 flat copper strip. For the 430MHz to 490MHz band 87% probability of calling. antennas can be a helical coil, discontinuous directional ring radia- Another method, from IEC No. 49, and mandatory for pagers, tor, etc. Unfortunately, because of the limited space within the pager when determining the sensitivity of equipment used on the German and close proximity of other components and earth tracks, u.h.f. Cityruf system, is as follows. The signal generator level is adjusted antennas are not able to operate at their optimum. While antennas SO that less than 10% of the tone only calls are successful. The signal may result in good calling sensitivity (e.g. 5uV/m) measured in an is continuously repeated SO as to ascertain each time whether a tone open field attached to a pole or on a strip line in the lab, performance only call is successful or not. The level is increased by 1dB until maybe considerably reduced when worn by the user due to the effect three successive calls are successful. The level is then noted. The of the body. level is reduced by 1dB and the new level noted. The signal is then sent 20 times. The level of the signal: 45.8.4 Performance measurement methods 1. Remains the same if a tone only call is successful. There are two particular difficulties in performing measurements on 2. Is increased by 1dB, and the new level noted if a tone only call is not successful. the paging receiver: 3. Is reduced by 1dB and the new level noted if 3 successive tone 1. The calling signal must be the pager's digitally modulated only calls are successful. address. 2. The integral antenna prevents direct connection from the signal The pager sensitivity is deemed to be the arithmetic mean of the generator. noted levels. As direct line connection of the modulated call is not possible, the Not withstanding the difficulties, the measurement method which pager must be placed in a uniform field strength capable of being has been the most commonly used is to set the signal generator varied. Three methods are commonly used: 45/14