Radio basics introduce the basics of radio you need to know
1. Radio transmission
AM radio broadcasting It is divided into three major bands: long-wave, medium-wave and short-wave, and the signals are transmitted by radio waves in the corresponding bands respectively. There are only two large bands of radio broadcasting in my country, medium wave and short wave. The frequency band used by medium wave broadcasting is roughly 550kHz-1600kHz, which mainly relies on ground wave propagation, but also accompanied by some sky waves.
FM radio broadcasting mostly uses ultra-short wave (very high frequency) radio waves to transmit signals, with a frequency of about 88MHz-108MHz, mainly relying on space waves to transmit signals.
At present, terrestrial radio and television are divided into two frequency bands: VHF (Very High Frequency or Meter Wave) and UHF (Ultra High Frequency or Decimeter Wave). In my country, the frequency range used by VHF band TV is 48.5MHz-3MHz, which is divided into channels 1-12, and the frequency range used in UHF band is 470MHz-956MHz, which is divided into channels 3-68. They are basically all propagating by space waves.
Second, the development of radio
Civil radio and radio were invented at the beginning of this century. Over the past hundred years, radio broadcasting and radio technology have undergone tremendous changes.
The broadcasting method started from the AM broadcasting era and has gone through the stages of FM broadcasting, FM STEREO broadcasting, and digital audio broadcasting (DAB). Currently, scientists are studying digital broadcasting (DRM) on short-wavelength bands.
The frequencies used in civil broadcasting have gone through the stages of long wave (LW), medium wave (MW), short wave (SW), ultra-short wave FM (FM), satellite FM broadcasting, etc. The propagation distance and coverage of broadcasting The range is also from close range to the use of artificial earth satellites for global broadcasting; radios range from ore radios, tube radios, transistor radios, integrated circuit radios, to digital tuning radios using microprocessors; the basic circuit form of radios, also from direct amplification type, to superheterodyne type, multiple frequency conversion circuit. The size of the radio has also changed from bulky and small to miniature, while the sound quality is getting better and better…
In the 1990s, the basic circuit of the radio and the commonly used signal amplifying components were mainly civil broadcasting standards and frequency bands
1920s-1960s tube circuit/direct amplifier, heterodyne longwave/mediumwave/shortwave
1950s-70s transistor circuit/heterodyne, multiple frequency conversion medium wave/shortwave/FM
70s-80s IC/heterodyne, multiple frequency conversion, digitally tuned MW/shortwave/FM
90s IC/heterodyne, multiple frequency conversion, Digital tuning medium wave/short wave/FM/digital broadcasting
3. Classification of radios
The common radios on the market mainly have the following classification methods :
■By band classification can be divided into:
FM/AM two-band, FM stereo/AM two-band, FM/medium wave/shortwave 3-5 band, FM/medium Wave/shortwave 8-12 band, FM stereo/medium wave/shortwave 8-12 band, TV sound and other radios.
■According to the technical characteristics of the circuit, it can be divided into:
traditional superheterodyne, with digital electronic clock and clock control function (LCD type/LED type/fluorescence type display), analog Tuning/digital display frequency and time, frequency synthesis (PLL) digital tuning (digital, memorized frequency), using double frequency conversion technology (high sensitivity and excellent selectivity), high sensitivity shortwave/single sideband (SSB receiver) ).
Fourth, FM/AM/full-band radio
1. AM radio:
AM radio uses amplitude-modulated radio waves (High-frequency carrier) to transmit program content, the amplitude modulation is the amplitude of the original constant-frequency high-frequency carrier signal, which changes with the amplitude of the modulating signal (audio signal). The AM radio is to receive these amplitude modulated radio signals and restore them to sound waves after demodulation.
2. FM radio:
FM radio uses frequency modulated radio waves to transmit program content. The so-called frequency modulation is the frequency of the original high-frequency signal of constant amplitude and constant frequency, which changes with the amplitude of the modulated signal (audio signal). FM radio is to receive these frequency-modulated radio signals and restore them to sound waves after demodulation.
