LoRa RFM95 not working at low bandwidths

[sbaratheon]

Hi

I am using two RFM95 LoRa Radios to transmit and receive messages and would like to maximise the range of the radio. I am currently using the following settings:

#define FREQ 915E6
#define TX_PWR 20
#define SPREAD_FACTOR 12
#define BANDWIDTH 125E3 // 
#define CODING_DENOM 5

This all works fine but when I decrease the bandwidth of the radios to 62.5E3 (which should increase the range) the radios seem to stop working. The datasheet seems to state that this bandwidth should be supported.

I just wondered whether anyone knew why this might be? I found the following (from someone who I believe posts here):

A low bandwidth signal, such as 7.8kHz will give the best (longest distance) performance, but with the low cost components used in the Hope and Dorji LoRa® modules a transmitter and receiver may not talk to each other at this bandwidth. The receiver has a frequency capture range, this is the percentage difference allowable in relation to the bandwidth that the transmitter and receiver can be apart in frequency in order for the receiver to pick up the transmitters signal. This percentage is typically 25% of the bandwidth.
...
Low bandwidths of 20.8kHz have been used for long range trackers and they do give better performance, but care is needed as devices used at this bandwidth may not talk to each other unless a frequency calibration factor is measured and applied. Once communication is established at low bandwidth a form of automatic frequency control can be used to adjust the receivers base frequency to keep it within the capture range.

Thanks

EDIT: In case it matters, I am testing the radios at very short range (<30cm) and without aerials.

[srnet]

Unclear what the question is ?

Your own post has the all the information in it that explains why low bandwidths are a problem.

[sbaratheon]

srnet:
Unclear what the question is ?

Your own post has the all the information in it that explains why low bandwidths are a problem.

Well, it seems a bit odd to me that bandwidths don't work when the datasheet says they should. Is there any further information about this?
What is a frequency calibration factor?

[srnet]

sbaratheon:
Well, it seems a bit odd to me that bandwidths don't work when the datasheet says they should.

Not odd at all, the datasheet, for the SX1278, is very clear on what needs to be done to get a particular bandwidth to work, and they do.

Modules such as RFM98 are fitted with low cost crystals, which are just not accurate enough for low bandwidths to interoperate between two devices. Given the tolearances of these crystals 125Khz is a practical limit at 868Mhz\915Mhz. For lower bandwidths you really need to use a module with a TCXO, not common in the SX127x modules but fairly common in the SX126x modules.

You can work around the issue to an extent by adjusting the set frequencies of modules so that when set to say 915Mhz there are actually on 915Mhz. And then using AFC to correct for frequency drifts.