Friis Transmission Calculator

Overview

The Friis equation is used to find the ideal power arriving at the receiving antenna given the transmitted power, the signal frequency, travel distance, and the gains of the antennae. To use this calculator, just enter the required parameters and press the "calculate" button. 

Equation

$$P_{RX} = P_{TX}G_{TX}G_{RX}(\frac{c}{4 \pi D f})^2$$

Where:

$$P_{RX}$$ = power arriving at the receiver (watts)

$$P_{TX}$$ = power at the transmitter (watts)

$$G_{TX}$$ = gain at the transmitter (absolute)

$$G_{RX}$$ = gain at the receiver (absolute)

$$c$$ = speed of light  = 3 x 10⁸ m/s

$$D$$ = distance between transmitter and receiver (meters)

$$f$$ = signal frequency (hertz)

Applications

The Friis equation is useful in calculating the signal power arriving at the receiver. However, in actual situations, a signal is composed of different frequencies, whereas the Friis equation only uses one frequency. The Friis equation provides a good approximation of the received signal power if the center frequency is high enough. This is because the frequency variation across the signal bandwidth is a smaller fraction for high carrier frequencies.

Further Reading

Textbook - Principles of Radio: Basic AC Theory

Technical Article - Fundamentals of Wi-Fi Antennas

Worksheet - Design Project: Radio Transmitter