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!








References:
1.  Yagi on microwaves and antennas
http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=1670051&tag=1

2.  Build your own Yagi antenna
http://blog.makezine.com/2010/05/01/collapsible-fabric-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
http://copaseticflows.appspot.com/findsat

4.  Esteemed discussion of the Yagi antenna
http://dx.doi.org/10.1109%2FJRPROC.1928.221465
(1928). Discussion, Proceedings of the IRE, 16 (6) 740-741. DOI:

Comments

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…

The Valentine's Day Magnetic Monopole

There's an assymetry to the form of the two Maxwell's equations shown in picture 1.  While the divergence of the electric field is proportional to the electric charge density at a given point, the divergence of the magnetic field is equal to zero.  This is typically explained in the following way.  While we know that electrons, the fundamental electric charge carriers exist, evidence seems to indicate that magnetic monopoles, the particles that would carry magnetic 'charge', either don't exist, or, the energies required to create them are so high that they are exceedingly rare.  That doesn't stop us from looking for them though!

Keeping with the theme of Fairbank[1] and his academic progeny over the semester break, today's post is about the discovery of a magnetic monopole candidate event by one of the Fairbank's graduate students, Blas Cabrera[2].  Cabrera was utilizing a loop type of magnetic monopole detector.  Its operation is in concept very simpl…

Kids R Kapable

Just a little note to concerned ‘grownups’ everywhere.  If you look at a kid—and I mean really look—I don’t mean notice a person shorter than you, I mean make eye contact, notice their facial expression and observe their body language—If you look at a kid, don’t assume they need your help unless they’re obviously distressed, or ask for it.  You might think this is difficult call to make.  You might think, not having kids of your own, that you’re unable to make this determination.  You are.  You do in fact, already have the skills even if you’ve never been around kids  It’s a remarkably simple call to make, just use the exact same criteria you would for determining if an adult was in distress.  Because, guess what, kids and adults are in fact the same species of animal and communicate in the same way.  Honest.  If someone—adult or child—doesn’t need your help, feel free to say hello, give a wave, give a smile, but don’t—do not—try to force help on anyone that doesn’t want or need it.



Y…