My initial entry into QRP was via two kits offered by Doug DeMaw W1FB (SK) 24 years ago – a small 40 meter transmitter and a 5-watt amp.
Along with the kits, Doug included a handwritten letter to me with words of encouragement for my new interest in QRP. I sometimes wonder how many others he started on a similar path, taking pen to paper in those pre-internet days before the convenience of cut and paste.
The kits contained specific instructions and diagrams but Doug also included two bulletins he wrote with instructions on how to wind the toroids and transformers contained within the kits.
Those bulletins also have info on how the number of windings were determined, permeability and other info that was nothing short of magic to me back then.
Almost a quarter of a century later, this info is still pertinent (and often misunderstood) for homebrewers:
Bulletin T-1A (Toroids) 3.7 MB pdf
Bulletin BBT-1A (Broadband Transformers) 3.7 MB pdf