If you saturate the rod, the field you generate will have lotsof 3rd
harmonic components in it, but little more of the fundamental. I was
trying to emphasize that you will need as strong a magnitic field as
possible aat the transmitting antenna, and just below saturation is
that limit, when a ferrite core is involved.

I understand that. I added a second coil on the ferrite rod to measure the
antenna current and set it just below the point where I saw harmonics (or
say non-sinusial) waveform on the scope.


If the rod has a large lenght to diameter ratio (say , above 10) then I
think the uptimum coil arrangement on the rod also doffers considerably
for the transmitting and receiving cases, since the receiving case does
not deal with saturation.

In the receiving case, the end sections of the rod act as flux
collectors, and only the middle thirs or so has almost all the
collected flux passing through it, so this third is the optimum place
for the coil. /in the transmitting case, the rod has a tendency to
saturate at the center, first, with this arrangement, and you want
essentially the whole rod to approach satuation at the same ampere
turns. This will produce a field that acts as if it has been produced
by the full length of the rod. You can achieve something close ot this
by spreading the turns out, all over the rod, with an extra
concentration (a second or third layer layer, perhaps) at the ends.
Something like this (shown in cross section. View with fixed width
font i.e. Courier, so charcters are on grid pattern):

That is a very interesting configuration. Never seen such a design. I read
about a old-fashion remote controller system having a ferrite antenna
transmitter. There someone wrote, the transmitter antenna was a mignon
battery-shaped ferrite rod. e.g. much shorter but wider than mine. So an
optimum ferrite transmitter antenna is maybe more like a fat battery shaped.

- Henry