Codan Radio Transportable Radio Systems User Guide - Manual (Page 17)

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4.3.6 Free Space Loss
The free space loss indicates the propagation loss that will occur as the radio signal is transmitted through the air. No
other effects are considered. Free space loss is a function of distance and frequency as shown in the formula below.
Free Space Loss (dB) = 32.44 + 20 log Distance + 20 log Frequency.
Distance is measured in kilometres.
Frequency is measured in MHz.
The Free Space Loss for various distances at 150 MHz, 420 MHz and 800 MHz are shown in the table below.
4.3.7 Obstruction Losses
Obstruction losses or excess path loss is introduced by physical obstructions that disturb the radio propagation.
These losses are caused by three mechanisms:
• Refl ection — the bouncing of the original signal off an object causing a second signal to be received in addition to
the direct signal. For low antenna heights this can be caused by refl ections off the ground.
• Diffraction — the splitting of the radio signal as it hits a sharp edged obstruction and produces secondary signals.
• Scattering — the scattering of the original signal as it encounters an obstruction and is redirected in a variety of directions.
The following table summarizes the excess losses generated at 400 MHz due to different types of obstructions
for a 5 km path.
4.3 Calculating System Gain and Range
The maximum distance permissible between antennas is a function of the sum of the gains/losses in the path.
These gains/losses consist of the following elements:
• Transmitters
• Receivers
• Antenna
• Duplexers
• Cables
• Free Space Loss
• Obstruction Loss
• Fade Margin
4.3.1 Transmitter Gains
The output of the transmitter amplifi er can be increased to provide additional gain in the path as required. The table
below shows the relationship between power output expressed in Watts and in dB relative to 1 milliwatt which is
normally expressed as dBm. Power (dBm) = 10 log Power (milliwatts).
4.3.2 Receiver Sensitivity
The sensitivity of the receiver is another key element in determining the total gain of a radio system. LMR receivers
typically are sensitive to -118 dBm. Receiver sensitivity is usually expressed relative to a defi ned Bit Error Rate (BER) of
10–3 or 12 dB SINAD for analog systems.
4.3.3 Antenna Gains
One of the key characteristics of an antenna is the gain it provides. A variety of antennas are available to meet the gain
requirements of a particular installation. Typically monopole or dipole Omni directional antenna have a gain of unity (0 dB).
4.3.4 Duplexers
Duplexers have losses associated with combining the transmit and receive paths together. For a simple duplexer this
loss is typically 1.5 dB.
4.3.5 Cable Losses
Cables and connectors between the antenna and the transmitter/receiver can produce noticeable losses if not prop-
erly installed. For a well designed and installed system the cable (<50 ft) and connector losses should be less than 1 dB.
Single Obstruction same height as antenna 6
10 metre trees in a forest less than 1 km wide 6
One antenna on a hill, one antenna below trees 20
10 metre trees in a forest – 3 km wide 25
Irregular terrain between antenna 26
Suburban area - homes in a path 30
Wooded valley with both antenna below tree height 40
Obstruction Excess Loss (dB)
0.5 70 79 84
1 76 85 91
2 82 91 97
5 90 99 104
10 96 105 111
15 99 108 114
20 102 111 117
30 106 114 120
40 108 117 123
Distance (km) FSL (dB) @ 150 MHz FSL (dB) @ 420 MHz FSL (dB) @ 800 MHz
0.1 20
2 33
5 37
10 40
25 44
50 47
Power Output (Watts) Power (dBm)