New for July 2006! Still
New for February 2009! Magnetrons
now have their own page with expanded content.
Microwave tubes may seem "old
school" for 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
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
Developed as a microwave source
for radar during WWII. Cheaply produced magnetrons like the one
below are used in your microwave oven! On this new
page, the Unknown Editor attempts to explain how a magnetron
works, to a fifth grade class!
"Magnetron" is the
combination of the words "magnetic" and "electron"
and is an example of portmanteau.
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:
You can find a link to 706FY
on the page, here it is:
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
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.
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! This is an an example