Radio wave propagation (part 1)
Signal propagation in the long, middle and shortwave range
LW: Low wave / frequency: 30 kHz - 300 kHz
MW: Medium wave / frequency: 0,3 MHz - 3 MHz
KW: Shortwave / frequency: 3 MHz - 30 MHz
First of all, I have to say that this topic is complex and multifaceted. The explanations given in this chapter are roughly simplified.
Basically, a distinction can be made between sky waves and ground waves in the propagation of radio waves in the range of long, medium and short waves. It is always about the same transmission.
A ground wave is a radio wave that spreads along the surface of the earth and follows its curvature. In doing so, it is exposed to the absorbing influence of the soil over which it runs. The absorption increases with increasing frequency. If a large surface wave range is important, the antenna should be vertically polarized.
In contrast to this, the sky wave propagates in a straight line like light from a transmission mast and is shielded, for example, by mountains, but under certain conditions it is reflected in the ionosphere and thrown back to earth. Such a sending up and being reflected back is referred to as a “hop”. If the antenna is beamed vertically upwards, distances of up to 300 km - 400 km in radius can be achieved with one hop. If, on the other hand, the signal is radiated flat, distances of up to several 1000 km can be covered by single or multiple reflection. Frequencies from 150 kHz to 4 MHz have a greater range in the night than during the day because the signal attenuation in the ionosphere decreases in the dark and the sky wave becomes more susceptible to wear.
Frequencies between 3 MHz and 10 MHz can often be received day and night, as these have both propagation paths at all times. This property is used in the so-called NVIS (Near Vertical Incidence Skywave). The electromagnetic wave is radiated very steeply (at an angle of 80 ° to 90 °) into the sky. This method is used by broadcasters, radio amateurs, the military and aid organizations. Frequencies between 10 MHz and 25 MHz spread over great distances during the day (sky waves) in the appropriate "radio weather", whereas the spread in the dark tends to be limited to the ground wave. Frequencies from approx. 25 MHz increasingly behave like ultra-short waves. With increasing frequency, these behave more and more like light waves and therefore propagate in a straight line.
The "radio weather", which mainly influences the shortwave propagation, depends mainly on the geomagnetic field and solar activity. There are relevant information pages and real “radio weather” reports. In connection with the corresponding forecast software (e.g. VOACAP), the probability of a secure connection can be calculated for a given radio link, frequency and "radio weather situation".
Some might find it interesting that the radio can also be heard underground. The lower the transmission frequency, the more "penetrating" the wave. Anyone who wants to listen to the radio underground and is in the coverage area of a long or medium wave transmitter can consider themselves lucky. Of course, this also depends on the surrounding rock type and / or soil - but the chances are good. Low shortwave frequencies can still get through, but VHF does not. As written elsewhere, this also applies to reception in dense forest; there, too, high frequencies are more likely to be absorbed.
As far as the signal propagation in the snow is concerned, it can be said that the more compact it is, the more the water behaves; this can only be penetrated by long-wave signals.
Only very low frequencies are suitable for communication underwater. In order to enjoy a modern (data) connection there, a cable is usually connected to a float on the surface on which the radio antennas are located.