【 Weak current college 】 radio communications terminology 】

【 Audio 】 also known as the audio is heard by human ears. Usually refers to 15 ~ 20000 Hertz (Hz) of frequencies.
【 Voice frequency 】 refers to audio frequency range of languages. In the General phone access, usually 300-3400 Hertz (Hz) of frequencies.
【 RF 】 radio transmitter with antenna can effectively launch to the space of the frequency of the electromagnetic wave, collectively referred to as RF. If the frequency is too low, the low emission of effectiveness, it is customary that radiofrequency refers to 100 kilohertz (KHz) frequency above.
【 Video 】 TV signal contains frequency range from several tens of Hertz to few MHz, video is the frequency of collectively.
【 Carrier 】 up means information of a sine wave or periodic pulses, called the carrier (or carrier frequency), along with the signal wave of change, so that the carrier's range, frequency or phase change accordingly.
【 Signal 】 used to express or to bring the power of information.
【 Channel 】 press pass information properties by number of pathways. Including possible but have not yet achieved pathway.
【 Analog signal 】 in time to be continuous or on a parameter can have an unlimited number of values of the signal.
【 Digital signal 】 so-called digital signal, the signal is discrete and continuous. This is the signal only a finite number of steps or incremental change and value. In other words, a digital signal, just the number of calculation steps regardless of the size of the step signal (the most common is the binary coding).
【 Band 】 in radio technology, band this term has two meanings. The first is a division of electromagnetic wave spectrum, such as long-wave and shortwave, FM and other bands. The second refers to the transmitters, receivers and other equipment in the frequency range of the Division. If the operating frequency range is divided into several parts, each of these sections are also called bands, such as three-band radios, etc.
【 Channel 】 communication devices work the passband call channel. Usually a communication device in its frequency range has many a channel.
【 Passband 】 a circuit that allows a smooth adoption of the current range of frequencies, known as the circuit of pass band. General provisions in the current is equal to the maximum current value within the range of 0.707 times the upper and lower frequency in the passband width.
【 Frequency coverage 】 communications equipment working frequency range, known as frequency coverage. The maximum operating frequency and minimum working frequency ratio, known as the coefficient of frequency coverage.
【 Cutoff frequency 】 used to illustrate the circuit frequency characteristic parameters of specific frequency. When hold circuit input signal amplitude is invariable, changes frequency so that the output signal is reduced to the maximum value of 0.707 times, or a particular rating when the frequency is known as the cutoff frequency.
In the high-frequency end and with a low-frequency end cutoff frequency, called on the cutoff frequency and lower cutoff frequency. Two cutoff frequency range of frequencies between is called pass band.
【 Frequency stability 】 oscillator frequency of various reasons to change the frequency change of size and rated frequency ratio is called the frequency stability. It is a measure of the quality of communication systems is an important indicator. Improved frequency stability over frequency stabilization using parameters, crystal resonator and frequency synthesis, etc.
【 Residual wave radiated power allow the least 】 licit radical wave radiation outside the harmonic radiation, parasitic radiation and mutual modulation of any residual wave radiated power of the minimum allowed value, in decibels, or MW, micro-Watt said.
【 Band width 】 is sometimes called the necessary bandwidth. Means to ensure a certain emission rate and quality of information required to consume bandwidth allowed value, in Hertz (Hz), kHz (KHz), megahertz (MHz).
【 Selective 】 radio receivers will be required for radio signals from many different frequencies of radio signals in the capabilities, called selective. Receiver selectivity, the better, the more vulnerable to interference from other radio stations. Therefore, the selectivity is an important decision receiver quality parameters.
【 Sensitivity 】 radio receiver of a weak signal reception capabilities, called sensitivity. If a receiver can receive very weak signal, the receiver sensitivity is high, but sensitivity is low. Therefore, sensitivity is an important decision receiver quality parameters.
【 Fidelity 】 also called fidelity. Means the receiver output signal and the similarity of the input signal, the receiver for the signals in the frequency can be the same magnification, restoration, without distortion. If the radio receiver fidelity as possible, its output of language, music, the more realistic.
【 Transmitter output power 】 refers to electromagnetic transmitter (antenna) radio frequency power known as the transmitter output power.
【 Transmitter spurious radiation 】 in nominal output impedance of the load measurement, transmitter carrier power less than 25W, any discrete frequency spurious radiation power not exceeding 2.5uW. When the transmitter carrier power greater than 25W, any discrete frequency spurious radiation power less than 70dB launch carrier power.