3. Full-band radio:
Full-band radio, the earliest regulation is a radio that can receive all broadcast signals within the frequency range specified by the International Radio Commission, but because many meter bands are There is no radio station, so now people call the radio with AM (medium wave), FM (frequency modulation), SW (short wave covering the meter band in the range of 5.9MHz-21.85MHz) as a full-band radio.
V. Introduction of FM, AM, MW and SW.
There are AM and FM frequency bands on common radios or tape recorders. I believe everyone is familiar with these two frequency bands. These two frequency bands are for you to listen to domestic broadcasts. If there is SW on the radio In addition to domestic radio stations, you can also listen to foreign radio stations (such as VOA, BBC UK London Radio, NHK Japan Radio…).
In fact, AM and FM refer to two different modulation methods in radio. AM is called amplitude modulation, and is a modulation method that changes the carrier amplitude according to the modulating signal. It maintains the frequency characteristics of the high frequency carrier, but the shape of the envelope is similar to the signal waveform. The amplitude of the AM wave is determined by the strength of the modulating signal. The modulation method in which the carrier frequency is changed according to the modulating signal is called frequency modulation. The magnitude of the frequency change of the modulated wave is determined by the magnitude of the modulating signal, and the period of the change is determined by the frequency of the modulating signal. The amplitude of the modulated wave remains unchanged. The waveform of the FM wave is like a spring that is compressed unevenly, and the FM wave is represented by the English letter FM. Generally, medium wave broadcasting (MW) adopts the method of amplitude modulation, and before you know it, the equal sign is drawn between MW and AM.
In fact, MW is just a kind of broadcast that uses AM modulation. For example, the modulation method used by international shortwave broadcasting in high frequency (3-30MHz) is also AM, even higher than FM broadcasting. The aviation communication (116-136MHz) also adopts the AM method, but the AM band we usually refer to refers to the medium wave broadcast (MW). The fate of M is similar to that of MW. We used to use FM to refer to general FM broadcasting (76-108MHz, 88-108MHz in Taiwan, 76-90MHz in Japan), in fact FM is also a modulation method, just in shortwave Within the range of 28-30MHz, as amateur, space, artificial satellite communication applications, there are also those who use FM. As for SW, it can actually be said to be an anonymous name. The correct term should be high frequency (HF: High Frequency), which is more appropriate. How did the name short wave come from? In terms of wavelength, medium wave (MW) is between 200-600 meters, while the wavelength of HF is between 10-100 meters, which is indeed shorter compared to the above wavelengths, so HF is called short wave (SW: ShortWave).
Similarly, between 150KHz-284KHz lower frequency than MW, this spectrum is also used for broadcasting, in terms of wavelength, it is about 1000-2000 meters, and MW 200-600 meters is much longer, so the broadcast in this spectrum is called LongWave (LW: LongWave). In fact, AM modulation is used regardless of long wave (LW), medium wave (MW) and short wave (SW). For general radios, the FM, MW, and LW bands are all for you to listen to domestic broadcasts (except for a few countries such as Europe and Japan, most of the countries have eliminated civil long-wave broadcasting), while the SW band is mainly for you to listen to. International broadcasting. The short wave is divided into 13 meter bands. The frequencies within the meter band are used for civil broadcasting, and the frequencies outside the meter band are mostly used for military and other civilian communications. Therefore, civilian radio programs can only be received in the meter band range. Most of the radio stations for civil broadcasting are concentrated in the frequency range of 6-18 mega-frequency, that is, in the 49-16 meter band.
Meter band M MHz (megahertz) station distribution and listening effect
11 == 25.67-26.10 There are no radio stations in the international band.
13 == 21.45-21.85 There are very few international band radio stations, and some radio stations can be received in the afternoon.
15 == 18.90-19.02 international band radio is very few, only the United States uses
16 == 17.48-17.90 international band radio is more, better before noon to 9:00 pm, The effect is average in the morning and morning, and there are fewer radio stations from late night to morning.
