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Free space path loss is the expected reduction in signal strength between an ideal transmitter and receiver in open air as distance increases and frequency rises. A wireless link budget calculator helps translate that loss into received power so you can judge reach and reliability with a single view. You provide a carrier frequency and a path length, then add antenna gains and any losses to see how the link performs.
Results summarize free space path loss, received power, and effective isotropic radiated power, then extend into link margin, noise floor, and signal to noise ratio. You can set receiver sensitivity to see margin directly, select a noise bandwidth with a noise figure to get a realistic noise floor, and add a target received level to estimate the maximum range under free space conditions.
A typical check might be a home access point across a room where frequency is 2.4 GHz and distance is 15 m. The received level will be shaped by antenna gains and cable losses, and indoor attenuation will lower the result further. Use additional loss for walls or foliage and a polarization penalty when antennas are misaligned.
Stable comparisons come from consistent units and repeatable inputs. Set the same bandwidth when comparing links, keep gains and losses positive or zero, and measure distance along the line of sight. For deeper analysis, sweep distance or frequency to see how power and loss evolve.
The calculator models a line‑of‑sight radio link using a classical free space formulation. Quantities include carrier frequency, path length, antenna gains, cable and environmental losses, receiver sensitivity, noise bandwidth, and noise figure. Outputs are received signal strength in dBm, link margin in dB relative to a sensitivity threshold, thermal noise floor in dBm, and signal to noise ratio in dB. Wavelength and the first Fresnel zone radius at mid‑path are provided for clearance checks.
Computation proceeds from path loss to received power. Free space path loss grows logarithmically with both distance and frequency, so doubling either increases loss by roughly 6 dB. Received power results from the transmitted power combined with antenna gains and all stated losses, minus the path loss. Link margin compares the received power to the receiver sensitivity, while the noise floor is derived from bandwidth and noise figure. Signal to noise ratio subtracts the noise floor from the received power at the same bandwidth.
Interpretation is straightforward. A positive link margin suggests headroom at the selected rate or demodulation threshold, while a negative value indicates the receiver is below its stated sensitivity. Signal to noise ratio is bandwidth‑specific and reflects thermal noise only. The maximum distance estimate for a target received level applies strictly to unobstructed free space.
Comparisons are valid when using the same bandwidth, modulation targets, and polarization. The model assumes far‑field conditions and does not account for multipath, diffraction, rain attenuation, or interference. Use additional loss and polarization mismatch to capture known penalties.
| Symbol | Meaning | Unit/Datatype | Source |
|---|---|---|---|
| fMHz | Carrier frequency | MHz | Input |
| dkm | Path length | km | Input |
| FSPL | Free space path loss | dB | Derived |
| Ptx | Transmitter conducted power | dBm | Input |
| Gtx, Grx | Antenna gains | dBi | Input |
| Ltx, Lrx, Ladd, Lpol | Cable, other, and polarization losses | dB | Input |
| EIRP | Effective isotropic radiated power | dBm | Derived |
| Prx | Received power (RSSI) | dBm | Derived |
| B | Noise bandwidth | Hz | Input |
| NF | Noise figure | dB | Input |
| N | Thermal noise floor | dBm | Derived |
| SNR | Signal to noise ratio | dB | Derived |
| LM | Link margin vs sensitivity | dB | Derived |
| Ssens | Receiver sensitivity | dBm | Input |
| Dmax | Max distance at target RSSI | km | Derived |
| λ | Wavelength | m | Derived |
| r1 | First Fresnel radius at mid‑path | m | Derived |
Worked example. Inputs: 2.412 GHz, 15 m, 18 dBm transmitter, 3 dBi antennas, 1 dB cable loss each, 10 dB additional loss, 0 dB polarization. Sensitivity −72 dBm. Bandwidth 20 MHz, noise figure 7 dB.
Interpretation: The link clears sensitivity by about 20 dB, and thermal noise yields a strong SNR for robust rates under free space assumptions.
| Field | Type | Min | Max | Step/Pattern | Units | Error Text |
|---|---|---|---|---|---|---|
| Frequency | Number | 0 | — | Any | MHz or GHz | None |
| Distance | Number | 0 | — | Any | m or km | None |
| TX Power | Number | — | — | Any | dBm | None |
| TX Antenna Gain | Number | — | — | Any | dBi | None |
| TX Cable Loss | Number | — | — | Any | dB | None |
| RX Antenna Gain | Number | — | — | Any | dBi | None |
| RX Cable Loss | Number | — | — | Any | dB | None |
| Additional Loss | Number | — | — | Any | dB | None |
| Polarization Mismatch | Number | — | — | Any | dB | None |
| RX Sensitivity | Number | — | — | Any | dBm | None |
| Noise Figure | Number | — | — | Any | dB | None |
| Noise Bandwidth | Number | 0 | — | Any | Hz, kHz, MHz | None |
| Target RSSI | Number | — | — | Any | dBm | None |
Inputs accept decimal numbers with a period as the separator. Displays round most values to two decimals; some compact badges use one decimal. Chart sweeps use 80 logarithmically spaced points for smooth curves. JSON output preserves full internal precision; CSV rows use the displayed strings for readability.
| Input | Accepted Families | Output | Encoding/Precision | Rounding |
|---|---|---|---|---|
| Frequency, distance, powers, gains, losses, bandwidth, sensitivity | Numeric with units selectors | FSPL, EIRP, RSSI, LM, SNR, noise floor, wavelength, Fresnel radius | Displayed text, CSV, JSON | One to two decimals on screen |
Processing is client‑only. Copy to clipboard and file downloads occur locally. No data is transmitted or stored server‑side.
Core calculations are constant time. Distance and frequency sweeps are linear in the number of sampled points.
Free space path loss and received power are estimated from a small set of radio parameters.
Example: 2.412 GHz, 15 m, 18 dBm, 3 dBi antennas, 1 dB cable losses, 10 dB additional loss. Expect about −51.6 dBm received, ~20 dB margin against −72 dBm sensitivity.
No. Calculations run locally and nothing is transmitted or kept on a server.
Copy and download actions use your device only.It is exact for free space and far‑field conditions. Real links include fading and clutter, so treat results as optimistic unless you add realistic losses.
Frequency in MHz or GHz, distance in m or km, bandwidth in Hz, kHz, or MHz. Powers are in dBm, gains in dBi, and losses in dB.
Yes. Enter a lumped value in Additional Loss and, if antennas are misaligned, a penalty for polarization mismatch.
When link margin hovers within about 0 to 3 dB, small changes in fading or interference can tip performance. Aim for extra headroom.
Combine transmitter power with gains and subtract every loss, then subtract path loss. Compare the received level to sensitivity for margin.
Yes after the page is loaded. All computations are local and do not need a connection.
Throughput depends on modulation, rate adaptation, and interference. Thermal noise alone does not capture collisions or adjacent channel effects.