Ferrite rod transmitting antennas.

This has appeared before, but I think it is worth repeating.

Ferrite rods can be used on TX as long as the ferrite does not saturate. This means low power.  I like to think of the high permeability as a "shrinking of space".  Indeed this is how I visualise ferrites work on RX.

Being a loop, they can be very narrowband, but will work when frequency is fixed such as with WSPR or FT8.

I have no idea what the "gain" is, nor have I any idea of the ferrite material that I used.  It is some years since these experiments were carried out.  The results astounded me.

See https://sites.google.com/site/g3xbmqrp3/antennas/ferrite_tx

See also https://en.wikipedia.org/wiki/Permeability_(electromagnetism)