Microwaves tubes

Updated August 21, 2006

New for July 2006! Still very thin!

Here we will get a start on discussing microwave tubes. Microwave tubes may seem "old school" for you college whippersnappers, but one tube can do the work of many, many solid state power amps. The tube industry is alive and well, but there is not a great number of engineers in this field. If you are thinking about making a career out of tubes, this could work out, because the average age of a real tube engineer is somewhere around 90!

For a great book on microwave tubes, get a copy of Electronic Tubes by A. S. Gilmour (Artech Press). Check it out on our book page! Much of the material on this page uses this outstanding book as a reference.

Tubes differ from solid state devices in that they operate using thermionic emission, not weird semiconductor physics. They are called vacuum tubes for a reason, thermionic emission only works in the absence of air.

If you are imagining that microwave tubes look similar to the amplifier tubes in your grandfather's radio, you are wrong. Microwave tubes have special features such as resonant cavities that usually can't be built from glass because complex-shaped objects would not stand up to the pressure of holding a vacuum.

Gridded tubes

The classic gridded amplifier tubes are the triode and tetrode. Gridded tubes typically crap out before microwave frequencies, because the dimension from cathode to grid starts to really matter, with respect to the small wavelength at microwave frequencies. Grid to cathode spacings of just thousands of an inch are required, which makes a nightmare to manufacture, let alone presenting a big heat problem.

Crossed field tubes

M-type tubes (this term originated in France)

Magnetron

Developed as a microwave source for radar during WWII. Cheaply produced magnetrons like the one below are used in your microwave oven!

Here's a photo of an old tube that someone named Dick sent us to identify, it's dated 1943. It's a 906FY magnetron tube. We had some help in identifying it from this German web site:

http://www.hts-homepage.de/WesternElectric/WesternElectric700.html

You can find a link to 706FY on the page, here it is:

http://www.hts-homepage.de/WesternElectric/WE706BY.html

Linear beam tubes

In a linear beam tube, the electron beam travels in the same direction as the magnetic field. Linear beam tubes are often called O-type (the designation originated in France).

Traveling wave tubes

The TWT was invented by Rudolf Kompfner in 1942 in England, for this reason he appears in our Microwave Hall of Fame. The classic tube uses a helix, but coupled cavity are also used.

RF energy travels down the helix, at nearly the same speed as the electrons that are traveling from the cathode to the collector. This causes the electron beam to bunch, which in turn amplifies the signal traveling down the helix. Trust us, this is a very cool concept, but we don't have time right now to draw a diagram so come back later!

The TWT is often used in lab test equipment to generate signals up to 100 watts.

Klystrons

The klystron was invented by the Varian brothers in 1939, they are in our Microwave Hall of Fame. The word "klystron" is derived from the Greek word "klyzo", which refers to waves breaking on a beach. We learned this from Gilmour's book!