2m 4 element yagi, matching system question

[KE5SWU]

I am building my first yagi and I have used K7MEM's modeling web site to get the dimensions.

Boom Diameter - 1"
All Elements 1/4" diameter
All aluminum bonded boom and elements

Cumulative Length, Element, Element Length
0, REFL, 39-13/16"
16-3/16", D.E., 39-1/16"
22-1/4", D1, 36-3/8"
36-13/16", D2, 36-1/16"

With a bonded system will the gamma match work? It appears to be the correct choice. But how does one define the correct lengths for the matching element?

Thank you.
James

[VK3NEA]

The gamma should be abt 0.04 -0.05 of a wave length long.
It's dia 1/3 to 1/2 that of the driven el.
Approximately 0.007 of a wave length center - center from the driven element
hope this help's
Alan.

[KE5SWU]

Thank you, off to the garage I go. I will take some pictures and post up the results. Just to clarify when you say the diameter needs to be 1/2 to 1/3 of the driven element, is that the diameter of wire used inside the matching tube or the OD of the tube itself? On my 13b2 the elements are the same diameter.

[KM3F]

For the gamma match, go to a hardware store that has alum stock.
Get a length of 1/4" alum rod, 1' of clear tubing that will slip over the rod and inside the chosen outer gamma rod.
This becomes your tuning assembly along with a clamp/strap at the outer end.
The gamma need only be about 6 to 8" long and is not critical.
Test and adjust to lowest VSWR by moving the rod in and out and moving the clamp/strap as needed then trim off the excess.
Start with all the elements at their design spacing and length.
You should find the reflector and first director to be the most sensitive to adjustment.
The gamma match is working as an equivelent to a one turn loop in series with a capacitor. This is what you are tunning into resonance at 50 ohms. It just so happens that a 1/2 wave driven elment is ((part of that loop)) and so gets excited by the RF power applied by also being a resonant length.
The rest of the elements do their thing by coupling additive and subtractive phase signals that shape the pattern mainly in the plane of the boom foward toward the smaller elements.
That's all there is as to how it basicly works.
Good luck.

[KE5SWU]

For the gamma match, go to a hardware store that has alum stock.
Get a length of 1/4" alum rod, 1' of clear tubing that will slip over the rod and inside the chosen outer gamma rod.
This becomes your tuning assembly along with a clamp/strap at the outer end.
The gamma need only be about 6 to 8" long and is not critical.
Test and adjust to lowest VSWR by moving the rod in and out and moving the clamp/strap as needed then trim off the excess.
Start with all the elements at their design spacing and length.
You should find the reflector and first director to be the most sensitive to adjustment.
The gamma match is working as an equivelent to a one turn loop in series with a capacitor. This is what you are tunning into resonance at 50 ohms. It just so happens that a 1/2 wave driven elment is ((part of that loop)) and so gets excited by the RF power applied by also being a resonant length.
The rest of the elements do their thing by coupling additive and subtractive phase signals that shape the pattern mainly in the plane of the boom foward toward the smaller elements.
That's all there is as to how it basicly works.
Good luck.


Thank you, its a wonderful day today in the mid 70's and I will take your information out to the backyard this evening and try to get this antenna to tune up. Thank you again.
-James, KE5SWU