ChatGPT suggestions on how to fix deaf SDR Play with GNU Radio at 1420 MHz where 100 MHz in same setup seems fine

Short answer: at 1420 MHz you’re probably running out of front-end signal, not software. FM at ~100 MHz is insanely strong so it “looks fine,” but your L-band setup needs proper antenna, gain, and low-loss plumbing.

Here’s a tight checklist that usually fixes “deaf at 1420 MHz” with SDRplay + GNU Radio:

1. Antenna & feed

• A random wire / long dipole that works at 100 MHz is effectively invisible at 1420 MHz.
• For H I (1420.405 MHz) use a small Yagi, patch, helical, or a purpose-tuned λ/2 dipole (~10.6 cm tip-to-tip; ~5.3 cm per arm).
• Keep coax short and low-loss (LMR-240/400 or better). RG-58 will eat your signal at 1.4 GHz.

2. Low-noise amplifier at the antenna

• Put a 20–40 dB LNA (≤1 dB NF) right at the feed, then a narrow 1420 MHz bandpass if you can.
• Power it with the SDRplay bias-T (if your LNA supports 4.7–5 V) or an external bias-tee.

3. SDRplay front-end settings (SoapySDR/gr-sdrplay3)

• Turn AGC OFF and set gains manually.
• Raise LNA state for the L-band (start mid-high, back off if clipping).
• Increase IF gain (IFGR) until your noise floor sits a few dB above the quantization floor but below overload.
• Make sure you’re on the correct antenna port (RSPduo: usually “A”) and that any MW/FM notches aren’t misapplied (they don’t help at 1.4 GHz).
• Use a sane sample rate (e.g., 2–6 MS/s for discovery; decimate in DSP for narrowband work).

Example GNU Radio (Soapy/SDRplay) parameters to try first:

• freq: 1420.405e6
• sample_rate: 2e6
• rf_gain: 45 (or use separate controls: lna_state ~ 6–8, ifgr ~ 20–30 dB)
• agc: false
• biasT: true (if powering an LNA)
• antenna: “A”

4. Sanity checks

• Verify the LNA actually has power (measure current or feel slight warmth).
• Measure coax loss (or temporarily bypass long runs).
• Inject a tiny test tone near 1420 MHz (−60 to −80 dBm) to confirm the chain hears anything.
• Try a strong known L-band beacon (e.g., GNSS at ~1575 MHz) to verify general L-band sensitivity.
• Watch for overload/ADC clipping—if raising gain doesn’t move the noise floor, your LNA isn’t on, or you’re on the wrong port.

5. About swapping the dish for “two cables on poles”
   That sounds like a long HF/VHF dipole. It will be essentially deaf at 1420 MHz. If you want a quick no-dish antenna, build a resonant 1420 MHz half-wave dipole (~10.6 cm total length), a small 3–6-element Yagi, or a simple patch; add an LNA at the feed.