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Bench testing without lying to yourself: near-field saturation

The most common way a bench test misleads you: you put the TX and RX inches (or a few feet) apart, run high power, and the link is worse than it was at range — low throughput, dropped frames, garbage video — and you start blaming the NIC, the antennas, or interference. Usually none of those. The signal is simply too strong for the receiver, and adding power or swapping antennas makes it worse.

Two mechanisms, both power problems that masquerade as sensitivity problems:

  • Front-end saturation. The LNA and ADC clip. The chip's AGC drops its gain to the floor and still can't keep the signal in the linear range, so the constellation smears — even though the raw signal level is enormous.
  • Multipath self-jamming. Your own strong signal bounces off nearby walls and arrives delayed. To the demodulator those delayed copies are in-band noise. Past some power level, adding TX power adds as much reflected noise as signal, and the link jams itself. This is why a link that is fine outdoors falls apart in a room at ten feet.

The tell: EVM, not SNR

The counterintuitive part — and the reason people miss it — is that SNR can look perfectly healthy while the link is saturating. Measured by sweeping an 8812AU's TX power from low to full while a second adapter reported per-frame metrics (tests/saturation_knee_sweep.sh):

TX power RSSI (raw) SNR EVM
low 50 18 dB −28 dB (clean)
knee ~60 18 dB −28 dB (best)
high 70 18 dB −20 dB (degrading)
full 73 18 dB (unchanged) −13 dB (collapsed)

SNR sat flat at 18 dB across the whole sweep and told you nothing. EVM improved as power rose, then reversed at the saturation knee — the moment more signal started making the constellation worse. That turnover is the signature.

Is 25 mW too much next to each other?

Yes. The numbers, at 5.8 GHz:

  • Free-space loss is only ~28 dB at 10 cm, ~57 dB at 3 m (10 ft).
  • 25 mW = +14 dBm. At 10 cm the receiver sees +14 − 28 = −14 dBm.
  • The RTL88xx front end starts compressing around −10 to −20 dBm input.

So 25 mW at 10 cm lands you squarely in compression — which is exactly the "RSSI −10, link dead" report. These receivers are linear and happy around −40 to −70 dBm at the input. To bench two adapters safely you want ≥ 40 dB of loss between them: drop TX to ~1 mW (0 dBm), add a 30–40 dB attenuator, or both. Ten feet in a reflective room is past hard clipping but squarely in the multipath-desense regime.

How to bench without the trap

  1. Back the power off. With the runtime TX-power API (src/TxPower.h) this is one call — no low-power firmware needed: dev->SetTxPowerOffsetQdb(-80) drops 20 dB (25 mW → ~0.25 mW). Sweep the offset down until EVM stops improving; that knee is your linear operating point. tests/saturation_knee_sweep.sh does exactly this and prints it.

  2. Add loss instead of distance when you can't move things apart: a 30–40 dB SMA attenuator on the conducted path, or just physical separation and cross-polarised antennas.

  3. Watch EVM, not SNR. A strong RSSI with poor EVM is the saturation fingerprint. DEVOURER_RX_ENERGY_MS=500 DEVOURER_LINKHEALTH=1 on rxdemo classifies each window and says so in plain language:

    {"ev":"link.health","verdict":"SATURATED","rssi_dbm":-33,"snr_db":19.5,"evm_db":-22.5,
  cause="strong RSSI but poor EVM — receiver front-end overload and/or the
  strong signal self-jamming via reflections (near-field). SNR alone can
  look fine here"
  fix="REDUCE TX power (SetTxPowerOffsetQdb, e.g. -40..-80 qdB), add an
  attenuator, or increase distance — do NOT add power/antenna"

  4. Only trust a "weak link" verdict at range. If the link doctor says WEAK (low RSSI, low SNR, AGC wide open) then more power or a better antenna is the answer. If it says SATURATED, the answer is the opposite — and no amount of antenna tuning will fix a receiver that is clipping.

The link doctor (the link.health event)

src/LinkHealth.h maps the RX sensor tuple to one of: SATURATED, INTERFERENCE, WEAK, MARGINAL, HEALTHY, NO_SIGNAL, each with a one-line cause and the fix. The discriminators, all from telemetry devourer already exposes:

  • SATURATED — strong peak RSSI and poor EVM (SNR can look fine). Back off power / attenuate / add distance.
  • INTERFERENCE — not-strong signal, degraded, and a high false-alarm rate: external energy raising the floor. Hop channels.
  • WEAK — low RSSI, low SNR, AGC at its ceiling. Genuine range limit: more power / antenna helps here.
  • HEALTHY — good SNR + EVM in the linear range.

It rides the DEVOURER_RX_ENERGY_MS cadence (so set both). The thresholds are calibrated from on-air measurement (the sweep above and the AWGN sweep in tests/j3_dig_penalty_sweep.sh) and unit-guarded in tests/link_health_selftest.cpp.