there are two basic propagation ways of short wave: one is ground wave and the other is sky wave.
as mentioned earlier, the ground wave propagates along the earth's surface, and its propagation distance depends on the characteristics of the surface medium. The conductivity of sea surface medium is the most favorable for radio wave propagation, and short-wave ground wave signals can travel about 1 kilometers along the sea surface. The conductivity of land surface media is poor, and the attenuation of radio waves is large, and the attenuation of radio waves is different for different land surface media (the attenuation of wet soil ground is small, and the attenuation of dry sand ground is large). Short-wave signals can only propagate for tens of kilometers along the ground at most. Ground wave propagation does not need to change the working frequency frequently, but it is different from sky wave propagation because of the obstruction of obstacles.
the main transmission route of short wave is sky wave. After the short-wave signal is sent by the antenna, it is reflected back to the ground through the ionosphere, and then reflected back to the ionosphere from the ground. It can be reflected many times, so it travels far (hundreds to tens of thousands of kilometers) and is not blocked by obstacles on the ground. But the sky wave is very unstable. In the process of sky wave propagation, path attenuation, time delay, atmospheric noise, multipath effect, ionospheric fading and other factors will cause signal weakening and distortion, which will affect the effect of short-wave communication.
Introduction to shortwave radio
1. Traditional pointer-tuned shortwave radio
If the types of radios are divided according to the received bands:
Single-band medium-wave radios: MW 525-16 kHz
FM-AM radios MW 525-16 kHz, Fm87.5-18mhz
FM/Medium/shortwave radio * * MW 525-16khz, Fm87.5-18mhz
SW: 3.9-12.mhz (75-25m)
(or 6.. 49-16 meters)
(or 9. -- 16. MHz, 31-19 meters)
In two short wave bands, SW1： 2.2--7.5 MHz, SW2: 7.5-23. MHz
or SW1: 5.9-9.5. SW2: 9.5-18. MHz
According to the meter band, SW1, SW2, SW3, SW4, SW5, SW6, SW7 ......
Multi-band shortwave radio (each short band covers an international short-wave meter band)
Traditional radios and recorders generally have only one or two short bands, but each band covers a wide frequency. The advantage is that the circuit is simple, but it is difficult to ensure that the sensitivity and selectivity of each point in the covered frequency range are uniform. Therefore, it is often that some meters are very good to listen to, while others are very poor. In addition, because of the wide frequency coverage, it is very crowded between radio stations and it is inconvenient to close the station. Therefore, some radios need to be improved by adding short-wave fine-tuning knobs.
There are also some traditional radios with well-designed short-wave circuits. The radios also have high sensitivity and selectivity, and the production and debugging are very accurate. It is also very convenient to use, which is not interesting, at least saving the trouble of changing the frequency band. In addition, traditional radios mostly use 3-4 batteries and relatively large-caliber speakers, which sounds good. No wonder many old shortwave fans still like traditional radios.
2. Multi-band shortwave radio divided by meter band
Modern shortwave radio is often divided into 6-1 short bands, and each short wave covers only one meter band (please refer to the meter band table of international broadcasting below). For this kind of well-designed shortwave radio, sensitivity and selectivity can be easily guaranteed. Moreover, dividing short waves by meter band seems to widen the interval between stations, and receiving short waves is as convenient as listening to medium waves, especially for radio stations.
If shortwave radio is divided by meter band, it is insufficient because there are too many short bands. For people who like different radio stations and programs, it is a bit troublesome to switch short bands frequently.
In addition, if the shortwave radio is designed according to the meter band, if it is to cover all the short-wave frequency range, the short-wave band of light needs 13 bands, and each band should be designed reasonably, and a lot of electronic component materials are used, which makes the circuit too complicated and expensive. The imported famous brand shortwave radio I have seen has 15 bands at most, and the price is close to that of 1 yuan.
It is worth mentioning that in the domestic market, there are some shortwave radio, which are called 18-band and 24-band, and the price is quite cheap. You don't know whether the designers are deceiving themselves or others! In addition, there are many shortwave radio with 8 and 9 bands, which are called [products recommended by consumers]. Due to vicious competition in the market, short-wave circuits have almost no other components except band switches. Instead of buying this kind of radio, the author suggests that it is better to buy a traditional shortwave radio with 3 or 4 bands.
