help me to understand gama matches

[BOOTY MONSTER]

im thinking now about trying to make my homebrew into something resembling a sigma4 . i keep reading about how they are better than 5/8 wgp's . i need to be sure i understand how the gama match actually connects to the antenna . the wolf .64 has the best pics i can find with a gama on a omni antenna .

the plate that the so-239 is connected to has direct physical and electrical connection to the ground elements . the gama rod comes off the center of the 239 then the top attaches at the top to the main verticle element with a bracket. it looks like adjusting the length of the gama (and sliding the connecting point on the top of the gama to the mast to follow that length) is how the antenna is tuned . IF.....some of that gama is sticking above that bracket that connects it to the main verticle does that also affect tuning ? should it be ignored or should it be cut off ?

also .... is the tube/section that the ground elements attached to insulated from the main verticle with some sort of sleeve like the maco 5/8ths are ?in the assembly pdf it doesnt look like it is .
http://www.cbtricks.com/ant_manuals/avanti/av174/index.htm


if anyone has any clear pics of how the gama and verticle and ground elements attach at the base/feedpoint that would be wonderful !!

i guess ill be reading this thread a few times over tonight , LOL
and yep , i know ill also need to get tubing to make my main verticle longer
http://www.worldwidedx.com/cb-antennas/36412-modified-vector-4000-a.html

 

[mackmobile43]

The gamma is a variable capacitor, there is an insulating medium between the rod that connects to the so-239 and the outer tube of the gamma usually a teflon sleeve.

So when you adjust the gamma you're just changing the capacitance or impedance between the ground and the radiating element.

The tapping point of the gamma (where it connects to the radiator) does have something to do with the (thingy, quote (doc") but I'm not sure how it does so bob or Marconi surely has that portion of the answer.

 

[Marconi]

I wouldn't touch building this one from scratch with a 10' foot pole and for sure as cheap as they are. There are just too many variables to consider in getting it built right and then tuning it so it will really act like it should, or otherwise it works like any other mediocre antenna or worse.

I think Mack has one for sale, why not get one made right and then you have everything you need to build one right. Then you can piddle with it all you want to see if you can get it to work better like Bob85 is able to do. That way maybe Bob could help you with your tweaking efforts. If you build one yourself there are just too many variables to consider and still be able to get some good outside advice.

Bob might could help, but I would not know where to start, I just don't understand the Sigma IV design well enough.

I have a feeling that every little part, tubing segment length, diameter, and angle is critically important to its tune and performance.

 

[BOOTY MONSTER]

thanks guys . i had no idea there was a insulator in the gamma match . im sure theres a certian spacing for it that will change with the different diameters of the upper and lower rod . im also sure theres a calculation or program for figguring that out too . that's just way too complicated for me to try .

ill stick with just adding ground elements to what i already have .

thanks again

 

[W5LZ]

In a nut shell, a gamma match is just a differently 'shaped' tuner, a combination of inductive and capacitive quantities which transform the 50 ohms of the feed line to whatever the input of the antenna is. That transformation can be done in several ways, capcitively, inductively, or a combination of both.
The 'ring' on that antenna is just a matching coil/inductance of a different shape.
- 'Doc

- 'D

 

[Captain Kilowatt]

Theory time: Think about this for a minute. The base of the antenna is grounded just like the center of a yagi element is also grounded. If we connect the center conductor of the coax also to this point it will be shorted to ground and have an impedance of zero ohms. The very ends of the radiating element have an impedance of practically infinite impedance so it stands to reason that somewhere along the length of that radiating element the impedance will be 50 ohms. The impedance versus distance is NOT linear BTW. A gamma match allows the feedpoint to be moved out the element to the 50 ohm point however since the gamma match adds inductance it needs to be canceled out by an equal amount of capacitance.That is the reason for the two sections of tubing insulated from each other that make up the gamma match. The tubing forms a capacitor along with the tubing. The more overlap the tubing sections have the more capacitance the gamma match has. A gamma match can also be formed using a single piece of tubing and a small variable capacitor but the capacitor must be in a water proof housing.

 

[bob85]

booty you give in too easily, i won't pretend sigmas are an easy antenna to understand or homebrew,
you don't need to understand how it works to make it work,

shockwave gets good results using a maco gamma.

