0kW, but the transmitters are usually located away from centres of population. Field strength levels in the 1V/m to performance. 10V/m range may be experienced occasionally, but at medium This situation is now changing, both because of electromagnetic frequencies coupling to circuit components is inefficient, SO these pollution which is bringing to light more cases of poor EMC, and transmitters do not pose a great problem except when coupled via because new legislation will compel manufacturers to consider their long cables. TV and FM band transmitters are more often found products' EMC. The technical constraints on equipment EMC are close to office or industrial environments and the greatest threat is that the equipment should continue to function reliably in a hostile to equipment on the upper floors of tall buildings, which may be in electromagnetic environment, and that it should not itself degrade line of sight to a nearby transmitter of typically 10kW. Field that environment to the extent of causing unreliable operation in strengths greater than 10V/m are possible, and although the building other equipment. EMC therefore splits neatly into two areas, la- structure may give attenuation between () and 20dB, achieving belled 'immunity' and 'emissions', and a list of some of the inter- immunity from even a 1V/m field is not trivial at these frequencies, ference types and coupling methods is shown in Table 23.3. where cable and track lengths approach resonance and coupling is correspondingly more efficient. 23.9.2 Immunity Portable transmitters (walkie-talkies, cellphones) do not have a high radiated power but they can be brought extremely close to The electromagnetic environment within which equipment will susceptible equipment. Typical field strengths from a 1 u.h.f. hand operate and which will determine its required immunity can vary held transmitter are 5V/m to 7V/m at half a metre distance. widely, as can the permissible definition of reliable operation. For instance, the magnitude of radiated fields encoun