What This Calculator Does
This tool computes a complete RF link budget — the accounting of all gains and losses between a transmitter and receiver. It estimates received signal power, fade margin, and link quality using standard formulas including Free-Space Path Loss (FSPL) and Two-Ray Ground Reflection.
Basic Workflow
- Set your frequency and link distance in the RF & Path tab.
- Enter Tx Power, antenna heights, and antenna gains.
- Select a propagation model (Free Space is standard for clear line-of-sight links).
- Set cable losses — use the cable calculator or enter dB values directly.
- Enter the receiver sensitivity from the radio datasheet, or switch to BW+NF+SNR mode to calculate it.
- Click Calculate to see the link result and fade margin.
- Use Fade Margin to plot signal vs. distance. Use 3D Surface to sweep antenna azimuth.
Main Input Fields
- Frequency [MHz] — RF center frequency. Affects FSPL — higher = more loss. Ex: 900 MHz (LoRa), 2400 MHz (Wi-Fi).
- Distance [km] — One-way link distance. FSPL rises ~20 dB per 10× distance increase.
- Tx Power [dBm] — Transmitter output power. 30 dBm = 1 W. Check local regulations for legal limits.
- Tx/Rx Gain [dBi] — Antenna gain relative to isotropic. Higher = more directional. 0 dBi = omni, 20+ dBi = dish.
- Cable Loss [dB] — Feedline insertion loss from radio port to antenna. Auto-calculated or enter manually.
- Rx Sensitivity [dBm] — Minimum detectable signal from radio datasheet. Ex: −90 dBm (Wi-Fi), −130 dBm (GPS).
- Bandwidth & Noise Figure — Used to derive sensitivity when in BW+NF+SNR mode (Capacity tab).
Understanding Fade Margin
Fade Margin (FM) = Received Power − Receiver Sensitivity. It is the safety buffer in dB above the minimum required signal level.
- ≥ 10 dB — Good. Typical minimum for reliable outdoor fixed links.
- 0 – 10 dB — Caution. Marginal link — may fail under adverse conditions.
- < 0 dB — Fail. Signal is below sensitivity — link will not close.
- For critical infrastructure, design for 20–30 dB to account for rain, multipath, and component aging.
SNR and Shannon Capacity
Enable the Capacity tab to calculate theoretical maximum throughput. The Shannon-Hartley theorem gives:
C = B × log2(1 + SNR) [Mbps]
- SNR [dB] — Signal-to-Noise Ratio at receiver input. Drives modulation selection and capacity.
- Noise Figure [dB] — Receiver noise added above thermal noise floor (kTB). Lower is better.
- Shannon Capacity — Theoretical maximum. Real-world throughput will be lower due to protocol overhead and coding.
- SNR Margin — Headroom above the SNR required for the selected modulation scheme.
Antenna Pattern & Direction
The Antennas tab models directional beam patterns, VSWR mismatch, and polarization. The 3D Surface plot shows fade margin vs. antenna azimuth sweep.
- Directional Simple — Gaussian/sinc approximation from beamwidth and front-to-back ratio.
- Omni — No angular attenuation (isotropic in azimuth plane).
- CSV Pattern — Upload a measured antenna pattern file for accurate modeling.
- VSWR — Impedance mismatch loss. 1.5:1 ≈ 0.18 dB, 3:1 ≈ 1.25 dB. Measure with antenna analyzer.
- Polarization — Cross-polarization (e.g., vertical vs. horizontal) causes 20–30 dB isolation. Always match polarization.
Output Results
- Rx Power [dBm] — Estimated signal level at the receiver antenna port.
- Fade Margin [dB] — Headroom above sensitivity. Green ≥10 dB, yellow 0–10 dB, red <0 dB.
- Path Loss [dB] — Total propagation loss including any Fresnel/obstacle contributions.
- Radio Horizon [km] — Maximum geometric line-of-sight range given antenna heights and k-factor.
- Shannon Capacity [Mbps] — Theoretical max throughput (requires Capacity tab to be enabled).
Engineering Disclaimer: Results are estimates based on simplified propagation models. They do not account for terrain, vegetation, building penetration, atmospheric ducting, interference, or regulatory constraints. Always validate critical RF deployments with field measurements, site surveys, and spectrum analysis.
