A link budget is a straightforward accounting of every gain and loss a radio signal experiences as it travels from a transmitter to a receiver. It adds up the power you start with, subtracts every source of signal loss along the way, adds back any gains from antennas or amplifiers, and checks whether enough power arrives at the other end for the receiver to decode the message. Think of it like a financial budget: you start with a balance (transmit power), earn some (antenna gains), spend some (distance, atmosphere, cable losses), and need a minimum amount left over to stay in the black.
Link budgets are used everywhere radio signals travel: cell towers talking to phones, satellites beaming data to ground dishes, Wi-Fi routers reaching laptops, and deep-space probes communicating with Earth. The math is done entirely in decibels (dB), which turns multiplication and division into simple addition and subtraction.
The Basic Equation
At its core, a link budget answers one question: is the received signal strong enough? The simplified form looks like this:
Received Power (dBm) = Transmit Power (dBm) + Gains (dB) − Losses (dB)
You then compare that received power to the receiver’s minimum sensitivity. If the received power is higher, the link works. If it’s lower, the signal is too weak to decode reliably. The difference between what you actually receive and the minimum you need is called the link margin, and engineers deliberately build in extra margin to handle real-world unpredictability.
Transmit Power and Antenna Gain
Every link budget starts with two numbers on the transmit side: the power fed into the antenna and the antenna’s gain. Transmit power is measured in dBm (decibels relative to one milliwatt) or dBW (relative to one watt). A handheld radio might transmit at 1 watt (30 dBm), while a satellite downlink or cellular base station transmits considerably more.
Antenna gain describes how well an antenna focuses energy in a particular direction rather than radiating equally in all directions. A higher-gain antenna concentrates the signal into a narrower beam, effectively making the signal stronger in that direction without increasing power consumption. Gain is measured in dBi (decibels relative to an ideal isotropic antenna that radiates equally everywhere). A large parabolic dish used in satellite ground stations can achieve 40 dBi or more at 12.5 GHz. The gain of any antenna is directly related to its physical size relative to the signal’s wavelength: a bigger aperture captures or focuses more energy. Doubling the diameter of a dish roughly quadruples its effective area and adds about 6 dB of gain.
The same principle applies on the receive side. A larger or more directional receiving antenna adds gain to the budget, partially compensating for losses during transmission.
Free-Space Path Loss
The single largest loss in most link budgets is free-space path loss, which is simply the signal spreading out as it travels. A radio wave radiates outward like an expanding sphere, so the energy per square meter drops with the square of the distance. Double the distance and you lose 6 dB of signal strength.
Free-space path loss also increases with frequency. Higher frequencies have shorter wavelengths, and a receiving antenna of a given physical size captures a smaller fraction of the incoming wavefront at shorter wavelengths. This is why millimeter-wave 5G signals (above 24 GHz) fade much faster over distance than lower-frequency LTE signals. The relationship is captured by the Friis transmission equation, which shows that received power is proportional to the square of the wavelength and inversely proportional to the square of the distance.
For a satellite orbiting at 550 km, the path loss at 12 GHz is enormous, often exceeding 170 dB. For a Wi-Fi router 10 meters away at 2.4 GHz, it’s around 60 dB. These numbers dominate the entire link budget, and everything else in the calculation exists to overcome them.
Other Sources of Loss
Free space is only the beginning. Real signals pass through cables, connectors, the atmosphere, and physical obstacles, each chipping away at the budget.
- Cable and connector losses: The coaxial cable between a transmitter and its antenna absorbs some energy as heat. Every connector introduces a small mismatch. These losses are typically a few dB total but matter in tight budgets.
- Atmospheric attenuation: Oxygen, water vapor, clouds, and rain all absorb radio energy. These effects generally increase with frequency. Ka-band signals (26-40 GHz) suffer significantly more atmospheric loss than S-band (2-4 GHz) or X-band (8-12 GHz). Rain is particularly damaging at higher frequencies, a phenomenon called rain fade.
