Link Budget Calculation: A Worked 2.4 GHz Point-to-Point Example
Before buying radios and antennas for a wireless link between two buildings, or choosing a LoRa module for a farm sensor, you should answer one question on paper: will enough signal reach the receiver? A link budget answers it by adding every gain and subtracting every loss between the transmitter output and the receiver input, all in decibels.
This tutorial explains each term, then works through a complete 2 km point-to-point link at 2.4 GHz, including noise floor, fade margin and the Fresnel zone clearance that decides how high the antennas must be.
The Link Budget Equation
Pr = Pt + Gt - Lt - FSPL - Lmisc + Gr - Lr (all in dB / dBm) Pt transmitter output power (dBm) Gt,Gr transmit and receive antenna gains (dBi) Lt,Lr cable and connector losses at each end (dB) FSPL free-space path loss (dB) Lmisc extra losses: foliage, walls, rain, polarisation mismatch Pr power arriving at the receiver input (dBm) EIRP = Pt + Gt - Lt (effective isotropic radiated power) Margin = Pr - receiver sensitivity
The receiver sensitivity is the weakest signal at which the receiver still meets a specified error rate, and it is always given in the radio's datasheet for each data rate. The margin is how much extra signal you have above that minimum to survive fading, rain and ageing.
Free-Space Path Loss
Even in a perfect vacuum, power spreads over a growing sphere, so the fraction captured by a receiving antenna falls with the square of distance. With distance in kilometres and frequency in megahertz:
FSPL (dB) = 20 log10(d_km) + 20 log10(f_MHz) + 32.44 Rules of thumb: Double the distance -> +6 dB loss Double the frequency -> +6 dB loss (for antennas of fixed gain)
| Frequency | FSPL at 1 km | FSPL at 5 km |
|---|---|---|
| 433 MHz | 85.2 dB | 99.2 dB |
| 866 MHz | 91.2 dB | 105.2 dB |
| 2400 MHz | 100.0 dB | 114.0 dB |
| 5800 MHz | 107.7 dB | 121.7 dB |
Free-space loss is the best case. Ground reflections, buildings and trees add more, which is why real links need margin.
Worked Example: 2 km Link at 2.4 GHz
Two buildings 2 km apart are to be linked with outdoor 2.4 GHz radios. The datasheet values used here are assumptions for the example; substitute those of your own equipment.
| Parameter | Value |
|---|---|
| Transmit power Pt | +20 dBm (100 mW) |
| Antenna gain each end | 14 dBi (directional panel) |
| Cable and connector loss each end | 2 dB |
| Frequency | 2400 MHz |
| Distance | 2 km |
| Receiver sensitivity at chosen rate | -85 dBm |
| Receiver noise figure | 6 dB |
| Channel bandwidth | 20 MHz |
Step 1: Path Loss
20 log10(2) = 6.02 20 log10(2400) = 67.60 constant = 32.44 FSPL = 106.06 dB
Step 2: Received Power
Pt +20.00 dBm + Gt +14.00 dB - Lt -2.00 dB = EIRP +32.00 dBm - FSPL -106.06 dB + Gr +14.00 dB - Lr -2.00 dB = Pr -62.06 dBm
Step 3: Margin and SNR
Margin = -62.06 - (-85) = 22.9 dB
Noise floor = -174 + 10 log10(20e6) + NF
= -174 + 73.0 + 6 = -95.0 dBm
SNR = -62.06 - (-95.0) = 32.9 dBA margin of about 23 dB is comfortable for a clear line-of-sight link. Many designers aim for at least 10 to 20 dB depending on how much fading and rain they expect and how reliable the link must be.
Step 4: Maximum Range
How far could the link go while keeping a 10 dB margin? The extra allowed loss is 22.9 - 10 = 12.9 dB. Each 6 dB doubles the distance, so 12.9 dB corresponds to a factor of 10^(12.9/20) = 4.4, giving about 8.8 km in ideal free space. In practice, Earth curvature and Fresnel clearance usually limit you first.
Fresnel Zone Clearance
Line of sight is not just a clear straight line. Radio energy travels in an ellipsoid around that line called the first Fresnel zone. Obstacles inside it, such as rooftops, trees or the ground itself, cause extra loss. Keep at least 60 percent of the zone clear.
Radius of first Fresnel zone at a point d1 from one end, d2 from the other: r (m) = 17.32 x sqrt( (d1_km x d2_km) / (f_GHz x d_km) ) At mid-path of our link: d1 = d2 = 1 km, d = 2 km, f = 2.4 GHz r = 17.32 x sqrt(1 / 4.8) = 17.32 x 0.456 = 7.9 m 60 % clearance = 4.7 m
So the straight line between the antennas should pass at least 4.7 m above any obstruction at the middle of the path. Lower frequencies have larger Fresnel zones, which is why sub-GHz links need taller masts for the same distance.
Common Mistakes
- Using the sensitivity for the lowest data rate while planning to run at the highest. Sensitivity worsens as rate increases.
- Forgetting connector and cable losses, which at 2.4 GHz can be several dB for a long thin coax run.
- Exceeding legal EIRP limits by adding high-gain antennas to a full-power radio. Check the rules for your band.
- Assuming visual line of sight means Fresnel clearance.
- Mixing km with metres or GHz with MHz in the FSPL constant. The 32.44 constant is only for km and MHz.
Summary
A link budget is a column of additions: transmit power, plus antenna gains, minus cable losses and path loss, gives received power. Compare it with receiver sensitivity to get the margin, and with the noise floor to get SNR. Free-space loss is 20 log d + 20 log f + 32.44 with km and MHz, and grows 6 dB per doubling of distance. In our 2 km, 2.4 GHz example, 100 mW and 14 dBi antennas deliver -62 dBm with 23 dB of margin, provided about 4.7 m of mid-path clearance is kept.