My LoRa application is set to transmit 18bytes of data every 15min. The duty cycle can be longer depending on if the data values changed or not, if the values not change it will skip the transmission.
How long is the air time expected? and how long the LoRa module will be active? I found on the scope that for the unconfirmed data up cycle, the LoRa module is active for around 1.52s. Is that ok? And between transmit and receive window, does the LoRa module go into sleep/low current mode? ADR=false for this application.
What spreading factor are you using? Did you include the LoRaWAN headers, MIC, and 4/5 coding granularity in your airtime calculation?
A node intended to be battery powered should sleep at least between the last receive window and the next transmission; but that it should doesn’t meant a particular implementation will.
Like mentioned above: that depends on the spreading factor. For 18 bytes of application payload, on SF9BW125, a total packet size of 31 bytes needs 246.78 milliseconds time on air.
What did you measure? What module is it?
Assuming US915, your airtime should never exceed the maximum dwell time of 400 ms; for 18 bytes application payload SF10 is not even allowed. So I guess you know that whatever you’ve been measuring is not just the transmission time. However, if there is no downlink in RX1, then I’d also expect the module to try RX2, so the full cycle without a downlink would take the time on air, plus 2 seconds, plus a bit for the preamble detection to know that there is no downlink in RX2. For that, 1.52 seconds seems too short.