H I 21‑cm with small horn + RTL‑SDR: protocol, calibration plan & request for replication feedback

Goal: a replicable 21‑cm detection pipeline (SNR ≥ 6) using a 60–90 cm horn/dish, 1 dB NF LNA, 1420 MHz BPF and RTL‑SDR/Airspy. Observing: plane scans (|b| ≲ 5°), 2–4 h per field, 1–5 s spectra, median stacking. Calibration: Y‑factor or sky‑drift; GPSDO (Phase II) or beacon checks (Δν ≲ 1–2 kHz). Processing: polynomial bandpass, Hanning, Gaussian/Voigt fit, LSR option, FITS + FAIR metadata.

Looking for feedback on: • Practical BPF notch choices around local RFI; • Gain stability and temperature control on the LNA; • Robust SNR definition and uncertainty estimates for amateur‑grade front‑ends; • Minimal B (kHz) vs. τ to hit SNR targets via the radiometer equation.

Data & code will be posted on an OSF/Zenodo DOI for independent checks. No fundraising — just method critique and replication.