Wednesday, March 20, 2013

The Magnetron Part II: Did I Mention Yagis?

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.

The next reference I foudn in the MIT Radiation Labs microwave magnetron handbook was to Yagi[1].  For the ham radio foks, yes, that Yagi!  The Yagi of Yagi-Uda beam antennas.  The handbook mentioned that whhile the cyclotron magnetrons of the type discussed yesterday were  enerally 'feeble in their output abilities, some people like Yagi had put them to fruitful use.

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[2] for transmitting ham radio signals via satellite[3].

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

The importance of the new type of 'beam' antenna was immediately recognized as pointed out immediately following Yagi's article by the discussion written by J. H. Dellinger: Chief of Radio Division, Bureau of Standards, Washington, D. C. [4].

Finally, for today, I leave you with the antenna rig used to test the Yagi.  Want to find out the elevation of your radio wave beam?  Get a crane!

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:

3.  Track amateur radio satellites

4.  Esteemed discussion of the Yagi antenna
(1928). Discussion, Proceedings of the IRE, 16 (6) 740-741. DOI:

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