8
n920BT Operating Manual
WARNING
:
MAXIMUM EIRP
FCC and IC Regulations allow
up to 36dBm effective
isotropically radiated power
(EIRP). Therefore, the sum of
the transmitted power (in dBm),
the cabling loss and the antenna
gain cannot exceed
36 dBm with respect to the
isotropic radiator.
WARNING
:
The N920BT can only be used
with any antennas listed in
Appendix A.
WARNING
:
Be careful with dBi vs dBd gains
on antenna specifications.
Antenna manufactures may not
clearly indicate the gain on the
antenna if it is dBd or dBi. Note
1dBd = 2.15dBi
.
When installing the cable, always begin fastening at the top near the
antenna connector/surge arrestor. The cable must be supported at the top
with a hose clamp or wrap lock, and at 5 ft intervals down the length of the
tower. Over-tightening the fasteners will dent the cable and reduce
performance. If properly grounded surge arrestors are not installed at both
the top and the bottom of the cable, then the cable should be grounded to
the tower at these locations using a cable grounding kit. If the tower is non-
conductive, then a separate conductor, physically separate from the cable,
should be run down the tower.
Antenna
Before choosing an antenna, you should have some knowledge of the path
loss and the topology of the equipment. If the equipment is in a fixed
location and is to communicate with only one other unit also in a fixed
location, then a Yagi antenna is suitable. Choose a Yagi with enough gain
to ensure adequate gain margin. When deploying the Yagi, point the
antenna towards the intended target, ensuring the antenna elements are
perpendicular to the ground for vertical polarization.
In applications where there are multiple units that you must communicate
with or units, which are in motion, you may select an Omni-directional
antenna with appropriate gain.
See appendix A for a list of approved antennas that can be used with
the N920BT radio modem. If you require another type of antenna
please contact Microhard Systems Inc. The N920BT CANNOT be used
with any antenna that does not appear in Appendix A.
Microhard Systems Inc. can provide you with approved antennas to ensure
FCC and Industry Canada compliance.
FCC Regulations allow up to 36dBm effective isotropically radiated
power (EIRP). Therefore, the sum of the transmitted power (in dBm),
the cabling loss and the antenna gain cannot exceed 36dBm with
respect to the isotropic radiator.
EIRP is calculated as follows:
EIRP = Tx Power(dBm) - Cable/Connector Loss(dB) + Ant Gain(dBi)
Antenna Gains must be in dBi when calculating the 36dBm EIRP limit.
1dBd = 2.15dBi
Use the guidelines in the previous section for calculating cable and
connector losses. If cabling and connector losses are 2 dB, then the
maximum allowable gain of the antenna will be 8 dB.
Examples of Antenna and Power settings to maintain 36dBm EIRP
Antenna
Power Level
Minimum Cable / Loss
EIPR
8.15 dBi Omni
30dBm
25 feet LMR195 / 2.675dB
35.475dBm
12.5 dBi Patch
23dBm
10 feet LMR195 / 1.07dB
34.43dBm
14.15 dBi Yagi
21dBm
10 feet LMR400 / 0.39dB
34.76dBm