 

[BOOTY MONSTER]

i hear ya bob . the cheapest i can find a maco gamma match for is about $55-60 shipped plus ill need to get two more 6 ft sections of tubing to extend my radiator and then theres the ground radial system so itll wind up costing well over $125 for the parts . its gonna cost me about $50 to add a nice clean looking (i hope) 8 element ground plane to my current verticle and tuning loop and thats about what my budget can handle this month for toys . not to mention for $150 i could just order a 4000/LW-150 . the reason i didnt get a 4000/150 to start with is concerns about it lasting here in Virginia since we do get some pretty strong hurricanes here every now and then .

so its a combination of lack of confidence on my part and budget . i know my antenna works very effectively with just 4 12 gauge wires and im not expecting 8 aluminum rods to make a lot of difference . but it still allows me to have something different from everybody else and itll only be about $20 more than 4 of those ground radials . i do understand that a 5/8 WL antenna cant perform better than a 5/8WL antenna and that the sigma 4 and its wanna be copies are better antennas .

ive only built 3 antennas including a wire dipole .

a man's got to know his limitations

 

[bob85]

i know where you are comming from booty, i did not realise that gammas cost so much, the stock vector gamma is crap but it works,
i can get teflon tube foc but i don't know if there are standard tube sizes that fit the free teflon,

if adding radials keeps you out of trouble and occupied its all good,
i guess you need to do the drooping and flat radials , no doubt the aluminum 8 will look more professional than wires,
hey one day you may discover that adding radials that don't add to vertical radiation is less effective than adding radials that do.

 

[BOOTY MONSTER]

"hey one day you may discover that adding radials that don't add to vertical radiation is less effective than adding radials that do"

LOL

ok BOB , i get the impression you could suggest a better way of doing the ground elements , either the number of them or the angle of them . id appreciate any suggestions or tips you may have on adding ground elements to my 5/8 .

i have read that adding more than four 1/4 wave ground elements to a elevated 5/8 has very little effect , but your comment above makes me wonder if its possible to have too many ground elements . i know marconi has had improvements adding more ground elements to his 1/4 wgp's . but he recently commented the past results may not be quite what he thought since becoming more familiar with his equipment and not being able to reproduce them .

so BOB , in youre opinion whats the most efficent ground plane setup for a elevated 5/8 on 11 meters . length ? angle ? ammount ?

i was thinking eight since i have to spend $30+ and the material was so cheap . i could just get thicker elements that cost more . i wont be able to take this one up and down to play with it so i need to have it as optimal as possible when its put up .

heres a post about the material ill be looking at and how i plan to make a plate to attach the radials to on the antenna .

http://www.worldwidedx.com/home-brew-mods/56158-buying-aluminum-again.html

thanks BOB , or anyone else that cares to comment

 

[Marconi]

I have read that adding more than four 1/4 wave ground elements to a elevated 5/8 has very little effect , but your comment above makes me wonder if its possible to have too many ground elements . i know marconi has had improvements adding more ground elements to his 1/4 wgp's . but he recently commented the past results may not be quite what he thought since becoming more familiar with his equipment and not being able to reproduce them.

BM, if this is what you recall, then I must have said it, but that is not exactly how it happened.

I got frustrated because of some process errors I was making in my testing procedures and I just quite and went on to testing some other antennas. I haven't expanded-on, ruled-out, confirmed, or determined if my 2006, testing with my Field Strength Meter (FSM) was repeatable, and I sure haven't changed my mind that adding radials can improve radiation. That is what I set out to do, but failed. I would be following up on that this spring, but after becoming ill last fall and in the hospital, I'm just not up to doing the work.

I'm not even sure that my ideas for testing are valid, so I kind of have to depend on discussions with others in order to figure that out later. I know that I have additional questions even before I start. I just do the test and record the results I see, and then try to hash it all out by studying and comparing the results later---if I can. During this process, I do try and anticipate the results with the next step in the process and I try to understand cause and effect better as I go. I'm just a guy that likes to piddle and compare.

Since you just moved, how are you going to compare your antenna results if you make a big change? Like you say, this is a one install event and you want as much assurance as you can that the antenna is right the first time. What if it don't work out?

BTW, instead of wasting the cuts of 8 x 12' foot radials, why don't you use tappering a little and save some material. You could go bigger than 5/16th, which is very thin for 9' lengths in the horizontal.

 

[BOOTY MONSTER]

"BM, if this is what you recall, then I must have said it, but that is not exactly how it happened."

i could certianly have gotten confused after our conversation marconi . you shared a lot of information and gave me some good pointers to keep in mind . i apologize if i got confused or miss understood something .


