Ok, I understand now that the modules are measuring the noise signal strenght before sending someting.
Apparently the noise level is high, but with the other gateway I get low values.
It seems to be the linux gateway that is generating the noise ???
The RFM69 is powered by a separated LDO and capacitors, so I don't think it is a power supply problem.
The RFM69 linux driver is the same that arduino RFM69 driver? Could be some missing register parameterization?
