Suppression and mitigation techniques for interference from 900MHz microwave ovens Final Report (AY 3499) for The Radiocommunications Agency

1 Introduction

In the United Kingdom the frequency band 886 to 906MHz has been allocated as a band suitable for the operation of Industrial Scientific and Medical (ISM) equipment. ISM machines are at present allowed to emit 12OdBm V/m measured at 30m from the wall of the building housing the equipment over this frequency range. This presents a problem as it occupies part of the band allocated to mobile phone operators.

The main users of the ISM band are organisations operating industrial microwave ovens. The ovens are primarily used in food production although other uses such as vulcanising rubber are also on record. High power magnetrons are used as the source of microwave energy, the magnetrons are designed to operate at 896MHz.

GSM mobile phone operators are allocated the band 890 to 915MHz and 935MHz to 96OMHz. There are 124 channels in the GSM specification each channel is 2OOkHz wide. The channels are full duplex with an upstream/downstream spacing of 45MHz. Appendix 1 shows a plan of the channel numbering and how this coincides with the magnetron centre frequency.

For process purposes the ovens are normally conveyor fed. Consequently depending on the size of the product being treated a large aperture exists at each end of the oven allowing relatively high levels of microwave energy to be emitted.

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