[Solved] LoPy max. transmit power 20 dBm at 869 MHz?

[pjoerke]

Hi guys,

I'm just wondering why the maximum transmit power for European frequencies of LoRa is limited in the LoPys to +14 dBm. The frequency band from 869.40 MHz to 869.65 MHz allows transmit powers up to 500 mW / 27 dBm. The SX 1272 LoRa chip supports up to 20 dBm transmit power. So why can't we use this maximum output power?

Kind regards
Pascal

[pjoerke]

@jmarcelino I solved it. I have used an old firmware version on my gateway. I guess I missed this one module when updating all others. After updating I can set the TX power up to 20 dBm without getting any errors. Thanks for your help!

Kind regards
Pascal

[jmarcelino]

@pjoerke
If it's like other modules my guess is because the LoRaWAN specification defines the maximum EIRP at 14dBi. Of course for raw LoRa this is different.

Then again the LoPy may not be certified to operate at 20dBm which could e.g. increase spurious emissions in bands where it should not.

Still it is strange that you get that error, from the code:

static void lora_validate_power (uint8_t tx_power) {
    if (tx_power < TX_OUTPUT_POWER_MIN || tx_power > TX_OUTPUT_POWER_MAX) {
        nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_ValueError, "Tx power %d out of range", tx_power));
    }
}

and TX_OUTPUT_POWER_MAX is #defined as 20... so it should be OK.

I'll try it later when my poor overworked LoPys get a break.

[pjoerke]

@jmarcelino Thanks for the response.

So I thought, too. But when I start LoRa with tx_power = 20, I get a ValueError: "Tx power 20 out of range". In the pycom documentation it is noted that tx_power "accepts between 2 and 14 for the 868 band" (https://docs.pycom.io/chapter/firmwareapi/pycom/network/lora.html). So there is still a regulation. But why?

[jmarcelino]

@pjoerke

#define TX_OUTPUT_POWER_MAX 20 // dBm

so I'm pretty confident you can start Lora() with tx_power=20 - if you know what you are doing and using frequencies where it is legal to do so.