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Friis Transmission Equation

The Friis Transmission Equation is fundamental to understanding link budgets. It calculates the power received ( $P_r$ ) at a receiving antenna given the power transmitted ( $P_t$ ), antenna gains ( $G_t, G_r$ ), and distance ( $R$ ).

The Formula

The classic form (linear power) is:

\frac{P_r}{P_t} = G_t G_r \left( \frac{\lambda}{4\pi R} \right)^2

Logarithmic Form (dB)

RF engineers typically work in decibels (dB) because it turns multiplication into addition.

P_r (dBm) = P_t (dBm) + G_t (dBi) + G_r (dBi) + 20\log_{10}\left( \frac{\lambda}{4\pi R} \right)

The term $20\log_{10}\left( \frac{\lambda}{4\pi R} \right)$ represents the Free Space Path Loss (FSPL), which is usually a negative value (loss).

Interactive Link Budget

Below is a simple calculator to experiment with these values.

🛠Link Budget Calculator

Frequency (MHz)

Tx Power (dBm)

Tx Gain (dBi)

Distance (m)

Rx Gain (dBi)

Path Loss (FSPL):0.0 dB

Rx Signal:0.0 dBm

Signal Level Waterfall (Simplified)

Interactive Link Budget Calculator Placeholder

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