User's Manual

46 Antenna selection: Introduction to antenna terminology
Integrator’s Guide – RIM 902M OEM Radio Modem
effectively doubling the length of the antenna by creating a virtual “mirror
image” of the antenna beneath the plane.
Antenna requirements
The following are the minimum requirements of the antenna system used with
the RIM 902M.
Impedance: 50 :
Center frequency: 913.5 MHz, r 5 MHz ( O =32.8 cm, r 0.2 cm)
this is deliberately biased toward transmit in order
to help balance the two-way link between the radio
modem and the network base station
Frequencies of operation: 896 to 902M MHz (transmit)
935 to 941 MHz (receive)
Acceptable return loss: VSWR < 2.0 or RL <  10 dB (minimum)
VSWR < 1.5 or RL <  14 dB (recommended)
required across all frequencies of operation
Introduction to antenna terminology
This section introduces some of the terminology that is used to describe
antennas, and expands on the summary of antenna requirements, above.
Gain and ERP
Antennas produce gain by concentrating radiated energy in certain areas, and
radiating less energy in other directions. The amount of gain depends on the
radiation pattern, antenna match, and antenna efficiency. Antenna gain is
given as a rating of the maximum increase in radiated field energy density
relative to an ideal isotropic radiator, expressed in decibels (dB) of power gain.