3. SW dual conversion technology in shortwave radio
At first, direct amplification circuit was used in shortwave radio. Since 195s, primary frequency conversion circuit, which is usually called superheterodyne radio, was applied. In order to further improve the sensitivity, selectivity and anti-interference ability of the radio receiver, scientists have developed frequency conversion technology for many times. Of course, it was first applied in the field of radio communication, and then transplanted to advanced radios, thus greatly improving the performance index of shortwave radio.
Portable high-sensitivity shortwave radio generally adopts secondary frequency conversion, while more advanced professional short-wave communication receivers even adopt tertiary or quaternary frequency conversion technology.
4. Radio (PLL) with digital tuning technology of phase-locked loop
The radio with digital tuning technology of phase-locked loop is a high-tech product with modern microelectronics application technology, which integrates advanced, practical and novel features.
1. Single-chip microprocessor is used as the core of digital tuning system, and it contains many advanced functions such as phase-locked loop frequency synthesis, frequency preselection, multifunctional digital clock control and LCD digital display.
2. lock the frequency of the receiving station with a high-precision and high-stability Shi Ying crystal as the frequency reference, and there is no drift.
3. It has the function of frequency storage and memory.
Generally speaking, the more storage stations in digital tuned radios, the better. Advanced digital tuned radios should have digital and analog tuning knobs for direct input frequency, and secondary frequency conversion technology is often used in electronic circuits to improve performance indicators.
The radio with digital tuning technology has the disadvantages of complex circuit, difficult design, strict requirements on components, high cost and complicated production and debugging. Due to the use of many components, the static power consumption is larger than that of ordinary radios, and the sensitivity and selectivity of popular digital tuned radios are not necessarily much higher than that of good analog radios.
4. Radio with digital display frequency technology
This kind of radio adopts traditional analog receiving circuit, which is low in cost and easy to achieve high performance index. The difference is that the digital display screen replaces the pointer of the traditional radio to indicate the frequency, and the electronic clock control function is added; Compared with digital tuned radio, it saves electricity, can be designed more compact and convenient in size, and is a very practical radio variety with high price-performance ratio.
The disadvantage of this model is that it has no memory radio function, and the accuracy and stability of frequency are not as high as that of digital tuned radio because it adopts traditional analog receiving technology.
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most people will be interested in short wave, because it can listen to long-distance broadcasts and get broadcast information from all over the world directly. However, many people are confused because of the wrong method of listening to short wave, and finally have to give up. For how to start listening to short-wave radio, the following suggestions are for your reference.
◎ What is the difference between listening to short wave and listening to MW and FM in daily contact?
daily listening to MW or FM broadcasts rarely leads to the problem of being unable to find a radio station, because the broadcasting frequency of these stations is fixed, and many of them are broadcast 24 hours a day. For short wave, it's different. Except for many radio stations, there are two seasonal changes in broadcasting frequency and broadcasting time every year, and there is a great difference in the quality of received signals every day, which makes listening to short wave more complicated than MW and FM, but as long as you master the essentials, you can enjoy short wave programs freely.
◎ Listen to short wave-choose radio station, frequency and time
For short-wave listeners, the biggest problem is that short-wave broadcasts are usually concentrated in a certain period of time? Broadcast, resulting in a traffic situation somewhat similar to commuting time, is extremely crowded. But you can make yourself not a crowded person, because usually radio stations broadcast the same program on different frequencies at different times. For example, short-wave 15-18MHZ can receive many radio programs from noon to evening every day, and only a few radio programs can be received after 1 pm, and even the background noise of the radio becomes smaller; It is difficult to listen to the radio clearly in the daytime when the short wave is below 7MHZ, but you can listen to the program well in the middle of the night. The short wave can receive the radio all day, but the listening effect is the best in the morning and evening, with more radio stations and clear sound. Also, if you often listen to the radio, you will find that many radio stations regularly change the broadcast frequency every hour. Therefore, in order to listen to short-wave programs conveniently, it is necessary to make your own listening time frequency Schedule. Of course, you can also start by collecting the broadcast time frequency tables of various radio stations.
