This week I've been experimenting with many types of digital modes on 11mtrs. One particular mode has caught my interest "PSK31", which is basically the big brother of FT8 or JS8. However after playing with this mode for a few hours, I think it's better than these later modes like JS8.

FLDigi is a very powerful "FREE" software package, that not only supports PSK31, but many types of early digital modes that are all packaged up together in one place, like FSQ, MT63, Olivia, RTTY and CW.

Here's a screenshot of FLDigi 4.1.20



Also bundled with FLDigi is FLARQ 4.3.8 which allows you to File Transfer.

Download link  fldigi-4.1.20_setup.zip (6411.36 kB - downloaded 33 times.)

A bit lazy of me, but I decided to go for this off-the-peg lead, which should be good:
https://www.amazon.co.uk/dp/B007NNXWC0?ref=myi_title_dpW

This time with attachment hopefully ..

Here is that photo of those harmonics I mentioned, Will. Quite interesting -stopped me going off to bed there - looks to me as if the 750 is a 1st harmonic of the 1500 (fundamental f) and then at ~2250 central a first harmonic (wider because of the multiple) & then to the left of 3000 (central) one leg of the 2nd harmonic.

I see what you mean. You have not only the harmonics, but a sub-harmonic too.  I wouldn't know where to begin with answering why this is happening within FLdigi. Hopefully someone else may provide a clue as to why this is. For me with a direct sampling connection from the external speaker socket on the radio, I only every see the fundamental.