For that matter, did I mention magnets? Magnetrons need magnets! The magnetic field causes the electrons emitted by the hot cathode in the center of the tube to travel in circular orbits on their way out to the circular can shaped anode. The ,(generally), iron-cored magnet required is the reason your microwave oven is as heavy as it is. Which brings us back to Yagi. There's a picture of the magnet he used in his microwave transmission research below. The magnet is the bulky looking cylindrical shaped object in the back.
For the non-ham radio initiated, a Yagi Uda antenna is a type of radio antenna developed by Drs. Yagi and Uda in the 1920s that directs radio frequency radiation into a beam. Here's a picture of +Diana Eng with her homemade Yagi antenna for transmitting ham radio signals via satellite.
Yagi and Uda had discovered that by placing appropriately spaced and sized metal elements around their antenna, they could make it more sensitive to radio waves from a given direction. They called this building a 'wave canal'. See the excerpts below for Yagi's explanation.
In addition to pretty completely characterizing their new antenna design, Yagi also came up with a formula for the frequency of microwave radiation emitted by a cyclotron type magnetron tube
1. Yagi on microwaves and antennas
2. Build your own Yagi antenna
Yagi H. (1928). Beam Transmission of Ultra Short Waves, Proceedings of the IRE, 16 (6) 715-740. DOI: 10.1109/JRPROC.1928.221464
3. Track amateur radio satellites
4. Esteemed discussion of the Yagi antenna
(1928). Discussion, Proceedings of the IRE, 16 (6) 740-741. DOI: 10.1109/JRPROC.1928.221465