【 Adjacent channel power 】 for 160, 450MHz band, the adjacent channel power 16KHz band, a carrier power low 70dB. For the 900 MHz band, in the second adjacent channel 32 KHz-band power, should a carrier power low 65dB.
【 Average power 】 transmitter under the conditions laid down, in the ratio of minimum modulation frequency corresponds to a much longer period of time to feed to the provisions of the experimental load average power on.
【 Peak Pack power 】 transmitter modulation provided for in the conditions, in the high-frequency modulated envelope peak at a cycle to the provisions on experimental load average power.
SSB transmitter rated output power to the nominal peak Pack power.
【 Sideband suppression 】 in SSB signal generation, without side-band signal suppression capability called sideband suppression. With no side-band signal level and useful side-band signal level is the ratio of the number of decibels.
【 Band power 】 under the modulation, transmitter total power in the hands of nominal frequency on either side of certain specified frequency as the center of a provision in-band power of that part.
【 Crosstalk 】 in one pass, because other pathway signal energy impact of noSignal.
【 Noise, noise 】 transmission paths or device in addition to the useful signal any power harassment.
【 SNR 】 signal-to-average power and noise-to-average power ratio called the signal noise ratio, signal to noise ratio or clutter ratio. To DB SNR expression as follows:
Signal to noise ratio (DB) = 10
【 Noise coefficient 】 means under certain conditions, receiver or amplifier, the output of the total noise power and internal no noise source, due to the input terminals of thermal noise output noise power ratio.
【 Distortion 】 refers to a signal during the transfer process and original signal or standard deviation over. In the ideal amplifier, the output waveform in addition to zoom in, and input waveform is exactly the same, but in fact, cannot be output and input waveforms identical, this phenomenon is called distortion, also known as the distortion.
According to the different conditions of waveform distortion can be divided into amplitude distortion, frequency distortion, three phase distortion. On the range of different signal on a number of different known as amplitude distortion. On the frequency of different signal on a number of different known as frequency distortion. On the frequency of different signals, the enlarged resulting latency different called phase distortion (or delay distortion).
Distortion is also known as nonlinear distortion, frequency distortion and phase distortion called linear distortion.
【 Level 】 is an expression of power (voltage, current or power) the amount of relative size, the common unit for DB (or Nai Bay). Typically specifies a numerical quantity as a standard value, in other numerical and standard values compared to the value to represent the level value. For example take standard power 1 MW to zero level, when the power is 10 MW, the level value can be obtained by the following formula:
Level value = 10
Therefore, the 10 MW has 10 decibel level. If the level value is negative, it represents less than zero, this level is used to represent any two power relative size.
【 Audio response 】 input signal level does not change, the audio range, the receiver output level varies with the audio frequency, known as audio responses. To the highest level and the minimum level of representation than the number of decibels.
【 DB 】 is the abbreviation of Beier, equal to 1 Bell's 1/10, with dB that is used to measure amplifier or the attenuation of the common unit.
In the said power amplifier or the attenuation: DB =
In voltage (or current) been number = 20 DB
【 Sinai Bedouin 】 is a measure of gain or attenuation. It is the ratio of voltage or current ratio of the natural logarithm. The impedance of the circuit in two points are equal, it is the power ratio of one half of the natural logarithm. 1 El Pui equals 8.686 DB.
【 Interference 】 some emission, radiation, induction or a combination of unwanted energy on radio communication system for reception by the effect of the reception performance degradation, or not receiving the signal, this effect is known as interference. Interference according to their sources can be divided into: industrial disturbance, interference, cosmic top there was interference, human disturbance, and so on.
【 Interference 】 in radio communication system, is to determine whether the interference emission, radiation or induction. The resulting prejudice radio reception those messy waves.
【 Cosmic disturbance 】 from Galaxy and the Sun's radiation caused by the interference. This interference of higher frequency UHF-band, is an important source of interference. A measure, in 18-160 megahertz (MHz) band interference level within the Milky Way Galaxy and the frequency is proportional to the cube.
【 Pulse interference 】 its intensity is high, but the shorter the duration, frequency band is wide. One of the major sources of various industrial equipment generates electrical impulses, such as welding Sparks, automobile, aircraft, starting and running a variety of fire, medical, electrical equipment produced by the spark. Lightning can also cause interference pulses. On average each seconds on Earth took place a hundred times and lightning, it caused strong electromagnetic waves can spread very far.
