Sub‑GHz Troubleshooting: “Decode Looks Right but Replay Fails”
Getting a clean-looking decode is not the same as emitting a signal the receiver will accept. Replay failures usually come from frequency plan mismatches, timing/modulation differences, or security controls like rolling codes.
Permission-first: only test on devices you own or have explicit written authorization to assess.
Many real-world Sub‑GHz systems control safety/security functions—treat them as off-limits without written scope.
See Legal & Ethics.
1) Frequency plan + region gotchas
- 433 vs 868 vs 915: a “close” frequency is often a total miss for narrow receivers.
- Device regional variants: the same product can ship with different RF front ends by region.
- Tool configuration: confirm your tool is set to the exact frequency plan you observed.
2) Modulation and timing mismatches
Some decoders output a symbolic representation that looks correct, while replay requires faithful timing, preambles, and modulation settings to match the receiver’s tolerance.
- Preamble/sync: missing or malformed preambles can prevent the receiver from locking.
- Bit rate tolerance: a few percent timing error can be enough to fail.
- OOK/FSK assumptions: decoding under the wrong modulation can still “look structured” but replay wrong.
3) Rolling codes and backend validation
- Rolling codes: many remotes intentionally change code each press; replaying an old capture should fail.
- Challenge-response: some systems require a reader-side challenge or timing window.
- Expect “fails by design”: in defensive validation, replay failing is often the correct outcome.
4) RF power, antenna, and “too close” problems
- Antenna mismatch: tuned antennas matter; a wideband whip is convenient but not always effective.
- Distance: too close can overload receivers or cause near-field coupling quirks.
- Orientation: small changes in antenna angle can materially change success rate.
5) A repeatable validation workflow
- Capture 5–10 presses of a known-good lab remote, not just one event.
- Compare captures: if every press differs significantly, suspect rolling code (expected).
- Validate frequency + modulation on the receive side first (can you re-capture your own replay?).
- Replay at multiple distances (e.g., 0.5 m, 2 m) with a tuned antenna.
- Document the exact parameters used so results are reproducible later.
Validation criteria
- You can explain failure as either (a) plan/modulation mismatch, (b) RF/antenna environment, or (c) security control (rolling code).
- You can reproduce results across multiple presses/captures, not a single lucky event.
- You can re-capture your own replay on a second receiver (sanity check the emission path).