Some of the newer designed Yagi have an impedance different than 50 ohms. On this site there are antennas with an impedance of 28 ohms. These can be matched using conventional methods or you can just use two parallel 1/4 wavelength of 75 ohm coax. This will transform the 28 ohm impedance to 50 ohm impedance good for your RG-8.
Matching 28 ohm design antennas to 50 ohm coax
Whenever you stack antennas the impedance will change (two 50 ohms antennas will give 25 ohms). If you stack two 50 ohm antennas you can match them by transforming the 50 ohms at the antenna to 100 ohms at the coax feed point (2 - 100 ohms in parallel gives 50 ohms at the feed point). You have to use odd number of 1/4 wavelengths to use this method. A lot of antennas are stacked at 1 wavelength apart so the 1/4 wavelengths will be too short (you will need to use 3/4 of a wavelength). Remember when you are making matching section to use the velocity factor of the cable.