His idea was to receive ADS-B signals with his Yagi and a dipole antenna, then compare the data received in order to determine in which directions the Yagi receives better than the dipole. To do this he first creates a standard 2D map of plane tracks collected over a 24hr period for both the dipole and Yagi. A gaussian blur is applied to the two maps in order to fill in blank space and the data is normalized. Then he simply subtracts the dipole plot from the Yagi-Uda plot. The resulting difference plot reveals a mapping of where the Yagi receives better or worse compared to the dipole in a 2D plane.
However, SWR is only partial information and tells us nothing about the actual gain and directivity / radiation pattern of the antenna. The radiation pattern tells us in which direction the antenna receives and radiates power best from. For a Yagi, we would expect the best reception gain to come from the front, with much less gain on the sides and rear.
To set up the radiation pattern measurement, Jephthai connected the Yagi to the TX port of the NanoVNA via a long coax cable, and connected an omnidirectional whip antenna to the RX port of the NanoVNA. The NanoVNA and Yagi are separated by a reasonable distance of 18' to ensure that the far-field radiation pattern is measured instead of the near-field pattern. He then measures and collects the S21 reading over multiple rotations of the Yagi.
The data is then plotted revealing a two dimensional radiation pattern for the Yagi. As expected gain is highest in the front, and weaker on the sides and rear. Jephthai notes that the radiation pattern mostly matches what the MMANA antenna modelling software predicted too.