T-Beam (SX1276/SX1262): no companion USB build — and RX observation with a USB build of v1.18.0 #3681

Request: the v1.18.0 release ships companion_radio_usb images for most boards (Heltec V3/V4, T114, RAK4631…), but the LilyGO T-Beam only has companion_radio_ble (and repeater). We need a USB companion on a T-Beam for a server-side gateway (the node is plugged into a Linux host and driven with meshcore_py), so we had to build one ourselves: the Tbeam_SX1276_companion_radio_ble env with -D ENABLE_USB_INTERFACE instead of BLE_PIN_CODE. It would help to have official Tbeam_SX1276_companion_radio_usb / Tbeam_SX1262_companion_radio_usb envs and release images.

Observation with that build (T-Beam v1.2, AXP2101, SX1276, no TCXO; 910.525 MHz, BW 62.5, SF7, CR5):
after boot, and again after set_radio() (CMD_SET_RADIO_PARAMS), the node receives nothing — get_stats_packets() recv and get_stats_radio() rx_air_secs stay at 0 while a Heltec T114 (SX1262) transmits a few metres away, although the T114 does receive the T-Beam's adverts. As soon as the T-Beam transmits once (a zero-hop send_advert()), reception works normally (3/3, SNR +9 to +10 dB). A ±20 kHz frequency sweep changed nothing. The radio code is unchanged from the BLE env, so the official BLE image may behave the same; we have not tested it.

Our workaround: a zero-hop advert at startup and after any radio parameter change.

Related: with a fresh flash the node starts on 869.618 MHz (EU default) — in Canada/US that is the cellular band; same concern as #2807.