[Owen Hill]

Owen Hill  asked the Naked Scientists:

Hi Chris.

My name is Owen from Toronto, Canada.  My question is:

How does a microwave convert electricity to microwaves?

Love the show!

Owen

What do you think?

[lyner]

They have a device inside them which was developed for RADAR in the Second World War. It's called a Magnetron.
see http://www.thenakedscientists.com/forum/index.php?topic=18533.0

[teragram]

Basically a Magnetron is a high power amplifier. A microwave cooker is similar to a high powered radio transmitter, the energy from which is broadcast via the antennae. In the cooker, the antennae is inside the box, all the energy is released in the box and absorbed by whatever is being cooked.

[Pumblechook]

A Magnetron is not really an amplifier..  It is self oscillating and produces an unstable (frequency wise) output.   Travelling wave tubes and klystrons use similar technology and can amplifier a stable and maybe modulated signal although klystrons do have a self oscillating version called a reflex klystron.  I have never worked with magnetrons but I have with klystrons up to 40 kW. 

[lyner]

Show me the amplifier that is not. potentially, an oscillator and the oscillator which is not an amplifier. If there were no amplification, there would be no oscillation. Have you never constructed an amplifier which, when you turned your back, would not  oscillate or, at least, squeg.?
The magnetron is just like a klystron where the beam goes in a loop, passing the cavities again and again.
It's horses for courses - I, like you, have only dealt with Klystrons (UHF TV) but the theory of bunching and amplification is just the same. Linearity is not a problem with magnetrons as it is with Klystron amplifiers.

[Bored chemist]

My Grandfather's clock was too tall for the shelf so it stood ninety years on the floor...
It was an oscilator but the amplitude of the pendulum was maintained by the power provided by a falling weight.
A pendulum without the rest of the clock is an oscilator but, since it has no external power source, it can't be an amplifier in any traditional source.

Anyway, say I have a weak 2GHz signal. Can I amplify it with the 2.4GHz magnetron in my microwave oven?

[Pumblechook]

Magnetrons can't amplifier a low level signal.  That is the point.   Klystrons can.  Travelling wave tubes can.  Solid state devices can but the output power is low and maybe always will be. 

[lyner]

Magnetrons can't amplifier a low level signal.

That's because no one gives them an input connection! The noise in the electron beam contains sufficient 'signal' to get it going.
If you arranged the geometry a bit different, there would be insufficient closed loop gain for it to oscillate - which would not be much good for your TV dinner - but you could inject a signal into one of the cavities and get a bigger signal out. Just like a rather bad Klystron.

Our shelf is big enough for my grandfather's microwave.
"Ninety seconds without slumbering" rather than ninety years.

[Bored chemist]

OK, how do you connect a signal in to a magnetron?

[Pumblechook]

You straighten it out and call it a Klystron.

[lyner]

You put a loop into one of the cavities - just like in the output cavity. I bet it's been done.