Path loss

Radio Frequency (RF) signals need to get from one point to another to do any good for the transportation of intelligence. This is done with an RF (modulated) transmitter, sending the signal to its antenna, so it will radiate into the air. It's then picked up at the far end antenna, sent to a receiver and decoded/demodulated into audio to be understood by the receiving person. All of this comes with gains and losses of the RF signal. In the case of microwave telecommunications a common practice for engineering uses this process as shown on this typical drawing and other information obtained at various web sites:

The following rules are observed:

calculation of received signal level (RSL)

Unfaded RSL = Total Gains – Total Losses

Total Gains

Transmit power in dbm
Antenna gains (transmit and receive)

Total Losses

Free Space Loss in db
Atmospheric absorption loss (oxygen and water vapor)
Transmission line losses (waveguide and coaxial cable)
Branching losses (circulators, filters, etc.)
Antenna radome losses for standard antennas
Hot standby splitter losses (1 db for 10:1 splitter)
Transmit attenuators

System Gain (3 ds3 radio example above):

+29 dbm (Tx) – (-71 dbm) 10-6 Tx min.) = 100 db

(does not include antenna gain, free space loss or wg loss)
(ATPC Disable – Space Diversity Receivers)

As in the case of VHF-UHF Amateur RF paths this can be somewhat simplified. More specifically, would be Amateur RF links.

For calculation of received signal level (RSL)

Unfaded RSL = Total Gains – Total Losses

Total Gains

Transmit power in dbm; i.e. +40dbm
Antenna gains (transmit and receive); i.e. 6 db gain each = 12 db

Total Losses

Free Space Loss in db; i.e. at 440 Mhz 150 miles is 132 db path loss
Transmission line losses (coaxial cable); i.e. 2 db each end= 4 db)
Branching losses (circulators, filters, etc.): i.e. 2 db each end= 4 db)

+40 dbm (tx) + (12) – (132 db) - (8) = -88 db

A -88 dbm is enough signal to produce a good quieting signal into a modern FM receiver since most are spec at -115 dbm sensitivity, or better. Having said this; it's also good to attemp stronger signals for improving the fade margine.

Fade Margin

Fade Margin (FM) is the difference between the RSL and the squelch point of the far end receiver. For practical circuits the receiver should "fail" the circuit in the event the path drops enough to raise the noise floor in the detected audio no more than a 20 db signal to noise ratio, or a 20 dbm0 noise floor. With most modern LMR receiver's this would be around a -100 dbm RF level. Therefore, if this was observed as a standard squelch point the (above) RSL of -88 would produce a FM of 12 db. To note, SRG specs call for a minimum FM of 20 db; 30 db being desirable.