OK Iwo. But why a small receiving ferrite antenna works here? A
non-saturated (=linear, and that means the superposition theorem works)
antenna system is reciprocal as antenna theory predicts. So you should
explain where the difference is!

At LF, rarely is the problem "not enough amplitude of received signal",
so past a certain point there isn't much need to make the receiver
antenna more efficient. The problem is always "too much noise!". So
antenna designs are usually built around nulling out local noise, and
loop antennas will get rid of the mostly electric-field local noise.
And they have some directionality (notably sharp nulls) which can help
get rid of specific further-away noise.

The problem is to pump enough energy in the air to overcome the atmospheric
noise seen by the receiver.

BTW: The mentioned antenna array picture is NOT the DCF77 sender. DCF77 is
exactly two antennas there only. And they have a very good performance
thanks to there big outline. They radiate about 30KW out of the 50KW output
power of the final amplifier. If I get 1% performance I think I'm good.

- Henry