because of the proximity of high power pulse equipment the equipment via signal leads enclosure and cables (radars) to sensitive signal processing equipment in the same air- craft, ship or vehicle, and military EMC standards first appeared in RFI picked up directly by the the 1960s. The increasing use of walkie-talkies on process plant and equipment circuitry elsewhere has prompted users of safety critical instrumentation to Electrostatic discharge specify a minimum immunity from r.f. interference. Measuring and weighing equipment must be prevented from giving incorrect read- ings in the presence of interference, and domestic broadcast re- mitting in the far field, defined greater than antenna as ceivers should be able to work alongside home computers. Radio frequencies are not the only source of interference. Tran- 2 is the wavelength. The value of the field strength in volts per metre sients can be generated by power switching circuits, lightning, can be calculated from Equation 23.1, where P is the radiated power in watts and d is the distance from the antenna in metres. electric motors, spark ignition devices or electrostatic discharge. Microprocessor circuits are particularly susceptible to impulse in- terference and must be protected accordingly. (23.1) 23.9.1 The importance of EMC In the near field closer than the field strength can be much Poor EMC performance of a product can be extremely costly, both in terms of damaged reputation and in the measures needed to greater depending on the type of antenna and how it is driven. improve performance once a fault has been found. For this reason, many firms will test their products for EMC before releasing them 23.9.2.1 Radio transmitters even though there may be no applicable standard for that class of equipment. Others, whose markets may be less demanding or whose AM broadcast band transmitter power levels tend to be around products are not used in a crowded electromagnetic environment, have not until recently considered it necessary to specify EMC 100k W to 50