The Gamma match is the often used as a matching device to tune the SWR in a Yagi.
Why a Gamma-match ? :
Although im a fan of “direct” 50 Ohms Yagi’s.
There are many Yagi designs and commercial available versions which depend on a gamma-match to provide a 50 Ohms impedance.
Gain, bandwidth, F/B etc, all relate to each other. It is hard to find design a yagi where all values are the way would like them to have on the design frequency. Besides near by obstacles will have influence on them too.
From the “old” days the ideal impedance of a yagi was considered to be around 20..25 Ohms. (this isnt necessarily true now a days though).
Besides from a manufacturer point of view, it is a bit of a “safety margin” to have a gamma-match as the manufacturer can always count on it to “tune” the yagi if maybe it isnt perfect assembled by the costumer, or it was an early Monday morning fabrication hi.
A Gamma-match can match impedance below 50 ohms right up to that 50 ohms wich your transceiver wants to see.
HOW TO CONSTRUCT :
The described Gamma-match on this page is used for 11 meter Yagi’s with an Aluminium element thickness around 25mm !
The diameter of the gamma-rod should be around 1/2 the size of the diameter of the radiating element. The length of the gamma-rod should be in the order of 0.05 wavelength long.
(ps I am however confident that with slight adjustments it will work for other diameters)
F is an aluminum plate in an L- shape, and should be about 0,5 cm thick and wide enough to hold a N-connector female (chassis) or the equivalent PL version. The “L” shape is needed, so you can attach it to one of the U-bolts holding the radiating element to the boom. The hart of the connector should be 10 cm from the hart of the radiating element
A the N- connector female or the PL version
!!! is between the two aluminium tubes B and C must be NON-electrical guiding material for example:
the “plastic” used in coax-cable.
The length of it must be a bit longer then the length given “G” !!!
C The long aluminum tube about 1,3 cm thick and 70 cm long.
D An aluminium plate drilled with three holes 1) the driving element
2) the tube D
3) On top a screw to hold it in place.
The holes should be from center to center 10 cm separated
E the radiating element also called driven element.
B Aluminum tube with a length of 13,5 CM diameter 1,8 CM (C and D goes into it)
By adjusting the aluminum plate left or right, you should be able to get a low SWR.
This Gamma-match can handle up to a couple KW’s.