Short-wave band (HF band) radio signals below 30MHZ rely on the reflection of the ionosphere for long-distance transmission, and the transmission distance can be from hundreds of kilometers to tens of thousands of kilometers. Due to the particularity of the ionosphere itself, under normal circumstances, the ionosphere's reflection of radio signals above 40MHZ is very weak, and most of the signals will penetrate the ionosphere and be emitted into space. Therefore, under normal circumstances, radio signals above 40MHZ are basically transmitted in a straight line, and the transmission distance is generally within one or two hundred kilometers, which is what we often call "line of sight" transmission.

However, under the special conditions of the ionosphere and the atmosphere (troposphere), VHF and UHF signals above 40MHZ can be transmitted along the earth's surface to hundreds of kilometers away. We usually call this kind of transmission beyond "line of sight" long-distance transmission. Sometimes, radio signals in the VHF band can also be reflected by the special conditions of the ionosphere and propagate thousands of kilometers away. We can call this propagation ultra-long-distance propagation. The mechanisms of "line-of-sight" propagation, long-distance propagation, and ultra-long-distance propagation of radio waves are different. The various propagations mentioned below refer to the situation where the intensity of the radio signal received from a distant radio station is not much different from the intensity of the radio signal from the local radio station using ordinary receiving equipment (ordinary radios, receivers, and ordinary antennas).

Line-of-sight propagation

Radio waves basically propagate from the transmitter to the receiver in a straight line.

Long-distance propagation

The propagation of VHF and UHF radio signals above 40MHZ within hundreds of kilometers is mainly caused by changes in the characteristics of the atmosphere (troposphere) near the earth's surface, and there are mainly two propagation methods.

When radio waves in the VHF and UHF bands propagate in the atmosphere of the troposphere, when special conditions appear in the atmosphere, they will refract with the different temperature, air pressure, and moisture distribution of the atmosphere, causing the propagation path to bend and change, thereby forming a good channel for propagating radio waves. This propagation channel is usually called an "atmospheric waveguide". Atmospheric waveguides can be used to propagate VHF and UHF radio signals hundreds of kilometers away, and the signals are relatively stable. Generally, in the stable weather after the rain in summer, atmospheric waveguides are easily formed in the atmosphere. In coastal areas, when dry hot air masses on land move toward the sea, atmospheric waveguides are also easily generated. Atmospheric waveguides are generally more likely to be generated in low-latitude and mid-latitude areas, especially in coastal areas and on the sea surface. Signals propagated by atmospheric waveguides are more stable than signals propagated by reflection from the ionosphere, and atmospheric waveguides can often be maintained for several hours. This is a challenge for us radio enthusiasts and BCL to receive FM broadcasts and TV sound. Generally speaking, atmospheric waveguides are better for UHF signals than VHF signals, so this is also a challenge for radio enthusiasts who prefer to play walkie-talkies.

Under the propagation condition that there is no atmospheric waveguide in the troposphere, the signal of the radio station can often be received within a range of 150-400 kilometers from the FM and TV radio stations. This is caused by the scattering of radio waves caused by the inhomogeneities in the troposphere. The inhomogeneities in the troposphere are many vortex-shaped turbulent air masses formed by the air. Under the irradiation of electromagnetic waves radiated by the ground transmitting antenna, each inhomogeneity reflects electromagnetic waves like a passive reflecting antenna, and propagates the radio waves emitted by the ground to a distance. We call this kind of propagation tropospheric scattering propagation.

It should be emphasized that the scattering of radio waves in the troposphere exists at any time, but affected by the changes in meteorological conditions, its characteristics change greatly, resulting in a large change in the receiving field strength, which causes the randomness of remote reception. In addition, the radio signal intensity of tropospheric scattering is relatively small, and it often requires a high-gain directional antenna and a high-sensitivity receiver to receive it. Moreover, the propagation field strength in summer is greater than that in winter, and the propagation field strength in the morning and evening is better than that at noon. In addition, the effect of tropospheric scattering propagation is better when the frequency is above 100MHZ. Both atmospheric waveguide propagation and tropospheric propagation are related to the changing laws of meteorological conditions near the receiving area, which can be explored through long-term practice to find out the laws of receiving seasons and receiving times.

Radio enthusiasts all know that at an altitude of 60 to 1,000 kilometers above the earth's surface, there is an atmospheric layer called the ionosphere, and the ionosphere also has a layered structure, and the height and electron density state of each layer are also different. Internationally, it is often divided like this: a layer 55 to 85 kilometers from the earth's surface is called the D layer, a layer 85 to 150 kilometers is called the E layer, a layer 150 to 200 kilometers is called the F1 layer, and a layer above 200 kilometers is called the F2 layer. Since the electron density of the ionosphere is extremely high, it can reflect radio waves, thus achieving long-distance radio communication. However, the ionosphere can only reflect radio waves below 40MHZ under normal circumstances. Radio waves above 40MHZ will penetrate the ionosphere and shoot into space.

However, when the electron density of the ionosphere is abnormal, special areas with very high electron density will appear. These special areas of the ionosphere will reflect radio waves with frequencies higher than 40MHZ. Moreover, the higher the density, the higher the reflected frequency. At the highest, 150MHZ radio waves can be reflected. This is the ultra-long-distance propagation mechanism of FM radio and VHF. When the abnormal electron density area of ​​the ionosphere appears in the F2 layer, its reflection frequency can reach 60MHZ and the propagation distance can reach 2500 km to 5000 km. This situation is easy to occur in the peak year of solar activity or when the ionosphere is disturbed. In addition, it often occurs during the daytime in winter.

When the electron density anomaly zone of the ionosphere appears in the E layer, its electron density can be tens to hundreds of times greater than that of the surrounding normal E layer. At this time, it can reflect frequencies of 100MHZ to 150MHZ, and the propagation distance can reach 1000 km to 2500 km. The signal field strength of the E layer anomaly zone is relatively high, and it is relatively easy to receive. Sometimes, ordinary radios and walkie-talkies can receive signals from distant FM stations and walkie-talkies. The appearance of the E layer anomaly zone is also random. Generally, it appears 50% to 70% of the time during the summer daytime in mid-latitudes, and exists almost during the daytime in the equatorial region.