 Radio paging 1. The pager is strapped to a wooden pole facing the transmitting AAA cell with renewal periods of 1 to 2 months is now often used. antenna 1.5m up from the ground in an open field, and is sent a Rechargeable AA cells of 500 mAHrs capacity may be employed signal from a calibrated antenna 30 metres away. It is important and single or multiway charging racks are available. Non rechar- that the area is clear of trees and nearby buildings, to avoid geable cells, such as zinc, alkaline, manganese dioxide, start life reflections distorting the uniform electromagnetic field. with 1.5V and slowly decrease to around 0.9V. Rechargeable cells 2. The pager may be placed in the centre of a strip line. only start at approximately 1.25V but maintain this level, then 3. The pager is placed in a TEM (Transverse ElectroMagnetic) rapidly collapse when discharged. The paging circuitry has to take cell. account of these different characteristics especially at the lowest working temperature of -10°C. The first method provides an absolute result directly, but due to the nature of the calling process, several results should be averaged. 45.8.7 Battery economy Although the strip line and TEM cell fields may be calculated, it is usual to take the portable TEM cell out to the free field site, and To provide acceptable primary cell life times suitable paging code correlate the face-on level determined on pole, directly to the formats and receiver designs are required. Referring to Figure 45.8 required TEM cell level. and assuming the code is POCSAG, then the decoder circuit always The second and third methods require a radio noise free environ- has power on, but only switches on the radio circuit every 0.4 to 1 ment which, along with portability, is the main advantage of using second (depending on data rate) to search for preamble. If preamble a TEM cell as in the third method. is detected a sync word search commences while power is still maintained to the receiver. 45.8.5 Free field sensitivity When a sync word is found the p