What is the principle of radio? How are radio waves divided into different bands?

Radio is closely related to our lives. Not only are daily WIFI and 4G mobile phones inseparable from radio, but daily broadcasting, communication, transportation, and transactions are also inseparable from radio communication. With the improvement of living standards, radio has gradually become a personal hobby. Various types of radios, receivers, and communication equipment are changing with each passing day, bringing a lot of fun and convenience to many people. This article will give a brief introduction to some basic issues related to radio, hoping to serve as a starting point for discussion. If there are any inappropriate or erroneous points, you are welcome to criticize and correct them.

1. Division of radio wave bands:

(The picture shows the division of radio bands by frequency and wavelength)

Radio waves are composed of electric and magnetic fields. The directions of the electric and magnetic fields in radio waves are perpendicular to each other. The propagation speed of electromagnetic waves is close to 300,000 km/s (the exact value is 299,792,485 m/s). The usual band calculation is as follows: wavelength (m-meter band) = 300/frequency (MHz-megahertz). Radio wave propagation mainly has the following forms: ground wave, sky wave, space wave, and scattered wave.

(Conversion rules: 1GHz=1000MHz; 1MHz=1000KHz; 1KHz=1000Hz, where Hz is read as Hertz.)

In my country, the common broadcasting and communication bands for residents cover low frequency (Low Frequency), middle frequency (MF), high frequency (HF), very high frequency (VHF), ultra high frequency (UHF), etc. Each band has its own unique propagation characteristics. By using appropriate radio equipment and antenna feed systems and following the characteristics of each band, better results can be achieved. Under certain technical and natural conditions, there are often surprising results, such as long-distance FM radio stations (FMDX), cross-regional medium-wave communication stations, and long-distance intercontinental short-wave stations.

1. Ground waves are radio waves that propagate along the surface of the earth. They are generally used for medium and short distance communications. For example, part of the medium wave broadcasting signal is propagated by ground waves. Long waves are also one of the bands that can be propagated by ground waves.

2. Sky waves, which are radio waves propagating in the ionosphere, will undergo multiple refractions after entering the ionosphere from the ground, and the refraction direction will also change, and will eventually be transmitted to the ground, thus achieving communication. The most common examples are shortwave broadcasting and shortwave communication.

3. Space waves: The transmitted radio waves reach the receiving point through direct radiation and reflection from obstacles in space. For example, common FM radio, common VHF/UHF walkie-talkie communication, microwave communication, etc.

4. Scattered waves: The radio waves emitted by the transmitting point are reflected in all directions through various uneven media, and finally a part of them reaches the receiving point. Scattered communication is also an important communication method. Under certain conditions, it has strong anti-destruction and anti-interference capabilities.

(The height of the ionosphere and the conditions of each layer, covering the D, E1, E2, F1, and F2 layers)

5. Overview of the ionosphere: The atmospheric layer with an altitude of 10-20km is the troposphere, which is a common area for various weather events. The altitude of 10-50km is the stratosphere, where the meteorological conditions are relatively stable and it is also an active area for many aircraft. At an altitude of 50-400km, due to the strong solar ultraviolet radiation, the electrons in various gas molecules break away from the constraints of atoms and become free electrons and ions, forming the ionosphere. The different intensities of ultraviolet radiation and the different molecular masses of various gases form layers with different electron densities and thicknesses such as D, E, F1, and F2. Each ionosphere has different radio wave characteristics. Among them, the D layer of the ionosphere at an altitude of 50-90km exists during the day and disappears at night. It is not easy to reflect electromagnetic waves. The lower the frequency of the radio wave, the more it is absorbed. The E layer of the ionosphere at an altitude of 90-140km is usually less dense. In some cases, it can reflect higher-frequency radio waves. In June and December in the northern hemisphere, some regular transmissions are likely to occur, making low-power long-distance communications possible. The ionosphere at an altitude of 200-300km is the F1 layer, and the F2 layer at an altitude of 300-400km is the F2 layer. The density of the F2 layer is relatively high. The F1 layer and the F2 layer exist simultaneously during the day, and at night, the F1 and F2 layers merge into one layer. The F2 layer has a strong reflection ability for radio waves, which is also the basis for short-wave (HF) long-distance communication. However, the F2 layer basically belongs to the upper atmosphere, and its density is greatly affected by solar radiation and cosmic rays. Therefore, short-wave propagation has a certain instability. During the peak of solar activity, the density of the F1 and F2 layers is relatively large, and relatively small communication power can achieve long-distance communication. At the trough of solar activity, short-wave propagation is greatly affected, and the efficiency and distance of communication are reduced. When the atmosphere is impacted by high-energy particles from the sun, some communications are easily affected, resulting in communication interference and interruption.

