply not where the noticeable differences are. Expect to note differences in Common Mode Currents present on the transmission line and skewing of the pattern. Not extra db's in signal.

The best way to prevent CMC and pattern skewing is to use a matching network that threats both sides of the driven element exactly the same. The currents on both halves of the driven element must be perfectly equal with the exact center of the element at ground potential for best results here.

Unfortunately, the only matching network I'm aware of that performs this function, is also the most difficult to construct and properly tune. That would be the "T" match using two solid bars with no insulators, where both sides are feed equally with the proper 180 degree phase inversion. It's virtually indestructible too since there are no insulators involved other than inside the RF connector being used.

 

[DXman]

Good info.I am interested in the Maco 5 element and will probably get it going with the gamma match. Then try the hair pin match at a later date.

Hi Shadetree Mechanic,
I first started using a 5 element LongJohn by Hygain back in 1976, then after 24 years I replaced it with the Maco 5 element. Its now going on 20 years for the Maco with the gamma match and I must say it has served me well.

I do have the Maco tuned good for 11 meters. I am only 24' above sea level and just 36' above ground on tower. I run just 100 watt radio and I do very well going around the world with it - when the cycle is good of course.

I've done well all across Russia with it - 28 contacts.

 

[Marconi]

Here is an idea for another yagi design that provides for a balanced radiator that is direct fed and isolated from the boom. It requires no matching device. The model shows that a choke may not be necessary for CMC, unless the issue develops.

Below is a link to the Website with this design, including some Eznec modeling performance details and some pictures. Unfortunately there were are no dimensions, so I had to do a lot of guessing. But, I took my time trying to figure it all out and I think I got close tweaking this idea. The effect is in the angle of the driven element ends towards the reflector.

I ended up with a bit longer boom at 16' feet and my performance data is a little different, but it is close. This model is at 59' feet high, at 27.205 MHz instead of 27.555 MHz...which Henry use for his model. Some should recognize the author of this link.

http://www.hpsd.nl/f-yagi/4el-f-yagi

Below is my model which includes a mast which is not included in the linked model. The driven element is isolated (ISO) 2" inches above the boom and the other elements are supported by the boom. I can't tell for sure, but I think Henry's model has all the elements in the same plane and that may make the difference between our models. The feed point is noted by a faint (O) in the middle of the driven element.

Abbreviations in the title:
CDF = the antenna uses a direct fee in the center of the isolated driven element.
wISO = the driven element is isolation from the boom.
nC = model shows no choke necessary due to very low currents on the mast.

 

[Shockwave]

Angling the driven element this way would help match impedance but I fail to see how it addresses the CMC problems that would be typical in any direct fed coax design.

 

[Marconi]

Well Donald, this is just a model, but you are right.

If I used this idea and there were CMC effects, I would simply add an effective choke and hopefully solve the issue.

The model shows the current on the mast as very low, and IMO this is because the driven element is isolated from the boom and the mast by its design. If I add a feed line to the model that will reconnect the driven element to the ground again...and then this model will likely show some CMC.

 

[357magnum]

Thanks all for the info ! That Sirio 27-4 still boxed up in the garage since delivery . Just hope I get help to get it up before the snow flies !

 

[Woody-202]

Hi Shadetree Mechanic,
I first started using a 5 element LongJohn by Hygain back in 1976
View attachment 31744

That antenna was a BEAST. I bought a used one from a guy (Joker) down the road from me. His QTH was so close we just split it in two and a bunch of us just walked it over to my house

 

[kc4ubc (bill)]

I took the MACO Y-Quad 10/11 mtr Beam and redesigned it for 6mtrs and it works great. Now I prefer the Gamma match over a Balum unless I'm building wire antenna's.

 

[Henry HPSD]

Big fan of direct fed !

Agreed, the loss is low in most matching devices, but still....
there is...so if always wondered why anyone would see the "need" for it ?

Besides that i really dont think it is needed, i honestly think more then 75percent of all failure in CB yagi's lies within the matching device, perhaps thats whats in my mind....
Lets put it otherwise....give me 1 GOOD reason as "user" to use any matching device ?

The "downside" is, you can not "adjust" a yagi that much.
That most likely (in my opion) is one of the reasons manufacturers like to use a gamma-match / hairpin or whatever....it allows "failure for the manufacturer".

You know if you have a direct fed yagi, and the SWR isnt accurate....there is something going on. With all those other yagi's they all keep explaining how to adjust the matching system. Why adjust ? .. It shouldnt be adjusted...or at maximum, agreed... marginal adjusted.
If the antenna is free up in the air, it should be 1;1 no questions asked.
If it isnt..it should ring a bell.

Again, there is absolute no need for it.....(gamma-match etc)

There are a couple things you can do:

1- Direct 50 ohms, most likely this would be an owa style yagi
(roughly 5.5...6 meters boom 5 elements just below 10dBi just above 20..25dB FB is the most known example).
You could do.....with just 4 elements on that boom, with roughly the same figures, but it will be low impedance and narrow banded.

2-Design a 12,5 Ohm impedance yagi and add a folded dipole.
Thats the link you are refering to Marconi
(its a folded dipole not a bend radiator as your pdf provides).

3- design something in the 20 ohm regions and bend the radiator backwards
(k6sti V-radiator)

There are so many ways to go....

The only thing you can put on and will need is an RF choke.
RG 213, 5 turns 10cm diameter ( other diameters and numbers are not correct)
And you are good to go for as much power as the RG213 can handle.



Almost forgot...Sirs: ...shockwave&marconi, Not sure if there was a point, but agreed, there is no gain in CMC in aspect to a bent radiator..in fact the opposite would be more creditable (?) (due to change of angle in time) .

hope it was of use, 73 H/.



73 Henry.