As mentioned above, the ground wave propagates along the earth's surface, and its propagation distance depends on the characteristics of the surface medium. The conductive characteristics of the sea surface medium are most conducive to the propagation of electric waves, and short-wave ground wave signals can travel about 1000 km along the sea surface; The conductivity of land medium is poor, and the radio wave attenuation is large. The radio wave attenuation of different land medium is different (the attenuation of wet land surface is small, and the attenuation of dry sand surface is large). Short-wave signals can only travel along the ground for dozens of kilometers 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.
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 the propagation distance is far (hundreds to tens of thousands of kilometers) and will not be blocked by obstacles on the ground. But the sky wave is very unstable. In the process of sky wave propagation, factors such as path attenuation, time delay, atmospheric noise, multipath effect and ionospheric fading will cause signal attenuation and distortion, which will affect the effect of short-wave communication. Most people will be interested in short wave, because it can listen to long-distance broadcasts and directly listen to broadcast information from all over the world. But many people are confused because of the wrong way to listen to short waves, and finally have to give up. The following suggestions are for your reference on how to start listening to short-wave broadcasting.
◎ What is the difference between listening to short wave and listening to medium wave and FM in daily contact?
Daily listening to medium wave or FM radio rarely encounters the problem of not finding a radio station, because the broadcasting frequency of these stations is fixed, and many of them are broadcast 24 hours a day. For short waves, it is different. Except for many radio stations, there are two seasonal changes in broadcast frequency and broadcast time every year, and the quality of signals received every day is also very different, which makes listening to short wave more complicated than medium wave and FM, but as long as you master the essentials, you can enjoy short wave programs freely.
◎ Listen to short wave-select radio station, frequency and time.
For short-wave listeners, the biggest problem is that short-wave broadcasting is usually concentrated in a certain time period? Broadcasting, causing traffic conditions somewhat similar to commuting time, is extremely crowded. But you can't be a crowded person, because usually the radio station plays the same program at different times and at different frequencies. For example, short wave 15- 18MHZ can receive many radio programs from noon to night every day, and only a few radio programs can be received after 10 in the evening, and even the background noise of the radio becomes smaller. Short wave is below 7MHZ, so it is difficult to hear the broadcast clearly during the day, but in the middle of the night, you can listen to the program well. Short-wave 9- 12MHZ can be heard 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 broadcasting frequency every hour. Therefore, in order to listen to short-wave programs conveniently, it is necessary to make your own listening time and frequency schedule. Of course, you can also start by collecting the broadcast time and 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 heard well. The purpose of monitoring is to choose the best frequency of sound signal from several broadcast frequencies and record it, make a broadcast frequency table, and then listen to the program of this station, which is much more convenient. Unfortunately, many people who just started listening to short waves were greatly reduced by the noise from the radio, and even gave up listening to short waves. Indeed, the sound quality of short wave can't be compared with FM high fax broadcasting, but it is basically very close to medium wave (MW). However, because listening to short wave is influenced by many factors, it often appears worse than listening to 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? Discuss what important factors must be considered when listening to short wave:
Ionospheric factors
Medium wave broadcasting (commonly known as MW), the distance from the transmitting antenna of a broadcasting station to the receiving station is generally within a few hundred kilometers in diameter, and the medium wave wavelength is relatively long, so it is not easily affected by obstacles such as buildings. 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 thousands of kilometers or even tens of thousands of kilometers. Radio waves emitted by radio stations can only be received in the distance if they are refracted at the ionosphere near 100 km on the earth's surface. The ionosphere over the earth is like a changeable mirror. The reflection ability and existence height of short wave are changing at any time, and the transmission of short wave broadcasting becomes less reliable. Even so, there are still some changes in the ionosphere, because the main factors that form the ionosphere are ultraviolet rays from the sun and tiny particles with energy; Therefore, the change of ionosphere will be influenced by the following factors:
The intensity of solar activity: the so-called one-cycle change every 1 1 year.
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 and then to night in a day. Therefore, the influence of solar energy on the ionosphere is different during the day and at night.
In addition, because the ionosphere changes rapidly, it is a common phenomenon that short-wave sounds often appear like waves. Even if automatic gain (AGC) is used in electronic circuits to eliminate this phenomenon, in serious cases, you will still feel the sound loud and small. If you can get used to it, listening to short waves is also a special feeling!
◎ Indoor and outdoor short wave listening effects are different.
Because the wavelength of short wave is much shorter than that of medium wave, buildings are 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 stick antenna and 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 lamp, SCR dimming desk lamp, computer, TV, microwave oven, telephone line, etc.
