BBC Focus Magazine

[M Paul Lloyd]

A quick overview.
For many years now I have worked with microwave equipment for the defence industries, mostly satellite telecommunication and radar systems but the principle behind the magnetrons used in these applications is much the same as you would find in your average microwave oven. Amazingly the people that ‘we’ make hardware for really don’t know everything about how microwaves do what they do. So a bit like electricity and gravity, both of which we use but do not fully understand, so microwave ‘just works’ and so we use it whilst the boffins try to figure out why. Some of our standard designs such as the cavity magnetron and wave guide conduit have remained much the same since they were first designed in the 1940’s. Indeed a radar technician from WW2 would recognise almost everything in many modern day systems. The magnetron generates a specific wave form that is emitted into, and then literally bounces along, a rectangular metal duct before going its merry way. Microwave can be used to heat all manner of substances by exciting the molecules that the substance is made of. It is not just food that can be heated and recent experiments have shown it to be very efficient at smelting steel. Very short wave length microwave, as used in your average microwave oven, is not much use for telecoms and radar as it has a painfully short range but I am unaware of anything that could, or even would, be used to ‘focus’ a wave form of either long or short frequency, rather it just hurtles out at light speed inside the oven and excites the molecules it encounters. Having two magnetrons allows two ‘beams’ to be directed into the oven which would be a bit more efficient rather than having one at full power and some even have rotating deflectors at the end of the wave guide to avoid the need for a turntable, although this is often very inefficient. If you look at the big horn shaped things on the side of the Old post Office tower they are microwave emitter/receivers and the radar dish at your local airport will have a similar arrangement that channels the wave form into a section of rectangular waveguide. The Sky dish for satellite TV is a bit more advanced, as it uses cable technology but works on much the same principle although many military systems still use wave guide as more information can be extracted using some rather clever software.

Components.
A magnetron is a diode-type electron tube which differs from a normal electron tube (think of an old style cathode ray tube TV) as it has no grid. Instead a magnetic field within the space between the Anode and the Cathode serves as the grid. Although external configurations of various Magnetrons will differ, the basic internal structures remain the same, which includes the Anode, Filament-Cathode, Antenna, and Magnets.

The Anode, is a hollow Ferrite cylinder from which extends an even number of inward pointing Anode vanes rather like spokes of a wheel. The open trapezoidal or cloverleaf shaped voids between each of the vanes are the ‘resonant cavities’ that act as tuned circuits and the output frequency of the tube is determined by their size and shape of these cavities. The So that the anode can function correctly alternate segments must be bonded, or connected together so that each segment is opposite in polarity to its neighbouring segment. This means that the cavities are connected in parallel to the actual output.

The Filament, also serves as the Cathode of the tube, and is located in the centre of the magnetron, supported by the large and rigid filament leads, being carefully shielded in a sealed tube.

The Antenna, is connected to the anode by a lead that extends into one of the tuned cavities. The Antenna being placed inside a waveguide, a hollow, rectangular section, metal conduit, along which the Microwave energy is transmitted.

The Magnetic Field, is provided by strong circular magnets, which may be permanent or electro-magnets depending on the application. These are mounted concentrically around the magnetron so that the resultant magnetic field is parallel to the axis of the cathode.

The theory behind the operation of a Magnetron is based on the motion of electrons under the influence of electro-magnetic fields. For the Diode Electron Tube to operate correctly, Electrons must flow from the Cathode to the Anode in a controlled manner, and there actual trajectory is governed by two fundamental laws.

1. The force exerted by an electric field on an Electron is proportional to the strength of the field.
This means that Electrons are inclined to move from a negative potential towards a positive potential by the shortest possible route.

2. The force exerted on an electron within a magnetic field is at right angles to both the path of the Electron and the field itself.
The resultant force imposed on the Electron causes it to travel to the anode in a spiralling trajectory rather than by a direct path.
Effect of the Magnetic Field.
Assume the upper magnet of the magnetron is a North Pole and that you are looking down on it. With the lower, South Pole magnet being located underneath the magnetic field would appear to be coming towards you.

Now just as Electrons flowing through a conductive material cause a magnetic field to build up around that conductor, so an Electron moving in space will create a magnetic field around itself. On one side of the electron's path this self induced magnetic field adds to the magnetic field already surrounding it. On the other side of its path, it cancels out the equivalent amount of magnetic field. The magnetic field on one side is therefore weakened, and the Electron's trajectory becomes bent in that direction, resulting in a spiralling motion of travel to the Anode. This process commences with a low voltage (usually in the region of 3-4volts AC) being applied to the Filament (which is also the Cathode) causing it to heat up. This rise in temperature induces greater molecular activity of the cathode and so it begins to emit Electrons. At first these Electrons float just above the surface of the Cathode but being negatively charged they are strongly repelled by other negative charges and so become repelled by the negatively charged Cathode. The distance and velocity of their travel would increase in proportion to the intensity of the applied negative charge and momentum is induced by means of a negative DC voltage in the region of 4,000 to 5,000 volts so they move quite quickly.

This rotating cloud of Electrons, under the influence of the high voltage and strong magnetic field, form a pattern resembling the spokes in a spinning wheel. The interaction of the rotating charged particles with the surface of the Anode causes an alternating current to flow in the Resonant Cavities of the anode. This happens because as a ‘spoke’ of Electrons approaches a segment of the anode, it induces a positive charge to that segment. As the electrons travel from one segment to another the positive charge falls off in the first segment while another positive charge is being induced in the following segment, and so on. The physical shape of the Anode therefore dictates the frequency of the emitted microwaves which are then ‘channelled’ along a rectangular metal conduit to wherever they are needed.
Truly the roughest of sketches from one of my old and rather tatty note books.

And a few photos.




M Paul Lloyd© 2003