Two radios are useless to each other if the signal arriving at the receiver is buried under its own noise. Long before antennas are mounted or a single packet is sent, an engineer can predict whether the link will work — by accounting for every decibel of gain the system adds and every decibel of loss the path takes away. That accounting exercise is the RF link budget, and it is one of the most reliable predictions in wireless engineering.
This article walks through what a link budget contains, why free-space path loss usually dominates it, and how to read the final number that tells you whether your link closes or fails.
Why this calculation matters
Every wireless system — a Wi-Fi access point, a satellite downlink, a LoRa sensor in a field, a drone control link — lives or dies on the same question: does enough signal reach the receiver? A link budget answers that question on paper, in minutes, and it does so before any money is spent on hardware or tower climbs.
The payoff is in the decisions it drives. The link budget tells you how much transmit power you need, how much antenna gain to specify, how far apart the radios can sit, and how much margin you have left for rain, fading, and the imperfections no spreadsheet fully captures. Skip it and you risk a system that works on the bench at three metres and fails at three kilometres. Regulators also cap transmit power, so the budget often has to deliver range under a hard ceiling — another reason to get it right early.
Because every term is expressed in decibels, the whole budget reduces to addition and subtraction. That simplicity is the point: it makes the trade-offs visible.
The core method
A link budget tracks signal power from the transmitter output, through the antennas and the path, to the receiver input. In decibel form the central relationship is:
P_rx = P_tx + G_tx + G_rx - FSPL
P_rx is the received power, P_tx is the transmitter output power, G_tx and G_rx are the transmit and receive antenna gains, and FSPL is the free-space path loss. Gains add; losses subtract. A full budget would also subtract cable losses, connector losses, and atmospheric or rain attenuation, but the structure stays the same.
The dominant term is almost always the free-space path loss — the energy spread out as the wave expands away from the transmitter. A convenient engineering form is:
FSPL(dB) = 20*log10(d) + 20*log10(f) + 92.45
with distance d in kilometres and frequency f in GHz. The constant 92.45 folds in the unit choices and the physical constants. Two consequences are worth internalising. Doubling the distance adds about 6 dB of loss; doubling the frequency does the same. Higher-frequency links are inherently harder to close over the same distance, which is one reason long-range systems often favour lower bands.
The link succeeds only when the received power stays above the receiver's sensitivity — the weakest signal it can still decode. The difference between the two is the link margin:
Link margin = P_rx - Receiver sensitivity
A positive margin means the link closes. A healthy design keeps several decibels in hand to absorb fading and weather.
A worked example
Consider a 10 km link operating at 2.4 GHz. Start by finding the dominant loss term.
Step 1 — free-space path loss.
FSPL = 20*log10(d) + 20*log10(f) + 92.45
FSPL = 20*log10(10) + 20*log10(2.4) + 92.45
FSPL = 20*(1.0) + 20*(0.380) + 92.45
FSPL = 20 + 7.6 + 92.45
FSPL = 120.1 dB
So just spreading the signal over 10 km at 2.4 GHz costs about 120 dB. That is an enormous number — the received power is roughly a trillion times weaker than the transmitted power before any other loss is even counted.
Step 2 — assemble the received power. With the path loss in hand, the received power follows directly:
P_rx = P_tx + G_tx + G_rx - FSPL
The link closes only if this P_rx stays above the receiver's sensitivity. Suppose the transmitter delivers a modest 20 dBm and both antennas contribute meaningful gain; the antenna gains and transmit power must together overcome that 120 dB hole and still leave a comfortable margin above sensitivity. This is exactly where the budget earns its keep: it shows immediately whether the link is feasible, or whether you need a bigger dish, more power, or a shorter hop.
The lesson from the worked example is that path loss is the term to respect first. Antenna gain and transmit power are the levers you pull to overcome it.
Common mistakes
Forgetting that gains and losses are in decibels. Decibels are logarithmic, so they add — they do not multiply. Mixing a linear power ratio into a decibel sum is a common and costly slip.
Ignoring cable and connector losses. The run of coax between the radio and the antenna can quietly eat several decibels, especially at higher frequencies. Those losses belong in the budget, subtracted just like path loss.
Designing with zero margin. A link that closes with exactly 0 dB of margin will drop the moment it rains, a person walks through the path, or the signal fades. Build in a margin — often 10 dB or more for outdoor links — to absorb the real world.
Using free-space loss where there is no free space. The FSPL formula assumes a clear, unobstructed path. Through buildings, foliage, or over rough terrain the real loss is higher. Free-space loss is a best case, not a guarantee.
Mixing up units in the FSPL formula. The 92.45 constant is tied to distance in kilometres and frequency in GHz. Use metres or MHz without changing the constant and the result will be off by tens of decibels.
Try the interactive NovaSolver calculator
Doing the decibel arithmetic once is straightforward; exploring how range, frequency, and antenna gain trade off is where a tool helps. The Wireless Link Budget Calculator (Friis Equation) on NovaSolver computes received power, SNR, link margin, and maximum range in real time. You can set transmit power, frequency, distance, antenna gains, noise figure, and bandwidth, add rain fade and atmospheric loss, and compare modulation schemes from BPSK to 64QAM — a fast way to see whether a link closes and how much margin it keeps.
Related calculators
- Antenna Gain calculator — to estimate the G_tx and G_rx terms that feed straight into the link budget.
- Antenna Basics calculator — for the fundamentals of size, frequency, and resonance behind any antenna you specify.
- Antenna Radiation calculator — to understand the radiation pattern and how directionality concentrates power where you need it.
You can browse the full set in the RF and microwave tools hub.
Closing note
A link budget is nothing more than honest bookkeeping for radio waves: list every gain, list every loss, add them up, and check the total against what the receiver needs. The arithmetic is simple, but the discipline is what matters. Respect the path loss, keep an honest margin, and account for the cables and the weather, and you will know whether a wireless link will work long before you ever switch it on.
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