User's Manual

Interfacing and Controlling the RIM 902MAntenna selection: Positioning the antenna 49
RIM 902M OEM Radio Modem – Integrator’s Guide
Positioning the antenna
Proper positioning of the antenna will maximize the gain provided by the
antenna. In determining the proper position, the designer must carefully
consider the environment in which the device will be used. Physical devices can
vary significantly, and incorporating the antenna is an integral part of a
successful design.
The Mobitex network is based on vertically polarized radio-frequency
transmission. Therefore, the antenna should ideally be oriented so that it
provides vertical polarization. This is achieved by positioning the antenna
vertically upward when the RIM 902M is in use. In small, hand-held devices, it
may be convenient to design the unit in such a way that the antenna folds out of
the way when not in use.
The antenna should be located as far from the active electronics of the
computing device as possible. In general, metal construction in the case of the
computing device and its internal components may attenuate the signal in
certain directions. This is not desirable, as it would reduce the sensitivity and
transmit performance of the radio modem when the computing device is held or
positioned in certain orientations. However, the judicious use of metal in the
construction of a ground plane for an antenna can significantly improve the
antenna gain and the coverage area of the system.
If the computing device is designed to sit on a surface, then the antenna should
be as far from the bottom of the device as possible. This will reduce RF
reflections whenever the device is placed on a metal surface.
When the computing device is hand-held or is worn next to the body of the
user, the antenna should be positioned to radiate away from the body.
Otherwise, the body will absorb the radiated energy and the effective coverage
area of the radio will be reduced.
For best results, the antenna should be connected directly to the antenna cable.
If an extension cable is required, it should be low loss, as short as possible, and
have an impedance of 50 : . It is important that a proper matching connector
be used, as each connector in the signal path introduces a return loss and
reduces performance.