Ground and Reflected
The direct wave may extend beyond the horizon as a stable ground wave, and at longwave frequencies below 300 kHz ground wave propagation is dominant up to 1.000-1.500 km. For medium- and shortwaves, the ground wave reaches up to 400 km, with the first reflection normally hitting the ground some 750-2.000 km away from the transmitter. No reception is possible in the skip zone between the
ground wave and the first reflection.
>> Radio and ionosphere
Higher-layer reflections achieve longer hop distances.
Day and Night
The ionised layers develop and decay differently. During sunlight, the mid E layer forms and reflects medium-frequency shortwave stations, and the top F layer gives high-frequency stations exceptional range. The E layer dissolves after sunset, but this is compensated by the F layer sinking and reflecting lower frequencies at night. The bottom D layer absorbs low frequencies during the day and confines long- and medium-
wave stations to their ground wave range.
>> Ionospheric pertubations
The free VOAProp software predicts radio propagation.
Season and Sunspots
As the length of the days changes through the year, so does the thickness of the ionised layers. This means that high-frequency shortwave stations are reflected less by the top F layer during winter, and that mediumwave stations are less affected by D layer attenuation of their reflected wave. The rays of the sun have a cycle of their own with 11-year sunspot maximums creating very dense ionised layers, with corresponding minimums in between.
>> Daily sunspot number
Sunspot 1040 (Solar and Heliospheric Observatory).
Fading and Interference
A reflected wave hitting the ground or sea may reflect again. After a couple of hops, the reception zones tend to overlap. As the overlapping waves have covered varied distances, they may interfere to slowly cancel and reinforce the signal at the receiver. This fading is audible in analogue, but completely masked with digital modulation. By comparison, man-made interference is often more localised and easier to cancel using suitable aerials.
>> Down to earth antennas