【 Undulating interference 】 (also known as rolling noise) in consecutive interference range constantly changes, this kind of interference comes mainly from the following aspects: the Astral of radiation of the universe; device internal noise; such as conductor in electronic thermal movement of fluctuations in voltage, electronic devices, such as current fluctuations.
【 Top there was interference 】 refers to atmospheric deposition storage charge discharge of electromagnetic radiation, caused by lightning is one of the most powerful top there was interference. Top there was interference in the longwave performance most strongly, as the frequency increase, top there was interference effects fade, to the FM band would be very small.
【 Human disturbance 】 separable unintentional interference and intentional interference. The former is due to the economic and daily life is widely used by various electrical equipment, industrial interference. You can use the filter or shielding to prevent. Intentional interference such as enemy jamming, radio interference, etc., can improve the antijam technology and application of anti-jamming equipment to prevent.
【 Industrial interference 】 refers to various electrical appliances, mainly arcing and sparking devices, such as welding equipment, tram, with electric ignition systems of engine work arising from the interference. Industry interference of spectrum is usually very wide, so that the receiving device to prevent such interference is very hard, generally in the interference source measures to reduce the intensity of the interference.
【 Inter-modulation interference 】 also called cross-modulation. A modulation interference (such as jamming station) and the signal simultaneously to the receiver, because of the high-and-drop or nonlinear effect of frequency converter, the interference of the modulation signal to signal carrier, to form cross-modulation interference, resulting, called cross interference.
【 Intermodulation interference 】 when two or more interference signal simultaneously to the receiver, because of the nonlinear function, this combination of two disturbing sometimes exactly equal to the frequency or close to the useful signal frequency and successfully passed the receiver, the third-order intermodulation. This form of interference, known as intermodulation interference. Intermodulation interference and inter-modulation interference, usually resulting in a high place and frequency level.
【 E-mail fog 】 all kinds of electrical and electronic equipment used in the process, you are a large number of different wavelengths, and given the frequency of electromagnetic waves, which includes radio, infrared, visible light, ultraviolet, x-rays, gamma rays, etc. This kind of electromagnetic waves flooded in space, known as "e-mist" sources of pollution, that is, we oftenSpeaking of electromagnetic pollution.
【 Quantization noise 】 in language coding communication, after signal demodulation and original delivery discrepancy is the result of signal amplitude and time of quantification, such a distortion is called quantization distortion. Because this kind of distortion and clutter of interference, the sounds and symbols of thermal noise, so called quantization noise.
【 Shielding 】 typically use copper or aluminum, and other low resistance materials or magnetic material container (needs good grounding) will need to isolate all of the parts, and the influence of magnetic power line or restricted to a certain extent, or to make a specified space prevent external static induction or influence of electromagnetic induction.
【 Filter 】 filter is on frequency selective action of a network, it can make a band of AC passed, leaving the other frequency AC be greatly attenuated.
Many types of filter, band-pass filter, band-stop filter, highpass filter, low pass filter, wave filter, LC filter, mechanical filter, lens filters and ceramic filters, etc.
【 Notch filter 】 used to filter out a frequency signal for tuning circuits.
【 Radio control 】 is the use of radio signals on the distance of a variety of agencies in control technology. These signals are received from receiving equipment, you can order or driving other appropriate machinery to do various operations, such as a closed circuit, the move handle, starting motor, after conduct by the mechanical action required. Therefore, each control signal in the frequency and continuation of time on each other for control of ships, aircraft, missiles, application of sea-blank line is extensively on.
【 Radio telemetry 】 is measured on a distant object. Get the required data. Such as radio remote automatic weather station, located on a hill, you do not need to directly in the Hill weather station, you can know the required information, such as atmospheric pressure, air temperature, atmospheric humidity, wind, precipitation, etc. The meteorological elements, through a series of electronic devices, conversion to electrical signals and program codes sent to remote telemetry the weather station. Like to learn more about an area of ocean, placed a certain number of automatic buoys (or other objects), buoy is equipped with measurement of meteorological and hydrological parameters for sensor, the measuring parameters converted to transmit signal, using radio waves. After reception by the coast, coast station i.e., access to sea, this kind of method of radio telemetry.
【 Radio monitoring 】 adopts advanced technical means and certain equipment for radio transmission frequency, frequency error, emission bandwidth measurement, etc., to listen to the sound signals, on the illegal radio and interference source finding investigated.
