I have an RTL-SDR, but I'm just thinking about ways to help deployed units avoid congested frequencies.
My devices transmit every 60 seconds (at the moment) so perhaps if there's no ACK from 915mhz then I should do a re-transmission 20 seconds later on 910mhz. and if no ACK from 910mhz, then 20 seconds later I should do another re-transmission on 920mhz. If I get an ACK from any of them then I stick with that frequency for subsequent transmissions.
I'm concerned about staying within the permitted 902-928mhz frequency range to remain FCC compliant. I've done some tests with my RTL-SDR with the default bitrate (55555) with your RFM69 library using an RFM69HCW radio.
This has probably been discusses before but I'll share my results in case anyone is interested.
When the transmitter is positioned next to the SDR antenna (50cm) I see additional signals down at 913.85, 908.0, 907.55, 907.25 but they move around when I scroll the window. So I think they're artifacts caused by excessive RF gain in the SDR. They disappear if I reduce the gain.
If I put my transmitter 5 metres away from the SDR antenna then I don't see anything outside 914.7 - 915.3. Even with higher RF gain values. I also tested at 2m but the results were basically the same. So I think that with the 55555 settings I can probably use frequencies 903 - 927.
I wish I had slightly more professional equipment than a $30 SDR!I changed my radios to use the HopeRF default settings and repeated the test.
/* 0x03 */ { REG_BITRATEMSB, RF_BITRATEMSB_4800}, // default: 4.8 KBPS
/* 0x04 */ { REG_BITRATELSB, RF_BITRATELSB_4800},
/* 0x05 */ { REG_FDEVMSB, RF_FDEVMSB_5000}, // default: 5KHz, (FDEV + BitRate / 2 <= 500KHz)
/* 0x06 */ { REG_FDEVLSB, RF_FDEVLSB_5000},
The signal has a single peak at 915. Almost all the signal is between 914.7 - 915.3. When I scroll to the left I see another weak signal at 913.6. To the right I see one at 916.4
So I think this means I can probably safely hop around the frequency range from 904 to 927. I'll need to verify the signals from all the other frequencies of course.