User's Manual

48 Antenna selection: Introduction to antenna terminology
Integrator’s Guide – RIM 902M OEM Radio Modem
VSWR
 1
P
P
1 -
P
P
reflected
output
reflected
output
Equation 2: VSWR
VSWR = 1 or RL =
f
dB is a perfect match. In practice, there will always be
imperfections, which means that VSWR will be greater than 1 and RL will be a
negative number.
VSWR and RL normally vary as a function of frequency. The RIM 902M’s
frequency range includes 896 to 902M MHz (transmit) and 935 to 941 MHz
(receive). The minimum acceptable match across this range must be
VSWR < 2.0 or RL <

10 dB. For best performance, the recommended antenna
match at these frequencies is VSWR < 1.5 or RL <

14 dB.
Antenna size
The optimal antenna radiation efficiency is produced by an antenna measuring
one wavelength,
O
. The value of
O
for the RIM 902M is 32.8 cm, and is
calculated by dividing the speed of light c = 3 x 10
8
m/s by the center frequency
f = 913.5 MHz. Because the RIM 902M’s receiver is so sensitive, this value
includes a 5 MHz bias toward the transmit frequencies to help balance the
uplink and downlink between the radio modem and the network base station.
Antenna lengths of
O
/2,
O
/4, and
O
/8 also work well, and usually result in a
relatively well matched antenna.
O
/2 or
O
/4 can be electrically “shortened” by
adding load matching elements to control the antenna match. However, this
shortening will reduce the antenna efficiency and therefore the effective
radiated power.