What it does
A link budget adds up every gain and loss between a transmitter and a receiver to see whether enough signal arrives to decode the data. It is the first sum in any satellite, CubeSat or long-range radio design. This calculator works it through line by line, from transmit power and antenna gains to free-space loss, noise and the energy per bit. It gives the margin left over, the fastest data rate that would still work, and a chart of the margin through a satellite pass from horizon to overhead.
How to use it
- Pick an example or type your own frequency.
- For a satellite, type its altitude and the elevation you want to plan for. Low elevations are the worst case, as the satellite is furthest away. Or type a fixed distance.
- Fill in the transmitter: power in watts, antenna gain in dBi, and cable losses.
- Fill in the receiver: antenna gain, or a dish size and efficiency, plus cable losses and the system noise temperature.
- Add path losses: atmosphere and rain, polarisation mismatch (3 dB for linear against circular), and pointing.
- Type the data rate, the modulation and the bit error rate you need, and an implementation loss for real hardware.
How it is worked out
- EIRP = transmit power + antenna gain − losses (dBW).
- Free-space path loss = 20 log₁₀(4πdf/c).
- Dish gain = 10 log₁₀(η (πDf/c)²).
- C/N₀ = EIRP − path and other losses + G/T − Boltzmann’s constant (−228.6 dBW/K/Hz).
- Eb/N₀ = C/N₀ − 10 log₁₀(data rate).
- The required Eb/N₀ comes from the bit error rate formulas: BPSK and QPSK need 9.6 dB for 1 error in 100,000 bits, coherent FSK 12.6 dB and non-coherent FSK 13.4 dB. Coded links need less: choose “My own” and type the value from your coder’s data sheet.
- Margin = Eb/N₀ − required − implementation loss. Aim for at least 3 dB, and more for links that fade.
Worked example
A CubeSat 500 km up sends 1 W at 437 MHz from a dipole (0 dBi, 1 dB cable loss) using non-coherent GFSK at 9600 bit/s. At 10° elevation it is 1,695 km away and the path loss is 149.8 dB. A 12 dBi Yagi with 1.5 dB of feed loss, a 500 K system, 3 dB of polarisation mismatch, 1 dB pointing and 0.5 dB atmospheric loss give a C/N₀ of 56.8 dB-Hz and an Eb/N₀ of 16.9 dB. Against 13.4 dB required and 2 dB implementation loss, the margin is only +1.6 dB. That is enough when the satellite is higher, but marginal near the horizon. The same CubeSat at S-band with 2 W, a 6 dBi patch and a 3 m dish reaches 1 Mbit/s with 9.3 dB to spare.
Frequently asked questions
What noise temperature should I use? About 150 to 300 K at L and S band with a good low-noise amplifier, and 500 to 1000 K at VHF and UHF, where man-made noise is high.
Does it include Doppler or fading? No. Add a few dB of margin for fading and make sure your receiver tracks the Doppler shift.
Is anything sent anywhere? No. Everything runs in your browser and works offline.
Where the numbers come from
- Physical constants: CODATA recommended values (NIST).
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-08.