Link Budget

Link budget is the full accounting of every gain and loss along a radio path, used to predict the power arriving at the receiver and confirm a link will actually close. Starting from the transmitter output power, you add antenna gains and subtract cable losses, free-space path loss, and any obstruction or weather penalties. The result is the expected received signal level, which you compare against the receiver's sensitivity to find the leftover margin. It is bookkeeping, not physics wizardry — and it is the difference between a radio link designed on paper and one discovered by trial and error on a cold hillside.

How the math works

The basic relationship, computed entirely in decibels so multiplication becomes addition, is: received power = transmit power + transmit antenna gain − cable and connector losses − path loss + receive antenna gain. The single largest term is usually free-space path loss, which grows with both distance and frequency — every doubling of distance costs 6 dB, and higher bands pay more for the same range. Real paths add obstruction penalties on top: foliage, buildings, and terrain intruding into the Fresnel zone all take their cut. The difference between predicted received power and receiver sensitivity is the link margin. Experienced operators want a comfortable margin — commonly 10-20 dB — so the link survives rain, snow loading, foliage growth, and seasonal changes rather than failing at the first storm. A link that closes with 2 dB to spare in October is a link that dies in May when the leaves come out.

A worked intuition

Consider a modest LoRa node: 20 dBm of transmit power into a 3 dBi antenna, 1 dB of cable loss, and a receiver on the far end that can decode signals down to around −130 dBm at long-range settings. That leaves a total allowable path loss of roughly 150 dB — an enormous number, and the reason LoRa links can span many kilometres on under a watt. LoRa's exceptional sensitivity, able to decode signals below the noise floor, effectively buys budget that ordinary modulation cannot; the price is paid in data rate, since the most sensitive settings are also the slowest. Every element of the chain is a line in the same ledger: a higher mast reduces obstruction loss, a directional antenna adds gain, cheap coax silently spends dB you thought you had.

Two refinements separate paper budgets from honest ones. Receiver sensitivity is not a single number — it depends on data rate and acceptable error rate, so quote it for the settings you will actually run. And margins exist to absorb fading: real signals flutter around their average as weather, foliage, and multipath shift, so the margin is not slack, it is the operating reserve the link spends every day.

Why it matters for sovereign comms

For off-grid Bitcoiners deploying Meshtastic nodes, LoRa telemetry links, or point-to-point microwave to move data without infrastructure, the link budget is the planning tool that decides antenna choice, mast height, and node spacing before any hardware is hauled up a hill. It converts "will it reach?" from a hope into a calculation, and it tells you which fix is cheapest when a path falls short: more height, more antenna gain, a relay node in between, or slower, more sensitive settings. Communications you engineered yourself, on hardware you own, with margins you chose — that is infrastructure independence in its most literal form.

A link budget ties together several RF fundamentals covered nearby. See Attenuation (RF) for the loss terms, dBm for the units it is computed in, and Yagi Antenna for the classic way to add directional gain to one end of a stubborn path.