RF Link Budget Calculator
Estimate Free Space Path Loss (FSPL) and Received Signal Strength (RSSI).Friis Transmission Equation Model
Transmitter
Free Space Path
Receiver
Path Loss Rules of Thumb
Engineering Principles: Friis Transmission Equation
The Friis transmission equation relates the power received to the power transmitted between two antennas separated by a large distance. It is fundamental to link budget analysis in telecommunications.
All values in dB/dBm. Pt=Tx Power, G=Antenna Gains.
K depends on units. For d in km/f in GHz, K=92.45. For m/Hz, K=-147.55.
Standard formula used in this calculator.
Technical Notes
- •Equation assumes far-field conditions (d > 2D²/λ).
- •Obstacles, multipath fading, and atmospheric absorption are NOT included in FSPL.
- •Antenna gains must be isotropic (dBi). If using dipole (dBd), add 2.15 to convert to dBi.
Frequently Asked Questions
What is a Link Budget?
A Link Budget is a calculation of all the gains and losses in a transmission system. It accounts for the transmitter power, antenna gains, cable losses, and free space propagation losses to determine if the received signal strength (RSSI) is sufficient for the receiver to operate reliably.
Why does Frequency affect Path Loss?
According to Friis Transmission Equation, for a fixed antenna gain, path loss increases with the square of the frequency. This is largely because the 'effective aperture' (capture area) of an isotropic antenna decreases as frequency increases (wavelength gets smaller).
What is FSPL (Free Space Path Loss)?
Free Space Path Loss is the attenuation of signal strength as it travels through space, purely due to the spreading of the wavefront (geometric expansion). It assumes a clear line of sight with no obstacles, reflections, or atmospheric absorption.