Codan Radio Radio Repeater Systems Training Guide - Manual (Page 54)

TRAINING GUIDE | RADIO REPEATER SYSTEM
Appendix D: Calculating Signal Levels Page 46
To this point, we have looked at information concerning one end of the path (gains and losses) and
the path loss itself. Now we consider what is happening at the receive end of the path. Our received
signal strength (+8.6 dBW - 124.0 dB) is -115.4 dBW. Adding our receive antenna gain (another 6.0 dB
omnidirectional), and subtracting the coaxial cable loss (50 ft. or 15.2 m at 9.2 dB/100 m) and connector
losses results in a signal at the receiver input of -111.3 dBW. The effective threshold of a good receiver
is usually given as -137 dBW, which is a compromise between many variables. The absolute difference
between -111.3 and -137.0 dBW, 25.7 dBW, is the useful fade margin. The calculated signal level of
19.3 μV follows as shown.
The second path calculation sheet showing gains and losses on the path BRIDGE ARS to IRON MTN.
ARS assumes UHF (450 MHz) frequencies (hence the higher losses for cable and fi ttings). Because
high gain is a function of beam width, the licensing rules may insist on 10 dB or higher gains on your
linking paths, but may also insist that your ERP is reduced so that the fade margin is at some pre-
determined level. For this reason, it is advantageous to use a repeater whose Tx power output is
continuously adjustable. It may also be necessary to add a fi xed loss, such as a 6.0 dB attenuator in the
transmitter output line. In this case, do not forget to add on additional connector losses. In our example,
BRIDGE ARS to IRON MTN. ARS, our losses include -3.0 dB (by reducing output power to 0.5 W), and
fi lter losses at each end of the path of -1.2 dB (interference protection). The result is a 27.2 dB fade
margin or a signal level of 22.9 μV. An alternate approach, as suggested above, would be to insert a
6.0 dB attenuator plus -1.0 dB additional loss for connectors in the transmitter output while running the
transmitter at 2.0 watts. The result would be a fade margin of 26.2 dB or 20.4 μV. If permitted by your
local rules, reducing power would be the better alternative.
A useful exercise would be to calculate the fade margin and signal level for the path BRIDGE ARS
to IRON MTN. ARS at 850 MHz. To start you off, the path loss, from the nomogram in Appendix C, is
129 dB at 850 MHz. Use the co-ax and duplexer losses and the antenna gains from the information in
Appendices F, G and H.