How Much Output Power Can Be Used; Transmitter Power Vs. Antenna System Gain - Microwave Data Systems MDS 9810 Installation And Operation Manual

MDS 05-3301A01, Rev. C
Table 2 lists several types of feedlines and indicates the signal losses (in
dB) that result when using various lengths of each cable at 900 MHz.
The choice of cable will depend on the required length, cost consider-
ations, and the amount of signal loss that can be tolerated.
Table 2. Length vs. Loss in Coaxial Cables at 900 MHz
10 Feet
Cable Type (3.05 Meters)
RG-8A/U 0.85 dB
1/2 inch HELIAX 0.23 dB
7/8 inch HELIAX 0.13 dB
1 1/4 inch HELIAX 0.10 dB
1 5/8 inch HELIAX 0.08 dB

3.5 How Much Output Power Can be Used?

The transceiver is normally supplied from the factory set for a nominal
+30 dBm (1 Watt) RF power output setting; this is the maximum trans-
mitter output power allowed under FCC/IC rules. The power must be
decreased from this level if the antenna system gain exceeds 6 dBm. The
allowable level is dependent on the antenna gain, feedline loss, and the
transmitter output power setting. Power considerations for various
antenna systems are discussed below.
NOTE: In some countries, the maximum allowable RF output may be
limited to less than 1 watt (e.g., 100 mW /+20 dBm). Be sure
to check for and comply with the requirements for your area.

Transmitter Power vs. Antenna System Gain

To determine the maximum allowable power setting of the radio, follow
the steps below. It is imperative that professional system planning
include cable loss or attenuators to ensure that the FCC/IC limit of 36
dBm EIRP is not exceeded under any circumstances.
1. Determine the antenna system gain by subtracting the feedline loss
(in dB) from the antenna gain (in dBi). For example, if the antenna
gain is 9.5 dBi, and the feedline loss is 1.5 dB, the antenna system
gain would be 8 dB. (If the antenna system gain is 6 dB or less, no
power adjustment is required.)
2. Subtract the antenna system gain from 36 dBm (the maximum
allowable EIRP). The result indicates the maximum transmitter
power (in dBm) allowed under the rules. In the example above, this
is 28 dBm.
MDS 9810 Installation and Operation Guide
50 Feet 100 Feet
(15.24 Meters) (30.48 Meters)
4.27 dB 8.54 dB
1.15 dB 2.29 dB
0.64 dB 1.28 dB
0.48 dB 0.95 dB
0.40 dB 0.80 dB
500 Feet
(152.4 Meters)
42.70 dB
11.45 dB
6.40 dB
4.75 dB
4.00 dB
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