"Since you just moved, how are you going to compare your antenna results if you make a big change? Like you say, this is a one install event and you want as much assurance as you can that the antenna is right the first time. What if it don't work out?"

i wont be able to compare to my previous install , itll be impossible like you mentioned due to the different location and mounting height . i know of several cbers with base setups near this area so ill be comparing my ears to them and if i can hear %95 of everything they can ill assume all is well . from what i understand rx and tx of antennas is usually reciprocal , but the truth is in the doing .

ill tune it on the ground and probally talk on it a few days there . i did that at the other house and even 8 ft off the ground it did as good if not slightly better than the 1/4 wave above the roof , once it was up on the house there was a very noticable difference . so i figgure if i can get half assed pefrormance near the ground here that itll really come to life once its above the house like befor .

so no comparison


"BTW, instead of wasting the cuts of 8 x 12' foot radials, why don't you use tappering a little and save some material. You could go bigger than 5/16th, which is very thin for 9' lengths in the horizontal."

i dont know what you mean by "tappering" . i went to lowes yesterday and looked at their aluminum rod again to get an idea of the diameters and stiffness of the material . the 3/8 is very very stiff and i think its gonna be the absolute largest diameter ill want to try to mess with . even the 1/4 inch is larger and has more surface area than the 12 gauge wire i used befor . but i dont think its stiff enough to not get bent if a good size bird landed on the end of it .

 

[swanny]

good thread booty. Lots of info I was looking for about the homebrew sirio clone idea. Think I'm gonna buy the vector tho and homebrew a multiband dipole as my first antenna project sometime in the near future. Then, after some serious learning, I might dive back into a sigma iv homebrew project. As was said earlier, seems like every little thing about this antenna makes a difference, and that's just too many variables for my limited knowledge at this point. So much information on this forum

 

[Marconi]

i could certianly have gotten confused after our conversation marconi . you shared a lot of information and gave me some good pointers to keep in mind . i apologize if i got confused or miss understood something .

Not a problem Jeff, I just wanted to make the point that I haven't changed my mind about adding radials. I'll have to give you another short story one day on how I see radials working and what adding radials can do.

i wont be able to compare to my previous install , itll be impossible like you mentioned due to the different location and mounting height . i know of several cbers with base setups near this area so ill be comparing my ears to them and if i can hear %95 of everything they can ill assume all is well . from what i understand rx and tx of antennas is usually reciprocal , but the truth is in the doing .

Yep, it would be difficult to compare and develop any meaningful results between different locations. Some would rightfully argue we can't even compare different antennas at different locations on the same residential property, so moving across town and comparing would be a stretch.

ill tune it on the ground and probally talk on it a few days there . i did that at the other house and even 8 ft off the ground it did as good if not slightly better than the 1/4 wave above the roof , once it was up on the house there was a very noticable difference . so i figgure if i can get half assed pefrormance near the ground here that itll really come to life once its above the house like befor .

so no comparison

Jeff my thoughts here were to install your old antenna at the new location, even if you had to do it low to the ground, and then record some signals for how it worked at the new place---before you changed the construction to a new antenna. This way you would have something to compare too, even if it is arguably not the best way. You know, see if the new modifications improved the antenna or not.

i dont know what you mean by "tappering" . i went to lowes yesterday and looked at their aluminum rod again to get an idea of the diameters and stiffness of the material . the 3/8 is very very stiff and i think its gonna be the absolute largest diameter ill want to try to mess with . even the 1/4 inch is larger and has more surface area than the 12 gauge wire i used befor . but i dont think its stiff enough to not get bent if a good size bird landed on the end of it .

Forget about the ideas of tappering to help save material and make the ground plane adjustable. I thought you were talking about using tubing, but maybe you're shopping for aluminum rods to make your radials.

Good luck,

 

[bob85]

based on past experience , some reading and asking cebik, i doubt you will see much improvement playing with radial angle,
adding or removing 1/8wave radials on a sirio 827 effects the tuning, i would expect you to find the same,
i have read different ideas on radials, some say once the antenna is well clear of earth more than 4 is overkill while others claim a rapid onset of diminishing returns after 4,
eddie seems to get better results with his marconi when adding radials,

the ground conductivity in your area may effect which stup works best for you,

in my experience id go for 4 -6 horizontal 1/4wave radials, the longer radial antennas have in the past and present outperformed no radial or short radial antennas for me even when using the same style matching,
compare an old 5/8wave skipmaster with its 4 x 1/4wave radials against the contemorary 5/8waves with 3ft or shorter radials, or when the matching differes for instance the i10k outperforms the 827 in our tests, not sure how much of that is better decoupling from full size radials and how much is the matching arrangement,

i don't know how they would compare if both antennas were properly isolated.

if more radials is better then the saturn with its 9 x fat 1/4wave radials no coils ect should be the dogs bollocks groundplane.