Radiation interference from high-power motors and high-voltage power lines in neighboring factories;
The power line of trams on the road and the spark discharge radiation interference of motors of various motor vehicles;
There is high-power and 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, radio and television transmitting stations with high transmission power, etc.
◎ Set up short-wave outdoor antenna.
Speaking of external antenna, this is the most puzzling problem for beginners of short wave. Indeed, setting up a truly standard short-wave external antenna requires professional knowledge. For your convenience, here is only a simple and practical external antenna for your reference: prepare a common conductor with a length of 5- 15 meters, find a suitable place outdoors, and pull one end horizontally; Pull the other end indoors and wind it around the rod antenna of the radio (about 7- 10 turns), and you're done.
The so-called suitable location means that the high place is better than the low place, and the more empty the surroundings, the better. For example, it is better to stay away from the wall than to cling to it. As for the length of the line, if space permits, in principle, the longer the better (5- 15 meters long). No matter how thick or thin this wire is, it doesn't need to be peeled from beginning to end. If there is no suitable space to pull horizontally, throw the wire out of the window and let it droop naturally, but it is best to tie a heavy object at the end to avoid the danger of the wire being blown into the high-voltage line or other things when it is windy.
Because the outdoor antenna is pulled outdoors, we must pay attention to the problem of "lightning strike". Therefore, in a thunderstorm, please loosen the wire originally wound on the radio and put it in a safe place (such as outdoors) to avoid danger.
◎ Improve the effect and sound quality of listening to short waves.
In addition to the flickering short-wave phenomenon mentioned above and the use of outdoor antennas to improve the listening effect, you should also pay attention to the interference of your surrounding listening environment, such as fluorescent lamps, computers, televisions, microwave ovens, motors and motors of various motor vehicles on the road, spark discharge and other external interference factors. Of course, these interferences will also occur in any frequency band, but the short-wave radio signal is weak and more susceptible. You should try to find the above interference sources and try to avoid them.
When listening to normal short-wave broadcasting, I always feel that the sound is not ideal. This is because the audio output power of small shortwave radio is generally low. Once the nearby environment is noisy or because of other factors, it is necessary to turn up the volume, which will cause great distortion. Moreover, in the short-wave radio station, in order to improve the selectivity, the passband width of the IF amplifier is narrowed, which also limits the sound quality. Therefore, if you can put on headphones or connect a small speaker with a power amplifier from the headphone jack, the sound quality problem can be improved. Sometimes the sound quality can even be comparable to that of local medium-wave radio stations. The short-wave band is very wide, and radio signals are scattered all over the place. Acceptance requires certain skills.
1 requires a good reception environment and a sensitive radio. A good reception environment can be open space, high-rise buildings and other uncovered places. Indoor, especially in reinforced concrete buildings, the shielding effect is very poor and there are many interferences. In addition, the radio needs to unplug all antennas when receiving short waves. It would be better if there is an outdoor antenna set up separately.
2. Find out the transmission time and frequency of short-wave radio stations. These can be obtained from the internet, and you can go to the broadcasting forum, where there is a short-wave frequency table in the BCL area. Generally, the bands of 19 m and 4 1 m are relatively concentrated. Or listen to these bands at will. The place where you can hear strong folk music is the frequency of BBC VOA. Praise our great party. In addition, in a low voice, if you really want to listen to the broadcasts of these stations, you should carefully search the whole band. Sometimes these stations change frequencies, and these frequencies do not appear on the frequency table. If the receiving frequency of the radio is above 18000 kHz, you can search from 18000 to 23000 kHz. )
3. Short wave tuning. Take your time and be patient! The medium-wave band is from 535 to 1605 kHz, and the short-wave band is from 2700 to 18000 kHz, with one radio station every 9 kHz (although this is not the case in practice, it is only a theoretical provision). From the coverage, it is easy to know that the short-wave stations occupy a small position on the dial. Although the radio divides the short band into many areas (1234567890) for easy tuning, it is still necessary to turn the tuning handwheel carefully and slowly, and pay attention to the RF accessories. If you hear a sound similar to a broadcast, carefully turn the tuning handwheel left and right, and listen to the broadcast sound carefully to get the correct tuning.
4. Be mentally prepared. For ordinary radios, the broadcast quality of short wave reception cannot be compared with FM, or even with medium wave. Moreover, due to ionospheric fluctuation, the propagation of short wave band is far less stable than that of medium wave, and there will be strong and weak fading. Although the radio has automatic gain control, it cannot be completely exempted.
