Calorimetric radar absorptivity measurement using a microwave oven

The measurement of the microwave absorptivity of materials typically requires expensive and arcane equipment and methods. I show that, for moderately absorbing materials (loss tangent δ ' 0.005 or higher), useful measurements can be easily made using a thermometer and a normal domestic microwave oven: the heat absorbed for suitably designed sample size and geometry relates directly to the absorptivity. This technique, although not supremely accurate, is quick and inexpensive and may be useful for rapid investigations, educational projects and situations in which samples are awkward to handle. I show that water ice spiked with even only 0.2% ammonia is about three orders of magnitude more absorbing than is pure ice at −80 ◦C.