A link budget is the running tally of every gain and every loss between a transmitter and a receiver, expressed in decibels. Add the transmit power and the antenna gains, subtract the feeder and path losses, and what is left is the received power. Compare that received power against the receiver sensitivity, and the difference is the link margin. If the margin comfortably exceeds the fade margin your reliability target demands, the link will work.
Received power (dBm) = Transmit power + Antenna gains − Feeder losses − Total path loss
That one line is the whole idea. The rest of this article expands each term, puts real numbers through it, and covers the engineering that decides whether a link that closes on paper also survives in the rain.
Broken out term by term, in decibels:
Prx = Ptx + Gtx − Ltx − FSPL − Lprop + Grx − Lrx
Two intermediate figures matter because they appear on every datasheet and licence. EIRP is the whole transmit side combined, EIRP = Ptx + Gtx − Ltx, and it is the number your licence conditions cap. The link margin is the headroom at the other end, Prx − Receiver sensitivity.
Put real numbers through it and the whole thing fits on a few lines. This is a 5.8 GHz point to point backhaul link, 15 km long, with a clear line of sight, a 28 dBi dish and a 25 dBm radio at each end:
| Item | dB |
|---|---|
| Transmit power | +25.0 |
| Transmit antenna | +28.0 |
| Transmit feedline | −0.5 |
| Free space path loss | −131.2 |
| Receive antenna | +28.0 |
| Receive feedline | −0.5 |
| Received power | −51.2 dBm |
The radio requires −67.0 dBm to sustain the selected modulation and throughput, so the only thing left is to compare the two:
| Item | Value |
|---|---|
| Received power | −51.2 dBm |
| Receiver sensitivity | −67.0 dBm |
| Link margin | +15.8 dB |
That link margin is the number to interrogate: it is not a pass mark on its own, it is the budget the link has to spend on every fade it will ever see.
Rule of thumb
- Doubling the distance costs about 6 dB.
- Doubling the frequency costs about 6 dB.
- Every 3 dB is a factor of two in power.
- Aim for at least 20 dB of fade margin on an important microwave link.
Most of the table above is just reading values off datasheets. The one line that needs working out is the path loss, which depends only on frequency and distance:
FSPL (dB) = 32.45 + 20 · log₁₀(f) + 20 · log₁₀(d), with f in MHz and d in km
At 5800 MHz over 15 km:
FSPL = 32.45 + 20 · log₁₀(5800) + 20 · log₁₀(15) = 32.45 + 75.27 + 23.52 = 131.2 dB
The transmit side gives an EIRP of 25.0 + 28.0 − 0.5 = 52.5 dBm, about 178 W. Subtract the path loss, add the receive antenna gain and subtract the receive feedline loss, and the received power is 52.5 − 131.2 + 28.0 − 0.5 = −51.2 dBm. Against a −67.0 dBm sensitivity that leaves 15.8 dB of margin. On paper the link closes with room to spare. Whether it survives a storm or a summer of leaf growth depends on the considerations below.
Three things turn that paper margin into a link that actually stays up.
A 15.8 dB link margin does not mean the link runs 15.8 dB louder than it needs to all the time. It is the depth of fade the link can absorb before the receiver drops below sensitivity. Radio paths are not static: multipath can cancel the signal for seconds at a time, rain attenuates the path while it falls, and atmospheric layering bends the beam. The fade margin is the reserve you hold against all of it.
How much you need depends on the availability target, usually quoted as a number of nines. A link engineered to 99.99% availability, four nines, may be unavailable for about 53 minutes a year, and reaching that on a microwave path typically calls for 20 to 40 dB of fade margin depending on frequency, path length and climate. So the example link, with 15.8 dB, passes comfortably against a 12 dB requirement but fails against a 20 dB one, at which point the design has to change: larger antennas, a more robust modulation that lowers the sensitivity threshold, a shorter hop, or a less rain affected band.
Free space path loss is only the floor. The example ignores atmospheric and rain loss because both are negligible at 5.8 GHz over a clear 15 km path, but neither stays negligible. Atmospheric absorption and rain attenuation climb quickly through the higher microwave bands, and foliage, terrain diffraction and ground reflection all add loss the free space formula never sees. For anything but a clean line of sight, the free space figure is an optimistic floor rather than the answer. The Path Loss, Fresnel Zones, and the 80% Rule refresher covers the terrain side, and the depth of rain fade for a given rain rate, frequency and path length is what the rain fade calculator is for.
Receiver sensitivity is not arbitrary. It comes from the receiver noise floor plus the signal to noise ratio the chosen modulation needs to decode cleanly, which is why a faster modulation always needs a stronger signal. The noise floor itself is:
Noise floor (dBm) = −174 + 10 · log₁₀(B) + NF
where B is the receiver bandwidth in hertz and NF the noise figure in dB. The practical consequence is that the floor rises by 3 dB for every doubling of bandwidth, so a wide, high throughput channel is harder to close than a narrow one over the same path. The noise floor calculator works this through, including the cascaded Friis noise figure of a multi stage front end.
What is a link budget in RF? A link budget is an accounting of every gain and loss between a transmitter and a receiver, expressed in decibels, used to predict the received signal power and decide whether a wireless link will work. It adds transmit power and antenna gains, subtracts feeder and path losses, and compares the result against the receiver sensitivity to give a link margin.
What is the basic link budget equation? Received power equals transmit power plus transmit antenna gain minus transmit feeder loss minus path loss plus receive antenna gain minus receive feeder loss. All terms are in decibels, with received power in dBm. The link margin is then the received power minus the receiver sensitivity.
What is a good link margin? A link margin is good when it exceeds the fade margin your availability target requires. For a high reliability microwave link aiming at 99.99% availability, that often means 20 to 40 dB; for a short, low frequency or less critical link, far less. A positive margin alone does not guarantee a reliable link.
What is the difference between link margin and fade margin? Link margin is the measured headroom between the received power and the receiver sensitivity. Fade margin is the amount of that headroom you reserve to ride out fading from multipath, rain and atmospheric effects. A link is sound when the link margin is at least as large as the required fade margin.
How does frequency affect a link budget? Higher frequencies raise the free space path loss, at 20 · log₁₀(f), and increase atmospheric and rain attenuation, all of which cost margin. They also let physically smaller antennas reach high gain, which wins some of it back. The net effect depends on the band, path length and climate.
Build it in noIM₃
The link budget calculator runs this whole budget end to end, with EIRP, fade margin, noise floor and carrier to noise ratio, and presets for VHF and UHF land mobile, WiFi, LTE, satellite and microwave. When the path itself is the question, the link planner carries the budget onto real terrain with path profiles and Fresnel clearance.
A link budget is one equation applied with discipline: add the gains, subtract the losses, read off the received power, and check the margin against the fade margin the job demands. Get it right and the link runs through rain, foliage and fading. Get it wrong and it works on the bench and fails on the tower, which is the one place a link budget is meant to stop you ending up.
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