ated of selected stock prices 4dB to 6dB. A modern on site digital proprietary code format when a few miles from his office, by using an alphanumeric pager specification is shown in Table 45.1. on a private wide area system. 45.5.3 Small systems 45.5.2 Code formats A small modern paging system will probably consist of a transcoder There have been many different on site paging system manufac- and five tone or numeric pagers. The transcoder is a combined turers since 1956, and each has been free to devise its own code digital paging encoder consisting of a keyboard and a 10 digit format. Early proprietary formats used sequential audio tone modu- display, and a small 1W transmitter incorporating an integral micro- lation using two inductor coils 2cm X 1.3cm X 1.3cm in the range of phone. In the 25MHz to 54Mhz h.f. band a standalone base loaded 500Hz to 3.4kHz. For example a tone of 640Hz had a coil of whip aerial would be provided, but at u.h.f. a flexible stub antenna inductance 11.7H and a Q of 10. Vibrating mechanical reeds were is connected directly into the encoder. also used to achieve higher Q's and more frequencies, thereby 1W is sufficient r.f. power to provide good coverage in the office increasing system capacity, but at the expense of mechanical robust- building environment, or 200 metres to 300 metres in a hotel block, ness. These were ultimately replaced by active filters which could and up to 8Km. on an open site. Range varies due to attenuation of then be made smaller with high Q and high stability by using hybrid buildings especially if steel reinforced structures or sun reflective thick film circuits. windows are used. Attenuations at different frequencies can be By this time the EIA (Electronic Industries Association) had encountered as shown in Table 45.2, which give average relative standardised on two tone and five tone sequential tone formats in levels in dB. It has been found that, when walking with a pager, peak the 67.0Hz to 1687.2Hz frequency range. Since such a two tone to trough variatio