Visual calculator 05 · Propagation

Path loss and link budget

Power spreads over a sphere. The receiver catches only its effective aperture. In free space, that aperture is where the frequency dependence comes from.

Rx antenna

Example links (illustrative)

Frequency

Distance

Tx power dBm

Tx gain dBi

Tx line loss dB

Rx line loss dB

Rx gain dBi

Rx dish diameter m

Aperture efficiency

Rx sensitivity dBm

Rain rate mm/h

Path in rain km

Elevation deg

Gases, total dB

Polarization

Link budget

Each bar adds or removes dB. The last bar is the received power.

Received power vs frequency

Free space only. All three curves pass through this design.

Check

Method

FSPL = (4π d / λ)² isotropic antennas Pr = Pt Gt Gr (λ / 4π d)² Friis Ae = G λ² / 4π, dish: G = η (π D / λ)² S = EIRP / (4π d²), E = √(S η0) r1 = √(λ d1 d2 / (d1 + d2)) first Fresnel zone rain: γ = k R^α (ITU-R P.838-3), A = γ · path in rain Pr = Pt − Ltx + Gt − FSPL − Arain − Agas + Gr − Lrx

Far field, line of sight, matched polarization, no multipath. Gas absorption is an input (take it from ITU-R P.676). Rain uses the P.838-3 coefficients with polarization tilt and elevation (eqs. 4–5); path reduction factors (P.530, P.618) are left to you through the "path in rain" length.

Reference values

H. T. Friis, "A note on a simple transmission formula," Proc. IRE 34(5), 1946, pp. 254–256.
ITU-R P.525-4, Calculation of free-space attenuation, 2019.
ITU-R P.838-3, Specific attenuation model for rain for use in prediction methods, 2005 (coefficients, Tables 1–5).
C. A. Balanis, Antenna Theory, 4th ed., Wiley, 2016, Sec. 2.15–2.17 (effective aperture, Friis).