Ham Radio Forum

Hello,
I have made a 4 element Yagi with a straight dipole and centre feed. I have a 50 ohm receiver. A straight dipole has around 73 ohm impedance and it will be still lower in case of 4 element Yagi because of added elements. I have calculated the impedance to be about 25-30 ohms. How do I match this impedance with the receiver of 50 ohms? Is it possible with the help of balun? if yes then what type of balun will be required?

thanks
trupti

There is a very simple solution to that problem. Any value of load impedance can be transformed into any desired feed impedance by means of a quarter-wave section of transmission line that has a characteristic-impedance equal to the square root of the product of the other two impedances.

If you make a 37.5-ohm quarter-wave section of transmission line composed of two 75-ohm quarter-wave lengths of transmission line connected in parallel, and if you then insert that quarter-wave section of 37.5-ohm line between the Yagi antenna feed terminals and the 50-ohm transmission line going to the radio, it will transform 28.125 ohms to 50 ohms, because (37.5 * 37.5) / 50 = 28.125 ohms.

Note that the quarter-wave matching section must be quarter-wave-resonant to perform as described, so its length must be equal to the velocity-constant of the transmission line used in matching section times the free-space quarter-wavelength.

-Bob

thank you very much for the reply