19=== 15.10-15.80 International band radio stations are more, better before noon to 9:00 in the evening, the effect is average in the morning and morning, and there are fewer radio stations from late night to morning.
21=== 13.60-13.87 International band is best for evening listening, but there are not many stations.
25 == 11.60-12.10 International band radio is the most, all day, the best morning, morning, evening until 12:00.
31=== 9.40-9.90 has the most radio stations in the international band, all day long, best in the morning, morning, evening until 12:00.
41=== 7.10-7.35 international band radio station, many radio stations received in the morning, evening and late night, but easily interfered with loud noise.
49== 5.90-6.20 There are many radio stations in the international band, and many radio stations are received in the morning, evening and late night, but they are easily interfered and the noise is loud.
60== 4.75-5.06 area band has good nighttime effect, but not many radio stations, poor daytime effect and lots of noise.
75== 3.90-4.00 international band has good effect at night, but there are not many radio stations, and the effect is poor during the day, and there are many noises.
90== 3.20-3.40 area band has good nighttime effect, but not many radio stations, poor daytime effect and more noise.
120== 2.30-2.495 area band has good effect at night, but there are not many radio stations, the effect is poor during the day, and there are many noises.
In addition, the following bands are used as a secondary service, and there are also a small number of radio stations, 5.73-5.90, 6.20-6.295, 6.890-6.990, 7.35-7.60, 9.25-9.40, 9.90-9.99, 11.50-11.60, 12.10 -12.16, 15.03-15.10.
The long wave has the lowest frequency and is mainly transmitted by ground waves, with a long propagation distance and stable reception performance throughout the year. It is mainly used for domestic broadcasting in European countries, and there is no long-wave radio station in my country.
The medium wave is transmitted by ground waves during the day, and the situation is similar to that of the long wave, but it is greatly affected by the geographical environment. Reflection transmission, the propagation distance increases, the radio station interval is denser, and the mutual interference phenomenon also increases. At present, the medium wave is used as domestic broadcasting both at home and abroad.
The frequency of short wave is high, and the transmission distance of ground wave is limited. It mainly depends on the reflection of the ionosphere. After multiple reflections, it propagates to thousands of kilometers away, and it can even bypass half of the earth. Therefore, international broadcasts are sent by short wave. Since the electromagnetic wave can reach the radio after several reflections after it is sent from the radio station, the received radio wave is unstable, the signal strength fluctuates from time to time, and the listening sound fluctuates. Therefore, high-end shortwave radios have automatic gain control circuits to ensure the normal reception of the radio. . In the shortwave band, reception varies with time, season and weather. Generally, the effect is better when it is sunny in winter, and the effect is worse when it is cloudy and rainy in summer, and the fruit is better at night than during the day. In addition, there is a quiet zone in the short-wave propagation range, that is, the radio station cannot be heard in the area that is not covered between the two reflections. For different times and frequencies, the quiet zone is different, so a set of programs is often transmitted at the same time with several shortwave frequencies and adjusted with the change of seasons.
Ultra-shortwave broadcasting is an FM method, which is generally used for high-fidelity stereo broadcasting. The division of frequency range is more complicated. The reason is that it needs to be inserted in the gap of TV broadcasting channels. Japan is 76-90 MHz, my country and European and American countries are 88-108 MHz or 88-104 MHz. UHF signals, like TV broadcast signals, can only be transmitted in a straight line, so FM programs can only be received in the local area, generally the longest distance is not more than 100 kilometers. Due to the straight-line transmission, it is less affected by climate and less electromagnetic interference, so the reception is very stable; and because the longest distance of transmission is small, each region can form its own broadcasting network, so that the same frequency band can be reused.
So at night, the signals of the medium wave and short wave band can be transmitted farther and the reception is more stable, so there are more radio stations that can be received. This is the most important receiving band for ordinary radios. FM bands are all received locally and are not affected by time.
