| TX Power | {{ txPower }} dBm |
| TX Ant. Gain | {{ txGain }} dBi |
| TX Cable Loss | -{{ txCableLoss }} dB |
| RX Ant. Gain | {{ rxGain }} dBi |
| RX Cable Loss | -{{ rxCableLoss }} dB |
| Additional Loss | -{{ additionalLoss }} dB |
| Calculated FSPL | {{ fspl.toFixed(2) }} dB |
| Received Power | {{ rxPower.toFixed(2) }} dBm |
Free-space path-loss (FSPL) models how an electromagnetic wave weakens as it propagates through unobstructed space. Wireless engineers rely on this logarithmic metric to forecast link viability, choose antenna patterns, and size power amplifiers. Because FSPL depends only on frequency and distance, it offers a baseline before terrain, weather, or hardware imperfections are considered.
This calculator couples a reactive engine with a lightweight charting layer to transform your frequency, distance, and hardware figures into instant path-loss and received-signal estimates. It applies the Friis transmission equation, adds configurable gains and losses, then plots both distance-swept RSSI and frequency-swept FSPL so you can visualise margins while adjusting sliders in real time.
An installer planning a rural LoRa gateway might preview whether a ten-kilometre hop at 868 MHz still clears the fade margin once cable attenuation and enclosure losses are included. The read-back helps decide mast height before anything ships onsite. Always verify that chosen frequencies, emissions, and antenna configurations satisfy national spectrum regulations, because exceeding licensed limits invites enforcement action.
The calculator implements the Friis free-space model, a first-principles expression derived from conservation of energy in spherical wavefronts. It assumes isotropic radiation and neglects multipath, foliage, humidity, or device nonlinearities. Two independent variables—distance (d) in kilometres and frequency (f) in megahertz—govern spreading loss. Additional user-defined factors adjust the link budget: transmitter power, antenna gains, cable attenuation, and any lump-sum path impairments.
| RSSI Band (dBm) | Link Quality |
|---|---|
| > −50 | Excellent margin |
| −50 to −67 | Reliable throughput |
| −67 to −80 | Degraded but usable |
| < −80 | Risk of drops |
Higher negative values indicate weaker signals; beyond −80 dBm many protocols lower data rates or time out.
Example – 868 MHz, 2 km, 14 dBm TX, 2 dBi antennas, 1 dB cable loss:
Based on Friis (1946) and validated by ITU-R P.525-4, ETSI TR 103 256, and FCC OET Bulletin 65 guidance.
All computations execute locally; the tool does not send or store user data.
Follow these steps to evaluate a link budget quickly.
No. Calculations run entirely on your device; nothing leaves the browser.
Frequency accepts MHz or GHz, and distance accepts metres or kilometres. Gains and losses are always in decibels.
The model matches the theoretical free-space curve within 0.1 dB. Real-world variation depends on obstacles, humidity, and hardware tolerances.
No. This tool focuses on free-space propagation. Add obstruction losses manually using the “Other Loss” field.
Received power is referenced to one milliwatt; wireless signals rarely exceed that baseline, so decibel-milliwatt values are typically negative.