Slightly Update frequency ranges in SX127x modules

[G4lile0]

Description

This PR adjusts the lower frequency bound for the SX1278 from 410.0 MHz to 400.0 MHz.

Reasoning

While recent commits introduced strict frequency validation based on datasheet specifications, the SX1278 silicon is physically capable of operating in the 400–410 MHz range. This specific band is critical for:

• TinyGS: Satellite ground stations globally depend on the 400–403 MHz range.
• Radiosondes: Weather balloon tracking frequently occurs in the 400–406 MHz band.

The enforcement in commit 989c64a (referenced in #1724) broke compatibility for thousands of existing SX1278-based modules (like Ra-02 and RFM95W) used in these community projects. This change restores support for these applications while maintaining the integrity of the validation logic.

Related Issues

Fixes #1732

[jgromes]

We can loosen the limit a bit, but I would add a clear warning to the Doxygen comments for all setFrequency methods that those frequencies are outside of the manufacturer-recommended range.

How well are those frequency ranges tested in terms of Rx and Tx in Lora and FSK? Have there been e.g. some succesful digipeats in the 400 MHz satellite band?

[K4KDR]

Not digipeats, but countless successful RX & decodes from the very large number of LoRa satellites using that commercial 400 MHz satellite band. A lot of Project Teams are not able to get licensed & coordinated to use the 70cm Amateur sat. band, so they opt to get an experimental or commercial license for 400-405.

[jgromes]

@K4KDR I'm aware of those but I was wondering if anyone was succesful in transmitting - regardless, I checked Tx at 395 MHz and the signal looks OK.

@G4lile0 it might be better to move the cutoff there, to prevent issues with rounding/float comparisons. And of course, a a big note that the actual limit should be at 410 MHz ;)

[G4lile0]

Done :) Thanks

[jgromes]

Looking good now - merged, thanks!