Of course, the best and simplest way is to find someone with better skills to teach you. Writing so much may not explain all types of radios. If divided according to the received frequency band:
Single-band medium wave radio: MW 525-1600 KHz
FM and AM radio microwave 525-1600 kHz, FM 87.5-108 MHz.
FM/MW/shortwave radio * * MW 525- 1600 kHz, FM 87.5-108 MHz.
SW: 3.9- 12.00 MHz (75-25m) when there is only one short wave band.
(or 6.00- 18.00mhz, 49- 16m)
(or 9.00-16.00 MHz, 3 1- 19 m)
Sw 1: 2.2-7.50mhz and SW2: 7.50-23.00mhz.
Or sw 1: 5.9-9.50mhz, SW2: 9.50-18.00mhz..
According to the meter band, SW 1, SW2, SW3, SW4, SW5, SW6, SW7 ... (each short band covers an international short-wave meter band).
Traditional radios and tape recorders generally have only one or two short bands, but each band covers a wide frequency range (several meters). The advantage is that the circuit is simple, but it is difficult to ensure that the sensitivity and selectivity of each point in the coverage frequency range are consistent. So some rice is often very nice, and some rice is very poor. In addition, due to the wide frequency coverage, the stations are very crowded and it is not convenient to receive stations, so some radio stations.
There are also some traditional radios with well-designed short-wave circuits. The radio also has high sensitivity and selectivity, and its production and debugging are very accurate. It is also very convenient and interesting to use, at least it saves the trouble of changing bands. In addition, traditional radios mostly use 3-4 batteries, and relatively large-caliber speakers sound good. No wonder many old shortwave fans still like traditional radios.
Multi-band short wave radio divided by meter band
Modern shortwave radio stations are often divided into 6- 10 shortwave bands, and each shortwave covers only one meter band (please refer to the meter band table of international broadcasting below). For this well-designed short-wave radio, sensitivity and selectivity are easy to ensure. 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 at the densest 16650 stations.
Short-wave radio stations, divided by meter bands, are not enough 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 short-wave radio station is designed according to the meter band division, if all short-wave bands are to be covered, the short-wave band of light needs 13 bands, each band should be designed reasonably, and a lot of electronic components and materials should be used, which makes the circuit too complicated and expensive. The imported famous brand shortwave radio I have seen, all frequency bands of FM/MW/LW/SW are added together, and the maximum frequency band is 15, and the price is nearly 654.38+10,000 yuan.
It is worth mentioning that in the domestic market, there are some shortwave radio, which are called 18 band and 24 band respectively, and their prices are quite cheap. You don't know whether the designer is deceiving himself or others! In addition, there are many shortwave radio with 8 and 9 bands, which are called "Consumer Recommended Products". Due to the 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 buying a traditional 3-and 4-band shortwave radio. Short-wave radio stations initially used direct amplification circuits, and began to use frequency conversion circuits in the 1950s, which are usually called superheterodyne radio stations. In order to further improve the sensitivity, selectivity and anti-interference ability of radio receivers, scientists have developed frequency conversion technology for many times. Of course, it was first applied to the field of radio communication, and then transplanted to advanced radios, thus greatly improving the performance index of shortwave radio.
Portable high-sensitivity short-wave radio stations generally adopt secondary frequency conversion, and more advanced professional short-wave communication receivers even adopt tertiary or quaternary frequency conversion technology. The digital tuning technology of PLL radio is a high-tech product of modern microelectronics application technology, which is advanced, practical and novel.
1. The digital tuning system is based on single chip microcomputer, and has advanced functions such as PLL frequency synthesis, frequency preselection, multi-function digital clock control and LCD digital display.
2. Lock the frequency of the receiving station with the high-precision and high-stability timely crystal as the frequency reference, without drift.
3. It has frequency memory function.
Generally speaking, the more storage stations in a digitally tuned radio, the better. Advanced digital tuning radios should have digital and analog tuning knobs with direct input frequency, and secondary frequency conversion technology is often used in electronic circuits to improve performance indicators.
The disadvantages of digital tuned radio are complex circuit, difficult design, strict requirements for components, high cost and complicated production and debugging. Because many components are used, the static power consumption is higher 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. This kind of radio station 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 indicating frequency of the traditional radio and increases the control function of the electronic clock; Compared with digital tuned radio, it saves electricity, can be designed more compactly and has more convenient volume, and is a practical radio with high cost performance.
The disadvantage of this model is that it has no memory radio function. Because of the traditional analog receiving technology, the frequency accuracy and stability are not as high as that of digital tuning radio.