Xircom An Intel Company GEM3501 User Manual
Core Engine GSM/GPRS Modem Developer Guide Preliminary Draft: 7/6/2001
57
Part Number: 07100026, Revision: 002
Confidential
© 2001 Xircom, Inc., an Intel company All rights reserved.
All trademarks and copyrights are the property of Xircom, Inc., an Intel company
All trademarks and copyrights are the property of Xircom, Inc., an Intel company
practices. Emphasis on freedom from obstacles, and the aesthetic guidelines required by
site management should be taken into consideration during the install.
site management should be taken into consideration during the install.
NOTE: Coaxial cables must be professionally installed. Coaxial cables must be
routed and installed in a manner that insures that the jacket, dielectric and outer
shield are not crushed, kinked, cut, scraped or otherwise damaged.
routed and installed in a manner that insures that the jacket, dielectric and outer
shield are not crushed, kinked, cut, scraped or otherwise damaged.
15.4 Coaxial Cable Losses and Lengths
There are some important factors to consider concerning coaxial cable length. The FCC
requires that for mobile applications, the maximum output power can be no more than 2
W (+33 dBm) EIRP, and for fixed applications, no more than 5W (+37dBm). When
using a directional gain antenna and short coaxial cable connections, it is possible to
exceed either FCC requirement. In such cases, additional attenuation must be added into
the path gain. This can be achieved with either in line attenuators, or by adding more
cable length.
requires that for mobile applications, the maximum output power can be no more than 2
W (+33 dBm) EIRP, and for fixed applications, no more than 5W (+37dBm). When
using a directional gain antenna and short coaxial cable connections, it is possible to
exceed either FCC requirement. In such cases, additional attenuation must be added into
the path gain. This can be achieved with either in line attenuators, or by adding more
cable length.
To determine the amount of loss needed between the modem and the antenna, the
following formula can be used.
following formula can be used.
Attenuation needed = g – p
where
g = antenna gain (in dB)
g
= maximum path gain allowed by FCC based on the modem maximum output power of
+30 dBm, and where
p
= 3 dB for mobile applications, or
p
= 7 dB for fixed applications
To determine the proper minimum cable length the following formula can be used.
L = (g – p) / a
where
L = required minimum cable length
g = antenna gain (in dB; per antenna manufacturers specifications).
p = maximum path gain allowed by FCC based on the modem maximum output power of
+30 dBm, p = 3 dB for mobile applications, or p = 7 dB for fixed applications
+30 dBm, p = 3 dB for mobile applications, or p = 7 dB for fixed applications
a = attenuation per foot of cable (in dB; per cable manufacturers specifications).
As long as installation and operating restrictions previously provided are observed, and
antenna gain is limited to 7 dBi for fixed, or 3 dBi for mobile applications, there is no need
to introduce RF loss between the modem and antenna in order to comply with FCC MPE
limits.
antenna gain is limited to 7 dBi for fixed, or 3 dBi for mobile applications, there is no need
to introduce RF loss between the modem and antenna in order to comply with FCC MPE
limits.
NOTE: Cable loss beyond that required to meet the FCC requirements, with the
given antenna, will decrease the signal strength reaching the modem. This will
negatively impact the ability of the modem to communicate with the network.
given antenna, will decrease the signal strength reaching the modem. This will
negatively impact the ability of the modem to communicate with the network.