RF Link Budget
Calculate received signal strength (Friis).
What is an RF Link Budget?
An RF Link Budget is the accounting balance sheet of a wireless communication system. Just as an accountant tracks money in versus money out, an RF engineer tracks signal power transmitted versus signal power lost.
The goal is to ensure that after all the gains (amplifiers, antennas) and losses (cables, free space path loss), the signal arriving at the receiver is strong enough to be understood. If the received power (Prx) is higher than the receiver's Sensitivity, the link works!
The Friis Transmission Equation
The core formula for calculating received power in free space is the Friis Transmission Equation. It tells us that signal power drops proportionally to the square of the distance (inverse square law).
Prx (dBm) = Ptx + Gtx + Grx - FSPL
Key Components
- Ptx (Transmitter Power): The output power of the radio, usually in dBm.
- Gtx & Grx (Antenna Gains): Gain is how well the antenna focuses energy. Measured in dBi (decibels relative to isotropic).
- FSPL (Free Space Path Loss): The natural spreading out of the radio wave as it travels.
Path Loss Formula (FSPL)
FSPL (dB) = 20log₁₀(d) + 20log₁₀(f) + 20log₁₀(4π/c)
Basically: Doubling the distance loses 6 dB of signal. Doubling the frequency also loses 6 dB.
Practical Applications
- Satellite Internet (Starlink): Engineers must calculate if the signal can survive the 550km trip through the atmosphere to your dish.
- WiFi Planning: Determining how many access points are needed to cover an office building.
- IoT Sensors (LoRaWAN): Designing sensors that can transmit small packets over 10km using tiny batteries.
FAQ
What is Fade Margin?
The "rainy day fund." Real life isn't free space; there's rain, trees, and interference. Engineers add a safety buffer (typically 10-20 dB) called Fade Margin to the budget to ensure the link stays up even when conditions are bad.
dBm vs. Watts?
Watts are linear. dBm is logarithmic. 30 dBm is 1 Watt. 0 dBm is 1 milliwatt. -100 dBm is 0.1 picowatts! We use dBm because it turns multiplication/division into simple addition/subtraction.
Does higher frequency reduce range?
Generally, yes. According to the path loss formula, higher frequencies (like 5GHz WiFi vs 2.4GHz) suffer more loss over the same distance. They also penetrate walls poorly compared to lower frequencies.