CubeSat RF Link Budget Explained
A link budget is the accounting sheet that tells you whether your ground station will actually hear your satellite — and at what data rate. Get it wrong and your beautifully built CubeSat is a very expensive piece of space debris you can never talk to. This guide walks through every term and finishes with a worked UHF downlink example.
The one equation to understand
Everything in a link budget builds toward a single comparison: the signal-to-noise ratio you receive versus the signal-to-noise ratio you need. The difference between them is your link margin. Positive margin means the link closes; negative means it doesn’t. Aim for at least 3 dB of margin, ideally 6 dB or more to survive rain, pointing error and a spacecraft that isn’t perfectly aligned.
Term by term
| Term | What it means |
|---|---|
| Tx power | How many watts (in dBW or dBm) your radio transmits. |
| Antenna gain | How much the antenna focuses that power in a direction (dBi). |
| EIRP | Tx power + antenna gain − losses: the effective radiated power. |
| Free-space path loss | Energy spreading out over the distance to the ground (the biggest number). |
| G/T | Ground station “figure of merit”: receive gain divided by system noise temperature. |
| Eb/N0 | Energy per bit over noise density — what the modem needs to decode reliably. |
Free-space path loss dominates everything
At UHF (say 435 MHz) to a satellite 1,000 km away, free-space path loss is roughly 145 dB. That single term dwarfs almost everything else, which is why CubeSats lean on the ground station’s big antenna and low-noise front end rather than raw transmit power — there’s only so much a 1 W radio in a shoebox can do.
Worked example: 435 MHz downlink
Here is a realistic 9,600 bps GMSK downlink from a 3U CubeSat:
| Line item | Value |
|---|---|
| Satellite Tx power (1 W) | +0 dBW |
| Satellite antenna gain (monopole) | +0 dBi |
| Line / pointing losses | −2 dB |
| EIRP | −2 dBW |
| Free-space path loss @ 1,000 km | −145 dB |
| Atmospheric + polarisation loss | −3 dB |
| Ground antenna gain (cross-Yagi) | +15 dBi |
| Received power at LNA | −135 dBW |
| System noise (G/T & bandwidth) | — |
| Received Eb/N0 | +11 dB |
| Required Eb/N0 (GMSK, BER 1e-5) | +10 dB |
| Link margin | +1 dB (tight!) |
A +1 dB margin is too tight for comfort. Realistic fixes: add forward error correction (convolutional or LDPC coding can buy 3–5 dB of coding gain), drop the data rate, use a higher-gain ground antenna, or add a mast-mounted LNA to improve G/T. Each of those pushes the margin into safe territory.
Practical tips
- Budget for the worst-case range (low elevation passes), not overhead.
- Cross-polarised (circular) antennas beat linear — a tumbling CubeSat rotates its polarisation.
- An LNA at the antenna matters far more than one at the radio: it sets the noise floor.
- Coding gain is the cheapest dB you’ll ever buy — use it.
The maths is fiddly and every design change ripples through the whole budget. The RF link budget calculator in the app recomputes EIRP, path loss, G/T and margin instantly, and pairs with the ground station guide so you can size both ends of the link together.
Design it for free in your browser
Defiant Smallsat is a free, all-in-one CubeSat & SmallSat design platform. Put this guide into practice — size your budgets, model your orbit and plan your mission without installing anything.