16 Jun 2020
Ferrite rods will work on TX as long as the ferrite does not saturate and the material is fine for the intended frequency.
Low power usually means millwatts or low watts. Some years ago I gave it a try with a random ferrite rod (characteristics unknown) on WSPR with great success. To be honest, it was far better than I expected. I think of ferrites as distorting space with the effective size being proportional to the effective permeability of the ferrite.
I wonder what would happen on 7MHz FT8?
UPDATE 2003z: 12 spots of me today so far on 40m FT8 with just the tiny ferrite antenna. I guess the biggest problem is being buried by bigger stations on the same frequency without realising.