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Radio link budget

Radio link budget: the circuit drawn with the example's valuesd = 2 km97.24 dB lost on the wayTXRX
-85.24 dBm at 2 km on 868 MHz, margin 34.8 dB

A radio wave spreads like light from a bulb: twice as far, a quarter of the power.

Opens in the app on 1 December, with Circuit Notebook 1.0 See it worked out

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The example, worked out

Given

Transmit power
14 dBm
Transmit antenna gain
2 dBi
Receive antenna gain
2 dBi
Frequency
868 MHz
Distance
2 km
Other losses
6 dB
Receiver sensitivity
-120 dBm

Working

  1. the spreading of the wave, in free space
  2. everything adds in dB
  3. what is left over the sensitivity

Results

Free-space loss97.24 dB
Power received-85.24 dBm
Margin34.8 dB
Range at zero margin109 km
Received, in watts2.99 pW

The formulas

  1. FSPL = 20 log10(4π d f / c)the spreading of the wave
  2. Pr = Pt + Gt + Gr − FSPL − Leverything adds in dB
  3. M = Pr − Sthe margin over the sensitivity

The usual traps

  • Free space is the best case: walls and trees add 10 to 20 dB.
  • A margin under 10 dB works on a clear day only.

Questions

How far can my radio link reach?

FSPL = 20 log10(4π d f / c): the spreading of the wave. Pr = Pt + Gt + Gr − FSPL − L: everything adds in dB. M = Pr − S: the margin over the sensitivity.

What does it give for typical values?

-85.24 dBm at 2 km on 868 MHz, margin 34.8 dB. Free-space loss: 97.24 dB; Power received: -85.24 dBm.

What are the usual mistakes?

Free space is the best case: walls and trees add 10 to 20 dB. A margin under 10 dB works on a clear day only.

In the app

Your circuit, drawn live

Change a value: the schematic redraws with your numbers on the parts.

The part you can buy

Results rounded to standard values (E12, E24, E96) and power ratings.

The working, shown

Every step written out. Try it first by hand, then check.

Into your notebook

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Opens in the app on 1 December, with Circuit Notebook 1.0

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