Height-constrained cylindrical helical antennas for satellite-based M2M communications

Both practical considerations and government regulations are beginning to limit the height of terminal antennas used in satellite-based M2M applications to less than 2.5 cm. For Orbcomm-based systems that operate at downlink and uplink frequencies of 138 and 150 MHz (or a wavelength of 2 m), respectively, 2.5 cm corresponds to a terminal antenna height of just over one-hundredth of a wavelength. Realizing antennas that meet the operating frequency, efficiency and bandwidth requirements of such systems is very difficult, particularly in light of: 1) the manner in which close proximity to a large ground plane reduces the radiation resistance of the antenna and 2) the Chu-Harrington limit on the operating bandwidth of the antenna. In particular, the bandwidth of such heightconstrained antennas barely covers the uplink or downlink passbands of the Orbcomm system so separate transmit and receive antennas are required.