How Paging Coasters work
Visiting a large popular museum, they handed out coaster pagers with FCC ID “QBTLTK-P1”. The pager is a 4kHz 1200 baud FSK system running on 26.995 MHz with a 2 Watt transmitter. The coasters have a loop antenna with about 2.5uV sensitivity, so perhaps 25 m range in good conditions. The relative insensitivity of such a small antenna and high pager transmit power work against the high levels of interference at the pager frequency.
I wondered why they would use such a low frequency and hence low efficiency antenna for the coaster size. Pager companies now use UHF (400+ MHz) for these coaster pagers since an efficient quarter-wavelength PCB antenna fits readily into a hand-sized pager. A decade ago there were all-in-one Samsung S1T8513A chips available that could receive up to 50 MHz. 49 MHz transmitters are at a disadvantage in the USA as the 10000 uV/m -> -15 dBm transmit power vs. 33 dBm transmit power at 27 MHz.
Paging doesn’t require frequency hopping or direct sequence spread spectrum. The system designer should pick an RF frequency band where the pager receiver antenna will be efficient for the pager size. For coaster pagers, the UHF band is a good choice, and the friendly ~ 5000 uV/m ~ -20 dBm power limit on UHF license-free transmission under FCC Part 15.231 leads to ~100 m pager range, using simple POCSAG modulation and hence commodity chipsets.
Efficient antenna design in a pager receiver allows for the best performance for a given size. Selecting the right frequency and antenna design enables the use of license-free power levels to achieve sufficient range. Interference-free frequencies are crucial to avoid broadband noise from PCs and lighting blocking pager messages with static. 27 / 49 MHz RF frequencies are generally not a good choice for pagers, baby monitors, etc. due to interference. Pagers operating at higher frequencies, such as 150/450 MHz, can be simple, cost-effective, and provide long-range communication. The Motorola Minitor fire pagers, which can achieve over 20 km range with powerful licensed base station transmitters, typically operate in the 150 or 450 MHz bands.
