Skip to main content

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:


Popular posts from this blog

Cool Math Tricks: Deriving the Divergence, (Del or Nabla) into New (Cylindrical) Coordinate Systems

The following is a pretty lengthy procedure, but converting the divergence, (nabla, del) operator between coordinate systems comes up pretty often. While there are tables for converting between common coordinate systems, there seem to be fewer explanations of the procedure for deriving the conversion, so here goes!

What do we actually want?

To convert the Cartesian nabla

to the nabla for another coordinate system, say… cylindrical coordinates.

What we’ll need:

1. The Cartesian Nabla:

2. A set of equations relating the Cartesian coordinates to cylindrical coordinates:

3. A set of equations relating the Cartesian basis vectors to the basis vectors of the new coordinate system:

How to do it:

Use the chain rule for differentiation to convert the derivatives with respect to the Cartesian variables to derivatives with respect to the cylindrical variables.

The chain rule can be used to convert a differential operator in terms of one variable into a series of differential operators in terms of othe…

Lab Book 2014_07_10 More NaI Characterization

Summary: Much more plunking around with the NaI detector and sources today.  A Pb shield was built to eliminate cosmic ray muons as well as potassium 40 radiation from the concreted building.  The spectra are much cleaner, but still don't have the count rates or distinctive peaks that are expected.
New to the experiment?  Scroll to the bottom to see background and get caught up.
Lab Book Threshold for the QVT is currently set at -1.49 volts.  Remember to divide this by 100 to get the actual threshold voltage. A new spectrum recording the lines of all three sources, Cs 137, Co 60, and Sr 90, was started at approximately 10:55. Took data for about an hour.
Started the Cs 137 only spectrum at about 11:55 AM

Here’s the no-source background from yesterday
In comparison, here’s the 3 source spectrum from this morning.

The three source spectrum shows peak structure not exhibited by the background alone. I forgot to take scope pictures of the Cs137 run. I do however, have the printout, and…

Unschooling Math Jams: Squaring Numbers in their own Base

Some of the most fun I have working on math with seven year-old No. 1 is discovering new things about math myself.  Last week, we discovered that square of any number in its own base is 100!  Pretty cool!  As usual we figured it out by talking rather than by writing things down, and as usual it was sheer happenstance that we figured it out at all.  Here’s how it went.

I've really been looking forward to working through multiplication ala binary numbers with seven year-old No. 1.  She kind of beat me to the punch though: in the last few weeks she's been learning her multiplication tables in base 10 on her own.  This became apparent when five year-old No. 2 decided he wanted to do some 'schoolwork' a few days back.

"I can sing that song... about the letters? all by myself now!"  2 meant the alphabet song.  His attitude towards academics is the ultimate in not retaining unnecessary facts, not even the name of the song :)

After 2 had worked his way through the so…