GAMMA'S

[bob85]

is there a calculation to determine the best dimentions for an efficient gamma??,
i am going to rebuild my hybrid and would like a better gamma,
i can get teflon tubing and aluminum but i want an efficient gamma?

thanks

 

[Master Chief]

an efficient gamma??

That's what I say Bob......an efficient gamma?

Although I've taken heat for saying this in the past, it doesn't mean that I'm wrong. A high power gamma used in a low power setting will experience more loss than an appropriately sized gamma. So, what is your maximum power output going to be?

Any gamma will be lossy when compared to other type of feed systems.

But that's enough about the loss factors of a gamma for now. Why don't we try to find an answer to your question!

Here is text from Chaper 26 of the current ARRL Antenna Book:

Calculating Gamma Dimensions
A starting point for the gamma dimensions and capacitance value may be determined by calculation. H. F. Tolles, W7ITB, has developed a method for determining a set of parameters that will be quite close to providing the desired impedance transformation. (See Bibliography at the end of this chapter.) The impedance of the antenna must be measured or computed for Tolles’ procedure. If the antenna impedance is not accurately known, modeling calculations provide a very good starting point
for initial settings of the gamma match.

The math involved in Tolles’ procedure is tedious, especially if several iterations are needed to find a practical set of dimensions. The procedure has been adapted for computer calculations by R. A. Nelson, WBØIKN, who wrote his program in Applesoft BASIC (see Bibliography). A similar program for the IBM PC and compatible computers called GAMMA is included on the disk bundled with this book, in BASIC source code, with modifications suggested by Dave Leeson, W6NL. The program can be used for calculating a gamma match for a dipole (or driven element of an array) or for a vertical monopole, such as a shunt-fed tower.

As an example of computer calculations, assume a 14.3-MHz Yagi beam is to be matched to 50-Ω line. The driven element is 11/2 inches in diameter, and the gamma rod is a length of 1/2-inch tubing, spaced 6 inches from the element (center to center). The driven element has been shortened by 3% from its resonant length. Assume the antenna has a radiation resistance of 25 Ω and a capacitive reactance component of 25 Ω (about the reactance that would result from the 3% shortening). The overall impedance of the driven element is therefore 25 − j 25 Ω. At the program prompts, enter the choice for a dipole, the frequency, the feed-point resistance and reactance (don’t forget the minus sign), the line characteristic impedance (50 Ω), and the element and rod diameters and centerto-center spacing. GAMMA computes that the gamma rod is 38.9 inches long and the gamma capacitor is 96.1 pF at 14.3 MHz.

As another example, say we wish to shunt feed a tower at 3.5 MHz with 50-Ω line. The driven element (tower) is 12 inches in diameter, and #12 wire (diameter = 0.0808 inch) with a spacing of 12 inches from the tower is to be used for the “gamma rod.” The tower is 50 feet tall with a 5-foot mast and beam antenna at the top. The total height, 55 feet, is approximately 0.19 λ. We assume its electrical length is 0.2 λ or 72°. Modeling shows that the approximate base feed-point impedance is 20 − j 100 Ω. GAMMA says that the gamma rod should be 57.1 feet long, with a gamma capacitor of 32.1 pF.

Immediately we see this set of gamma dimensions is impractical—the rod length is greater than the tower height. So we make another set of calculations, this time using a spacing of 18 inches between the rod and tower. The results this time are that the gamma rod is 49.3 feet long, with a capacitor of 43.8 pF. This gives us a practical set of starting dimensions for the shunt-feed arrangement.

You can download a GAMMA program here:
http://www.ve3sqb.com/

 

[bob85]

thanks chief ,
heat lol dont take it to heart
i will look at the prog when i am awake,

power lets say 2kw,

efficient as in low dielectric losses and propper gamma spacing if that is an issue,
i certainly aint going to use garden hose or petrol pipe as i have seen locals do in the past,
i will find the best tapping point through trial and error as usual,
maybe the sirio gamma will do but if theres something better i will try and make it.

 

[Master Chief]

maybe the sirio gamma will do but if theres something better i will try and make it.

I'll be uploading pictures of the Jo-Gunn gamma today! I'll include dimensions. Here is a sample:

Check out the rest of the pictures here:

http://www.worldwidedx.com/gallery/album11

 

[bob85]

nice pics chief,
i like the look of it, the gamma and strap look much better than mine,

i intend to beef up my hybrid extending it down rather than adding 2 extra sections above the original avanti radiator as i did before,
i want to use scaffold tube for the base section and 2 box sections with welded brackets one moveable for the spreaders so i can play with leg angles and one fixed for the gamma + 4 basket legs,
this would stop any base section flex so the gamma does not move in the wind and make mounting to a scaffold stub mast very simple and strong,

thats why i am looking at better gammas and i also need to understand what effect making the base section just over 2" diameter will have on the gamma spacing,

the strap on the jogunn is the type i would like but it needs to fit around a fatter tube,

as usual i will find the best tapping point/antenna length by trial and error,

the only downside i can see at the moment is the extra weight,


ps
when you assemble that make sure you put the legs the correct way up :twisted:

 

[bob85]

chief i had a cursory look at the gamma prog and saved it for future reference ,

it would seem my problem is going to be the fact that every gamma i can find is set parallel to the driven element and diameter ratio/spacing does matter whereas the avanti design is set at an angle,
since i will be stepping up the radiator diameter a fair bit this i would imagine has a bearing on what would be an optimal gamma arrangement too :?

the jogunn you are rebuilding looks like it is also parallel?

now you have me wondering if there is any benefit/dowside to the avanti arrangement