I might have missed it, how does OP broadcast across 80mhz with a 20mhz SDR? I think my mental model about SDR capability is wrong, does sample rate not necessarily gate the range at which an SDR can TX/RX?
You’re right, I can’t believe I actually failed to mental math that subtraction problem correctly!
An SDR's sample rate only limits its bandwidth. Most devices include a mixer, which allows the transmitted or received signal to be shifted in frequency by an amount much larger than the bandwidth.