I sometimes play with simple medium wave receivers for my grandson.
As there is a very weak signal in our flat, I recently made a simple signal generator with Avr.
It produces a square wave frequencies modulated with 1 kHz (also square wave).
I used a MW ferrit antenne coil as the load.
It worked well but something confused me.
When I set 200 kHz, I expected to receive harmonic frequencies at 200, 400, 600, 800, 1000 etc. kHz.
But surprisingly I found only odd harmonics 200, 600, 1000, 1400.
The even ones are extremely weak (if any). See the picture below.
I would like to know why it is so.