# How is RF Power at Receive Antenna Output Terminals Calculated?

Is the Friis equation sufficient to make this calculation? Must other considerations also be evaluated?

## 1 Answer

Below is the output screen of a spreadsheet calculator using the Friis equation, displaying received r-f power based on transmit/receive system parameters when using a free space propagation path. Numeric values in the yellow-shaded cells may be entered by the user.

The spreadsheet also automatically calculates and displays

• the mid-path radius distance for clearance of the 0.6 Fresnel zone, as such Fresnel clearance provides the propagation loss of a free-space path
• the r-f voltage present across a 50 Ω output termination of a loss-free matching network at the receive antenna output terminals.

This Excel-type spreadsheet is available at https://nofile.io/f/19jUxxDHCJE/Friis+Transmission+Equation+Speadsheet+(Fry).xls — free to download and free to use.

Next below is a graphic illustrating the effect on the e-field produced by insufficient clearance for the Fresnel zone with various heights above ground of the receive antenna, for the conditions shown there.

Next below to illustrate the significance of Fresnel zone clearance is the result of a Longley-Rice calculation for the path loss from FM broadcast station WIOG to a receive site in northern Livingston county, MI.

For those coordinates and antenna heights AGL and the frequency, the terrain profile plus earth curvature along the path shows blockage of the 0.6 Fresnel zone radius starting at about the mid-point of the path.

If the path geometry exactly allowed clearance of the 0.6 Fresnel zone for its entire length then the received field intensity would exactly equal that of a free-space, unobstructed path, and as based on the Friis equation.

In this example the 0.6 Fresnel zone obstruction contributes about 8.9 dB of additional loss to that LOS path.