How can two linearly polarized Yagi antennas be used to produce circular polarization?

To produce circular polarization from linearly polarized antennas, you must create two electromagnetic fields that are physically perpendicular to each other and electrically 90 degrees out of phase. This is achieved by mounting two Yagi antennas on the exact same boom axis but rotated 90 degrees apart (so the elements form an 'X' or '+' shape). Their driven elements must be co-located at the same point on the boom. Finally, you feed the antennas with signals that are 90 degrees out of phase, usually by using a quarter-wavelength delay line on one of the feedlines. Stacking Yagis in parallel planes or feeding them in phase only results in linear polarization with higher gain or a different pattern. The combination of a 90-degree physical offset and a 90-degree electrical phase shift is absolutely required to make the combined electric field rotate.