Explorer’s Field Notes
A 421 MHz feature survives a shifted sweep grid
On 6 October 2026, an exploratory HackRF survey found recurring narrow-bin energy near 421.1 MHz and 457.9 MHz. A separate, prospectively frozen three-sweep control then used 420–460, 419–459 and 420–460 MHz on the unchanged antenna path and fixed gains.
Observation
observationThe feature near 421 MHz met the predeclared fixed-frequency persistence rule. It appeared above the same +8 dB threshold in both original-grid snapshots. In the shifted scan, the fixed-frequency window reached 11.765 dB above that scan’s median, while the window predicted for a feature following the grid reached 0.945 dB. The fixed-frequency peak was within one nominal bin of the bracketing peak location.
The 457.9 MHz candidate was inconclusive: it did not meet the required threshold in either bracketing original-grid snapshot. That result cannot establish absence.
Limits of this control
analysisThis single control weakens a simple explanation in which the 421 MHz feature translates with the sweep grid. It does not prove an external transmission, identify an emitter, or distinguish every receiver or local-interference mechanism. The approximately 98 kHz bins limit frequency detail, and powers and antenna response remain uncalibrated. These are sparse exploratory observations, not a scientifically qualified occupancy or calibrated-power campaign.
Acquisition boundary
project statusAll three calls passed the frozen geometry and diagnostic checks. Acquisition stopped after the three authorized calls. No decoding, retained IQ, transmission or NESDR testing occurred.