The propagation mode here is pure magnetic coupling, not a proper
electromagnetic "radio" wave, because the antennas are so small
compared to a wavelength. Dipole magnetic fields fall off with the
cube of distance.

A lot more turns on the rods, and resonating with a cap, will help
some. Longer rods would help some, too, but 1/d^3 is a cruel function.

How far do you need to go?

Hi!

I remember 1/d^4 for a full EM-field here.

The receiver is a WORKING time-code receiver. Working in distance at least
2000km from the time-code transmitter with an EIRP of 30KW. The time-code
transmitter have of course a VERY BIG antenna (120m height).
So I'm a little confused of your capacitor idea. That is true?: The
transmitter is NOT sending an electromagnetic wave but the same antenna
system at the receiving end reads it as an full established EM field?

How far: Hm, several km's if possible. Maybe I should go higher in
frequency?
What other small effective antennas work here?

I think the problem is not the minimum turns because I tested it with an
original ferrite rod - the same as in the original time-code receiver. It
have a lot of turns, probably 100 or more. The same behaviour with 100 turns
AND with 10 turns. No difference!

What I understand of ferrrite antenna theory is:
That the coil is simply an impedance transformer and bandpass (with a
parallel capacitor for narrow-band reception) to couple the preamplifier to
the antenna system (= ferrite rod).

But I miss something. Maybe something with differences between transmitting
and receiving with a ferrite antenna. The antenna is not pure reciprocal -
because the ferrite (or iron powder) can be nonlinear!

- Henry