In fact, short-wave radio stations usually broadcast on multiple frequencies at the same time, but usually not every frequency can be listened well. The purpose of monitoring is to select the best frequency of sound signal from several broadcast frequencies and record it, and make a broadcast frequency schedule, so it is much more convenient to listen to the program of this station later.
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It's a pity that many people who have just started listening to short waves are greatly reduced by the noise from the radio, and even give up listening to short waves. Indeed, the sound quality of short wave cannot be compared with that of FM high fax broadcasting, but it is basically very close to that of medium wave (MW). However, because listening to short wave is influenced by many factors, it often appears worse than medium wave. In fact, if all factors are favorable, the sound quality of short wave can be comparable to that of medium wave broadcasting. Below? To discuss what important factors must be considered when listening to short waves:
◎ Ionospheric factors
Medium wave broadcasting (commonly known as AM), the distance from the transmitting antenna of the radio station to the reception of the radio is generally within a few hundred kilometers in diameter, and the medium wave wavelength is relatively long, which is not easily affected by obstacles such as buildings. The short wave is different. The transmitting antenna of a radio station has a certain direction and elevation angle. Under normal circumstances, the distance of the receiver is often as far as thousands of kilometers, or even tens of thousands of kilometers. The radio waves emitted by the radio station can only be received in the distance by refraction of the ionosphere at a height of nearly 1 kilometers above the earth's surface. The ionosphere above the earth is like a changeable mirror, and its reflection ability to short wave and its existing height are changing at any time, so the transmission of short wave broadcasting becomes less reliable. Even so, there are still some changes in the ionosphere that can be summarized, because the main factors for the formation of the ionosphere are ultraviolet rays from the sun and tiny particles with energy; Therefore, the change of the ionosphere will be affected by the following factors:
the intensity of solar activity: the so-called change every 11 years or so.
the distance between the sun and the earth: that is, the change of seasons.
the thickness of the atmosphere through which solar energy travels to the earth is different: it changes from day to night, that is, from morning to dusk to night in a day, so the influence of solar energy on the ionosphere is different during the day and night.
In addition, because the ionosphere often changes rapidly, listening to short waves often appears as loud and small as waves, which is a common phenomenon. Even if automatic gain (AGC) is used in electronic circuits to eliminate this phenomenon, you will still feel loud and small in severe cases. If you can get used to it, it is also a special feeling to listen to short waves!
◎ The effect of short-wave listening is different indoors and outdoors
Because the wavelength of short-wave is much shorter than that of medium-wave, the building is a big obstacle to short-wave, that is, the signal intensity indoors will be much weaker than that outdoors, so the ideal way to listen to short-wave should be to listen to it outdoors with a radio pull rod antenna, and to listen indoors with an outdoor antenna. According to experience, in addition to irresistible natural environmental factors, setting up an ideal outdoor antenna is the first good thing to improve the short-wave listening effect.
Various reasons for interfering with listening to short waves:
Lightning interference in summer;
Indoor electronic fluorescent lamps, SCR dimming lamps, computers, televisions, microwave ovens, telephone lines, etc.
the radiation interference transmitted by high-powered motors through high-voltage power lines in neighboring factories;
the power line of streetcars on the road and the motor spark discharge radiation interference of various motor vehicles;
There is high-power high-frequency radio wave radiation interference near the listening place, such as pager transmitter (BB); Taxi 27MHZ radio intercom; Professional short-wave communication stations, wireless mobile phones, listening places near FM and radio and television transmitting stations with high transmission power, etc ...
◎ Setting up short-wave outdoor antennas
When it comes to external antennas, this is the most puzzling problem for beginners of short-wave. Indeed, it takes professional knowledge to set up a truly standard short-wave external antenna. For everyone's convenience, here, we only introduce a simple and practical external antenna for your reference: prepare an ordinary wire with a length of 5-15 meters, find a suitable place outdoors, and pull it horizontally at one end; in addition