The Link-Budget Margin
Ada re-derives this chapter’s own numbers step by step, at full precision
ADA · CALCULATION AUDIT
The Link-Budget Margin
Four arithmetic checks, ~3 minutes
The physics question is whether enough signal power still reaches the receiver after distance, frequency, enclosure loss, obstacles, and fade reserve have spent the budget.
The chapter puts a 2.4 GHz gateway 100 m from a sensor in a clear yard and asks whether the link really closes. Free-space loss comes to about 80 dB, so a 14 dBm transmitter with two 2 dBi antennas and 3 dB of feed loss lands near −65 dBm — a comfortable 27 dB above a −92 dBm receiver. But add 14 dB of obstruction and 8 dB of fade reserve and only 5 dB is left. This audit re-runs that budget to ask how much link-budget margin actually survives a real installation.
Companion to the chapter Propagation and Link Budgets — every number here comes from that chapter.
The worked-example values
Use only the worked-example values above: a 100 m path, 2.4 GHz carrier, 14 dBm transmitter, two 2 dBi antennas, 3 dB enclosure/feed loss, -92 dBm receiver sensitivity, 14 dB obstruction loss, and 8 dB fade/interference reserve.
The four checks
| Check | Calculation | Result |
| Free-space loss | 20 log10(0.1) + 20 log10(2400) + 32.44 = -20.00 + 67.60 + 32.44 | 80.04 dB, rounded to 80 dB |
| Received power | 14 + 2 + 2 - 3 - 80.04 | -65.04 dBm, about -65 dBm |
| Clean-path margin | -65.04 - (-92) | 26.96 dB, about 27 dB |
| Installed reserve left | 26.96 - 14 - 8 | 4.96 dB, about 5 dB |
Review implication: the link still closes on paper, but the final 5 dB is small enough that a field RSSI/SNR and packet-success record should decide release. The calculation explains what must be measured; it does not replace measurement.
Every number above is taken from the chapter’s own worked link-budget example and re-derived step by step.