Downlink message TX power

[jpmeijers]

I just noted that the console reports a tx power of 16.15 for downlink messages:

    "scheduled": {
      "data_rate": {
        "lora": {
          "bandwidth": 125000,
          "spreading_factor": 7,
          "coding_rate": "4/5"
        }
      },
      "frequency": "868300000",
      "timestamp": 115869749,
      "downlink": {
        "tx_power": 16.15,
        "invert_polarization": true
      },
      "concentrator_timestamp": "115869749000"
    },

Yes, this is indeed the 14dBm limit plus the gain of a dipole. But why are we taking the gain of a dipole into account here?

[descartes (Nick McCloud)]

@jpmeijers

But why are we taking the gain of a dipole into account here?

And how does it know what’s attached???

[adrianmares (Adrian-Stefan Mares)]

The difference actually comes from EIRP vs ERP. We carry around EIRP, and depending on the frontend we send to the packet forwarder either ERP or EIRP.

For example, the UDP packet forwarder and the MQTTv2 format expect ERP, so we send 14 (16 - 2.15), while for Basic Station we don’t send the transmission power at all (the antenna gain is a hardware specific configuration in station).

[jpmeijers]

Thanks for the explanation @adrianmares. We are so used talking about 14dBm and/or 25mW, that we often forget the E.R.P. part. Using EIRP on the backend rather than ERP indeed makes more sense.