The DCF77 signal radiated by the transmitting antenna reaches the receiver place in two ways: On the one hand, it propagates as ground wave along the Earth’s surface, on the other hand it reaches the receiver place as a sky wave after reflection on the ionospheric D-layer. In the case of straight propagation and one hop at the lower side of the ionosphere, maximum reach of the DCF77 sky wave is obtained when it leaves the transmitting place tangentially to the Earth’s surface and also incidents tangentially at the receiver place. Under these assumptions, the reach is approx. 1900 km during a day and approximately 2100 km at night. Receivers places at larger distances are reached by the DCF77 signal only after multiple reflections (e.g. two reflections at the D-layer, one reflection at the Earth’s surface) which are, however, associated with a strong decrease in the field strength. The map of Europe shown above illustrates the 2000 km circle around Mainflingen. Beyond this circle, reliable reception has been proved only in individual cases.
In summary, the following properties result for ground wave and sky wave:
1. The very stable ground wave has a large reach. Up to distances of a few hundred kilometres, its reception field strength is clearly larger than that of the sky wave. At distances below 500 km from the transmitter, field strengths of the ground wave above 1 mV/m can be expected.
2. In the distance range between approx. 600 and 1100 km, ground and sky wave may occasionally be of equal size which may lead to mutual fading when both signals are out of phase. On the other hand, equal phase may also lead to a strong temporary increase in the field strength. Both phenomena are also observed in Braunschweig
3. At distances of more than 1100 km, the ground wave fraction constantly decreases, and the sky wave, whose propagation at large distances is fairly constant especially during the day, prevails. At distances between 1100 and 2000 km, field strengths of the sky wave between a few hundred and approx. 100 µV/m are to be expected.