RF EngineeringJanuary 22, 20267 min read

RF Link Budget Analysis: A Step-by-Step Engineering Guide

Learn how to perform an RF link budget analysis from scratch. Covers free-space path loss, antenna gain, cable losses, receiver sensitivity, and link margin calculation with worked examples.

What Is a Link Budget?

A link budget is an accounting of all the gains and losses a signal experiences from transmitter to receiver. If the received power exceeds the receiver sensitivity (with sufficient margin), the link works. If not, you need to find more gain, reduce losses, or move the antennas closer.

The Fundamental Equation

P_{rx} = P_{tx} + G_{tx} - L_{cable,tx} - FSPL - L_{misc} + G_{rx} - L_{cable,rx}

All values in dBm or dB. The result is received power in dBm.

Link margin = P_rx − Sensitivity_rx

A positive margin means the link works. Standard targets:

Indoor WiFi: 10–15 dB margin
Outdoor point-to-point: 15–20 dB margin
Satellite link: 3–6 dB (every dB costs money)

Free-Space Path Loss

FSPL dominates every wireless link. It's not a loss in the physical sense — it's the geometric spreading of the wavefront:

FSPL = 20\log_{10}\left(\frac{4\pi d f}{c}\right)

For quick mental math: FSPL ≈ 20log(f_GHz) + 20log(d_km) + 92.4 dB

Examples:

2.4 GHz, 100m: 80 dB
2.4 GHz, 1 km: 100 dB
28 GHz (5G mmWave), 100m: 101 dB — 21 dB more than 2.4 GHz at same range

Receiver Sensitivity

Sensitivity is determined by noise floor and required SNR:

S_{min} = -174 + 10\log_{10}(BW) + NF + SNR_{min}

Where:

−174 dBm/Hz is thermal noise at room temperature (kT)
BW is receiver bandwidth in Hz
NF is receiver noise figure in dB
SNR_min is the minimum SNR for demodulation (e.g., 10 dB for BPSK, 25 dB for 64-QAM)

A typical 802.11n receiver at 20 MHz bandwidth with 7 dB NF and 10 dB SNR_min: S_min = −174 + 73 + 7 + 10 = −84 dBm

Worked Example: 900 MHz IoT Link

Goal: 500m range, outdoor, LoRa-style system.

Parameter	Value
TX Power	+20 dBm (100 mW)
TX Antenna Gain	2 dBi (simple dipole)
TX Cable Loss	−0.5 dB
FSPL @ 915 MHz, 500m	−85.7 dB
Environmental loss	−5 dB (trees, buildings)
RX Antenna Gain	2 dBi
RX Cable Loss	−0.5 dB
Received Power	−67.7 dBm
RX Sensitivity (LoRa SF7)	−123 dBm
Link Margin	+55.3 dB

Excellent margin — could extend range to several kilometers or use lower TX power to save battery.

Common Mistakes

Forgetting polarization loss. Misaligned antennas can lose 3–20 dB. Two linear antennas at 90° lose ~20 dB. Ignoring impedance mismatch. A 2:1 VSWR creates 0.5 dB mismatch loss. Easy to forget, difficult to recover in a tight budget. Using peak antenna gain in all directions. Antenna gain is directional. A 10 dBi dish only delivers 10 dBi in the boresight direction. Off-axis can be −10 to −30 dB. Not accounting for fading margin. Real wireless channels fade. Add 5–15 dB margin for Rayleigh fading in multipath environments.

Use our RF Link Budget Calculator to model your system, including received power vs. distance plots.

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