TOOL 06 / COMPLETE THE SIGNAL PATH

RF Link Budget

From transmitter output to receiver input. One budget, every gain and loss.

Link assessment ↓Equations & assumptions ↘

Build the link

Dot decimals or scientific notation; no commas or unit suffixes. Changing a unit reinterprets the number. Gains use dBi; all losses use positive dB.

Examples

Carrier frequency

1 kHz–1000 GHz. Gains and receiver data must apply at this frequency.

Path distance

0.001–1e9 m. Straight separation, with ideal free-space propagation.

Transmitter output power

At the transmitter connector, before the entered transmitter losses. Zero is supported in W/mW.

Transmitter → antenna

Transmit antenna gain / dBi

Gain toward the receiving antenna; includes antenna efficiency.

Transmit feeder loss / dB

Cable loss only; use Tool 03 if needed.

Other transmit insertion loss / dB

Connectors, filters etc. not already in feeder loss.

Receive antenna → receiver input

Receive antenna gain / dBi

Gain toward the transmitter. Assumes aligned polarization unless a known loss is entered once.

Receive feeder loss / dB

Signal attenuation before the receiver input.

Other receive insertion loss / dB

Remaining known fixed loss, counted once.

Receiver input threshold

Sensitivity source

Receiver sensitivity

Threshold at the receiver input AFTER the entered receive losses. −300 to +100 dBm; use a defined waveform, bandwidth and acceptance criterion.

Count each physical loss once. Do not also subtract antenna efficiency already included in gain, Tool 03 connector losses already included in a feeder total, or losses already included in a quoted sensitivity. Use receiver-input sensitivity and receiver NF at the reference plane shown here.

Display precision

Full precision is retained internally. Bounds are numerical calculation limits, not equipment ratings.

Consolidated assessment

IDEAL FREE SPACE

LINK MARGIN AT RECEIVER INPUT

—dB

Enter link inputs

Calculated signal minus the specified receiver threshold.

Free-space path loss—dB · isotropic-to-isotropic

EIRP—dBm · after transmit losses + gain

Receiver input power—dBm · after receive losses

Receiver sensitivity—dBm · at the same input plane

Transmitter output: —
− TX feeder / − other TX loss: —
= Power at TX antenna feed: —
+ TX antenna gain: —
= EIRP: —
− Free-space path loss: —
+ RX antenna gain: —
= Available RX antenna power: —
− RX feeder / − other RX loss: —
= Receiver input: —
− Receiver sensitivity: —
= Link margin: —
Total entered insertion loss: —

A positive margin only exceeds the chosen threshold in this ideal model. It does not establish a usable, legal or reliable installed link.

Copyable calculation record

Includes inputs, reference planes and model limitations. You can also select this text manually.

Free-space reference values

Exact c, unrounded constant

Frequency	1 km / dB	10 km / dB	Wavelength / m
145 MHz	75.6751	95.6751	2.06753
433 MHz	85.1775	105.178	0.692361
915 MHz	91.6762	111.676	0.327642
2.4 GHz	100.052	120.052	0.124914
5.8 GHz	107.716	127.716	0.0516884

Double distance or frequency: +6.02060 dB FSPL. ×10 distance or frequency: +20 dB. 1 W = 30 dBm = 0 dBW; 1 mW = 0 dBm = −30 dBW. dBi is an antenna gain reference, dB is a ratio or loss; dBm/dBW are absolute power levels.

Equations & assumptions

SIX FREE FOUNDATIONS · v0.6.0

The signal path

λ = c / f; c = 299792458 m/s (exact)

FSPL = 20 log₁₀(4πdf / c) [dB]

= 32.44778322… + 20 log₁₀(f/MHz) + 20 log₁₀(d/km)

EIRP = Pt − Ltx,feeder − Ltx,other + Gt [dBm]

Pr = EIRP − FSPL + Gr − Lrx,feeder − Lrx,other [dBm]

Margin = Pr − receiver sensitivity [dB]

P[dBm] = 10 log₁₀(P[W]) + 30; P[dBW] = P[dBm] − 30

f is frequency in Hz and d is distance in m in the first FSPL equation. Gains Gt/Gr use dBi in the path direction; insertion losses use positive dB. Power at receiver input is matched available power after all entered receive losses. Receiver gain is not part of this budget. EIRP uses an isotropic reference; it is not ERP or conducted transmitter power.

Optional Tool 05 sensitivity

F = 10^(NF/10); Te = 290(F − 1) K

N = k(Ts + Te)B; k = 1.380649e−23 J/K

Sensitivity = 10 log₁₀(N / 0.001 W) + SNRrequired [dBm]

At Ts = 290 K only, this becomes −173.975187… + 10 log₁₀(B/Hz) + NF + SNRrequired dBm. B is effective noise bandwidth. These are classical white-noise and input-referred receiver assumptions. Use where hf ≪ kTs is appropriate; no quantum or coloured-noise correction is modelled. Source temperature is at the receiver connector after external losses; the tool does not calculate cable thermal emission or a cascaded system NF. Derived sensitivity is theoretical, not a measured equipment specification. Required SNR must be justified for the selected waveform and criterion.

Model limits

Ideal free-space spreading with matched reference planes, directional gains and applicable far-field conditions. No terrain, Earth curvature, obstructions, Fresnel clearance, ground reflection, diffraction or multipath calculation.
No fading, rain/gas attenuation, interference, receiver blocking, availability assurance, radar or spectrum analysis. A user-entered insertion loss is a fixed scalar, not a prediction of these effects.
No antenna dimensions are entered, so far-field validity cannot be confirmed. A distance below 10 wavelengths gets a conservative heuristic caution; exceeding it does not establish the far field. Negative FSPL at a very short separation is shown mathematically and flagged, not treated as physical gain.
No regulatory EIRP/power limit, antenna pattern, polarization or impedance mismatch is inferred. Use applicable measured or specified values; include any known remaining mismatch once in other loss.
Input limits: frequency 1e3–1e12 Hz; distance 1e−3–1e9 m; antenna gains −100…+100 dBi; each loss 0…300 dB; TX power zero or 1e−30…1e30 W (−270…+330 dBm). Manual sensitivity 1e−33…1e7 W (−300…+100 dBm).
Derived limits match Tool 05: Ts 1…100000 K; B 1e−6…1e12 Hz; NF zero or 1e−12…100 dB; SNR −100…+100 dB. No silent clamping. Invalid active inputs clear all dependent results and the copy record; inactive sensitivity inputs are ignored.

Zero transmitter power gives zero received power, −∞ dBm and −∞ margin. A zero margin is exactly the selected threshold, with no reserve. Status uses full precision; a tiny margin may round close to zero on screen. No extra design/fade reserve is automatically subtracted.

Tools 01–05 remain available for power, wavelength, feeder loss, VSWR and receiver noise.

Primary references: ITU-R P.525-5, free-space attenuation; ITU-T K.91 §7.3.3, EIRP. Thermal-noise references: NIST SI defining constants and Keysight noise figure and temperature. External references need internet; calculations run in your browser after the page loads.