EmonTx v2 unreliable data transmission

Have a look around line 28 at the init call - the uint16_t off parameter is the frequency ‘offset’ into the chosen ISM band (defaults to 1600 in the 868.00 MHz case) The Tx/Rx frequency is set up once only when rf69_initialize() is called. Some compiler conditional magic maps this to the rf12_initialize() the legacy calling code is using.
IIRC changing offset by 1 gives about 4Khz delta in Tx/Rx frequency.

As you’re ‘transmit-only’, there’s a stripped-down and slightly modified version of JeeLib in the 3-phase sketch, that’s fully commented. You’ll probably need to read it and the Hope data sheet in parallel to work out what the constants are.

rfm.ino, lines 44 - 56 (Just leave or get rid of the frequencies you don’t need, and tweak the one you do.)