Field Distribution inside a Microwave Oven.

It is surely interesting to get an impression of the field distribution of Electromagnetic waves inside a microwave oven. At a conductive wall the electric field along the wall will be very small. A quarter of a wavelength in front of the wall the field could be very strong if it is not parallel to another wall and a multiple of half a wavelength apart from this second wall.

A pattern may come up with a spacing of half a wavelength between maxima. With a velocity of 300000km/s electromagnetic waves of 2,5GHz have a wavelength 120mm, so a pattern will have a pitch of 60mm. And there will be such a pattern as rotating tables are available (if it is not just for marketing reasons).

To visualise the pattern of standing waves one could use sparks or heat if heated spots could be made visible. Boiling eggs (tend to explode) or better: exposing a thin layer of egg white to a standing wave field could reveal a heat pattern. Fortunately there is fax paper or more precisely thermal paper on rolls that is used for fax machines.

Experiment

Thermal paper shows black marks with a high resolution at those locations where the temperature rises over 60°C and the pattern even remains. Electromagnetic wave do not directly heat paper but they do heat water or wet paper. So when slightly wetted thermal paper (manual pump sprayer) is exposed to a field of standing waves it will show the locations of the peaks of the waves. It just has to be coated in a sheet protector as microwave ovens have an air blow inside.

The experiment works smoothly with a microwave oven connected via variable transformer to adjust the power carefully. A µA-meter with a diode placed at a location close to the door can serve as a relative power meter. The paper is then placed inside the oven horizontally at different heigths.

With just little water for heating a first pattern appears within seconds and with the power kept low one can wait for the back spots to grow to cover half of the area. Before steam is produced inside the sheet protector the print out of the microwave field distribution is complete.