Free Space Propagation

Free space propagation, often referred to as "Free space Loss", is not actually "loss" but is, instead, the result of energy spreading out as it travels farther and farther from the point of origin. Only a portion of energy radiated is, as a result, available at a point of reception to be intercepted by an antenna. Free space propagation is a simplistic, best case calculation which assumes no additional losses between transmit and receive antennas.

This worksheet calculates how much energy is left at a known distance from a radiating source due to simple energy spread. Antenna system gains (antenna gain with transmission line loss subtracted off) on both transmit and receive ends of this "RF link" are also considered.

The calculator is initialized for a 2M path of 50 miles, 170W transmitter output into a transmission line (coax) of 1dB loss, an M2 18XXX yagi (15dB gain), a 5dB gain antenna at the receive end (like a good, stacked, omnidirectional array), and a 0.5dB coax loss (typical for a mobile). To run different numbers, simply highlight the applicable number and enter a new value. Make sure units of distance, frequency and power are properly selected, then click on "Compute". To run the default calculation again, click on "Reset Input" and then on "Compute".

Enter units of distance:     kilometers.     miles.
Enter distance from transmit antenna to receive antenna:  

Enter units of frequency:     MHz.     GHz.  
Enter Frequency:  

Enter units of transmitter power:     mW.     W.     KW.  
Enter transmitter power output:  

Enter transmit antenna gain in dB:  
Enter transmit transmission line loss in dB:  

Enter receive antenna gain in dB:  
Enter receive transmission line loss in dB:  

Click on and scroll down for results!  

An ND2X javascript calculator