New formulae for practical pager design

The antennas used in VHF and UHF pagers are electrically small loops, which are highly resonant devices that are resistant to detuning through user proximity. The antennas are often designed using a combination of computer simulation and experimental measurement techniques. However analytic formulae are also useful in suggesting prototype arrangements and in showing how the performance varies with geometry, matching and proximity induced losses. Philips have developed many world-leading techniques for designing VHF and UHF pager antennas. This paper summarises several previously unpublished results. The paper presents a simple formula for the efficiency of isolated air filled rectangular strip loops is presented. It discusses the effect of user proximity induced losses and mismatch. A simple method of mitigating the mismatch effects is outlined. Including PCB tracks in the loop can seriously degrade the antenna´s performance. However for manufacturing reasons it is best for the trimmer capacitor to be mounted on the PCB. The paper discusses how this can be done and explains how, despite high loss tangents at UHF, ferrites can still enhance the loop´s performance