- Polarization mismatch: If the transmitting and receiving antennas aren’t aligned in polarization, some signal is lost. In Starlink ground-station measurements, for example, the lack of a proper polarizer and feedhorn misalignment alone accounted for 4 to 5 dB of loss.
- Shadowing and obstacles: Buildings, terrain, and vegetation block or scatter signals. In cellular network planning, the shadow standard deviation typically ranges from 4 to 12 dB depending on the environment, with 8 dB being a commonly used value for urban areas.
The Noise Floor and Receiver Sensitivity
A receiver doesn’t just need to detect the signal. It needs to distinguish the signal from the background electrical noise. Every electronic component generates thermal noise (also called Johnson noise) from the random motion of atoms. This sets a fundamental minimum noise level called the noise floor.
Thermal noise power depends on temperature and bandwidth. At room temperature (about 290 Kelvin), the noise power spectral density is roughly −174 dBm per hertz of bandwidth. A receiver with a 20 MHz bandwidth, for instance, starts with a noise floor around −174 + 73 = −101 dBm before any additional noise from its own electronics. The receiver’s noise figure, which describes how much extra noise the receiver’s own circuitry adds, raises that floor further.
Receiver sensitivity is the minimum signal power needed to achieve an acceptable error rate at a given data rate. For digital systems, this is tied to the signal-to-noise ratio (SNR). A higher required data rate generally demands a higher SNR, which means the receiver needs more signal power. A low-noise amplifier at the front end of the receiver, like the one in a Starlink ground terminal with 60 dB of conversion gain and a 0.8 dB noise figure, helps by boosting the signal before downstream electronics can degrade it.
Link Margin: The Safety Buffer
No link budget assumes perfect conditions. Engineers add a link margin, extra dB of received power above the minimum sensitivity, to account for things that are hard to predict: weather changes, equipment aging, slight antenna mispointing, or temporary obstructions.
How much margin you need depends on the application. A cellular network designed to maintain coverage 90% of the time in an environment with 8 dB of shadow variation needs roughly 10 dB of shadow margin alone. Satellite links typically budget several dB for rain fade on top of that. A point-to-point microwave link on a clear line of sight might need less margin since fewer variables are in play.
If the final link margin is negative, the link will not work reliably. The engineer then has a few options: increase transmit power, use a higher-gain antenna, reduce the data rate (which lowers the required SNR), narrow the receiver bandwidth, or shorten the distance.
Putting It All Together
Here’s what a simplified link budget looks like in practice, laid out as a running total:
- Transmit power: +30 dBm
- Transmit antenna gain: +15 dBi
- Cable loss (transmit side): −2 dB
- Free-space path loss: −130 dB
- Atmospheric loss: −3 dB
- Receive antenna gain: +12 dBi
- Cable loss (receive side): −1 dB
- Received power: −79 dBm
- Receiver sensitivity: −90 dBm
- Link margin: 11 dB
You add every gain and subtract every loss. The received power of −79 dBm is 11 dB above the receiver’s sensitivity of −90 dBm, so there’s a comfortable margin. Each line item can be measured, calculated from physics, or estimated from manufacturer datasheets, which makes the link budget both a design tool and a troubleshooting checklist. When a link underperforms, you can walk through each term to find what changed.
Why Link Budgets Matter Beyond Engineering
Link budgets explain many things you encounter in daily life. Your phone signal drops in a basement because the building materials add loss that eats through the margin. Satellite internet slows down during heavy rain because rain fade temporarily steals dB the budget can’t spare. Your Wi-Fi router works better in the same room than through two walls because each wall is a loss term in the budget. 5G millimeter-wave signals promise blazing speeds but cover short distances because the path loss at 28 or 39 GHz is dramatically higher than at the sub-6 GHz bands used by 4G.
Understanding the link budget concept, even without doing the math, gives you a framework for thinking about any wireless system: there’s a power source, there are gains, there are losses, and there’s a minimum threshold. Everything in wireless design is an effort to keep the balance positive.