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SX1280 reported RSSI accuracy

I am doing some basic testing to check the power output performance of the SX1280 radio.

I have two identical boards with an SX1280 with signal routed to an SMA connector.

If I set the SX1280 to output at +13dBm with a CW, I get roughly this measured on an SA connected to SMA connector on both boards.

If I now connect both boards directly to one another and transmit LORA packet data from one to other and read the Params.LoRa.RssiPkt value (which seems to convert to dB inside sx1280.c), I consistently get 12dB less than transmitted values.

Eg, transmit packet at 0dB, received at -12dB through a very short wired connection.

What would account for that loss if connection to SA shows matching circuit is OK?

Is the modem interpreting the signal level in a way I am not understanding?

I have just repeated the test with two LAMBDA80 reference boards and got the same result.

Reading the datasheet for RSSI and SNR for LoRa, it seems I need to subtract the SNR from RSSI when SNR is negative to get correct RSSI.

Should I be adding the SNR to RSSI to get correct RSSI when SNR is positive? In my case I would get the correct value as SNR is 12. But this is not mentioned in the datasheet