(The above picture shows the solar activity cycle in recent years, including several peaks and troughs)

2. Propagation characteristics of each common main band:

1. Low frequency band (30KHz-300KHz, LW), wavelength 10km-1km, this band is what people often call long wave band. Long wave radio waves can be transmitted through sky waves and ground waves at the same time. Generally, the transmission distance at night exceeds the transmission distance during the day. In this band, in addition to some European countries and a few American countries that still use long wave broadcasting, there are still some lower frequencies in this band, which are mainly long-range anti-submarine and anti-ship communications. Because of the characteristics of long wave propagation communication, even at a certain depth underwater (depth <300m), key communication information can be clearly received. In the 1970s, the lower frequency very low frequency (frequency <30KHz) was also developed to achieve the above-mentioned long-distance communication goals because it has a stronger ability to penetrate seawater and ground. This band still has a certain communication transmission capability under extreme conditions, such as ionosphere destruction and nuclear explosions. However, due to the characteristics of this band, the construction of long-wave communication stations requires very large antennas (antenna lengths range from several thousand meters to tens of kilometers) and deep ground networks, as well as long-wave radio stations with high technical requirements, in order to ensure sufficient communication efficiency and signal strength. Therefore, there are relatively few civilian long-wave radio stations, and even fewer civilian long-wave signals with global coverage. In the evening in northern my country, long-wave broadcast signals from neighboring countries can sometimes be received. There are also some civilian long-wave radio stations still working in northern and western Europe, but the coverage area and range are relatively limited.

2. Medium wave broadcast band (300KHz-3000KHz, MF), wavelength 1 km to 100 meters, in Eurasia, common medium wave frequency is 520KHz-1620KHz, step 9KHz; in North America, common medium wave frequency is 520KHz-1710KHz, step 10KHz, some radios need to adjust different steps in different regions to receive programs. The broadcasting in this band is mainly regional broadcasting, such as covering a region or a province or a state. Medium waves can use ground waves and sky waves to propagate at the same time. At night, medium waves can achieve a farther propagation distance than during the day. It is common to hear signals from neighboring provinces or countries. There are also many medium wave long-distance reception enthusiasts active here. Compared with long waves, medium waves are relatively easy to achieve in terms of equipment technical requirements and antenna size requirements, and are also important frequencies for broadcasting in many cities. However, due to the characteristics of propagation, it is more susceptible to shielding by concrete buildings and various radio scattering interference in modern cities. In some areas, good reception cannot be achieved or there is serious background noise. In some areas of my country, such as East China, there are abundant medium wave broadcasting programs. A German enthusiast once used a farm to set up a nearly 1km long loop antenna, clearly receiving the medium wave broadcasting signals from China, and confirmed it with a listening card.

3. 160-meter band (1800Khz-2000KHz, MF), which belongs to the medium wave band in the amateur radio band, but common shortwave radio stations also cover this band. In the early days of amateur radio communication, it also started from this band. The 160-meter band is also a band with great challenges for long-distance communication. Compared with the current common residential environment, not only because of its huge antenna size, but also because of the propagation characteristics of this band, most enthusiasts cannot effectively use this band for transmission. Usually, the electromagnetic wave propagation in this band is mainly ground wave, and the effective coverage range is only within 200km-300km. In the general year of sunspots, it is only suitable for ordinary local and nearby area communications, but in the year when sunspots are more active, especially in the winter evening to early evening, the long-distance propagation of this band will be opened. The 100W transmission power of common equipment seems to be very weak in this band, and it is easy to be drowned by the large background noise in this band. If you need to achieve long-distance two-way communication, you need a large and efficient antenna, and seize the opportunity of opening the transmission. At present, only a small number of enthusiasts and clubs with good conditions are active in this band. Newer versions of radios may also cover this band. This band mainly communicates in the form of CW (Constant Amplitude Telegraph). If you have a radio with single-sideband function, you may be able to receive it by using the LSB lower sideband and fine-tuning it.