【 DF 】 determination of transmitting station is located. It is used to redirect incoming radio special direction. This kind of radio direction-finding stations, called by using a direction-finding stations, you can determine the direction of the radio is on. Use of the two are enough distant radio direction finder, you can determine the direction of the radio, but also to determine its location, as it should be located in two direction station by the intersection of the two directions. Therefore, radio navigation and radio detection aspects of wider application.
【 Phase 】 its phase on its reference phase deviation value along with the modulation signal proportional to the instantaneous change in modulation mode, called phase modulation, or called phase modulation. Adjust phase and frequency modulation. Phase modulation, frequency modulation accompanying occur at the same time; FM, also accompanied by a transfer to occur, but different variations of the two. Actual use is rarely used and phase, it is mainly used as get FM.
【 Pulse modulation 】 pulse modulation has two meanings. One is the pulse of its own parameters (margin, width, phase) with signal change process. Pulse amplitude with signal changes, known as pulse amplitude modulation; pulse phase with signal changes, known as pulse phase modulation; Similarly also pulse width modulation, pulse spacing modulation, pulse code modulation, etc. Of these, pulse code modulation of the immunity of the strongest, since communication applies the most promising. The second is the use of pulse signal to a process of high-frequency oscillation. Two meanings of different points are: the former pulse is itself a carrier, which high-frequency oscillation is the carrier. In general the pulse modulation usually refers to the former.
【 Electromagnetic 】 this is the space-borne alternating electromagnetic field. In a vacuum, electromagnetic wave propagation speed is 3 × 108 m/sec.
Electromagnetic radiation in the wavelength range very wide wavelength, the rendered form. Where the light waves are electromagnetic waves of a wavelength of extremely short, and the longer wavelength radio waves. Radio wavelengths shorter boundary (MMW) and wavelength long boundary (infrared) connection. Classification as electromagnetic waves (a) schedule.
Communications, General radio frequency band, using radio waves to communicate the following bands, but currently used is not very widespread, many communication projects are still under study.
【 Fading 】 electromagnetic wave propagation process, as communication media and communication channels with time-induced variations in received signal strength of the phenomenon known as fading. For example, in your words, sounds a little strong, weak, this is fading phenomenon.
The decline of the rate of change can be divided into two types of fast, slow decline.
1. fast fading: it is caused by multipath, its rate of change some general in zero
Seconds to several tens of seconds.
2. slow fading: it is only with meteorological conditions (e.g. temperature, pressure, humidity, etc.)
It is diurnal, seasonal and are closely related, it is fading, and generally refers to a change of hours.
Fading can also be classified in law, can be divided into flat fading and selective fading two types. Fading quality of communication have great influence on the design of communication circuits, to take into account this factor. 【 Telegraph communication 】 telegraph communication is the use of electricity in the distance a written information transmitted between a means of communication. Transfer there are two basic methods: one to put the characters into code, release-follow certain code to send out a signal pulse, in newspapers and received signal pulses into a character, this is called a coded telegrams;Is the text of the manuscript or electricity to other methods, this is called fax.
【 Code 】 they use several, no current pulse or positive and negative current pulse formed by different combinations of signals, in which each signal combination represents a set of letters, numbers, or punctuation.
【 Morse Telegraph 】 is composed of dots, two symbols of that point, the length of time to have a certain standard, that is:
1. point to a basic signal unit, a division of length should be equal to the length of three
The equivalent of three basic signal units.
2. in a letter and numbers, points, divided between the interval should be equal to the point of long
Degrees.
3, letter (digital) and letters (number) the interval between the length of the seven. By
In the character code for varying, so called non-uniform code.
【 Five unit code 】 is comprised of five, no current of positive and negative pulse or different current pulse signal composed of combinations, each signal combination represents a character. Because each pulse is equal to the time, every time the length of signal combination is equal, also known as a uniform code.
In the beginning and end of the telegram within the five units of code, in order to guarantee, receive both sync work, that is, release some action (send a character), sink part of action also once (your printed one character), in five coded pulse before sending a starting pulse, the sink component start. Similarly, in five coded pulse sender, then send a stop pulse to sink some stop, it can be seen for grant-Telegraph machine, actually transfer a character to a total of seven pulse, a starting pulse, five coded pulse and one stop pulse.
【 Fax 】 use of scanning technology to images (including photographs, charts, text)-is from the party to the Subscriber's method of communication. Send, as it is placed in the send fax machine, according to a certain order into many different shades of black and white dots, the role of via opto-electronic equipment, the dissimilarity of small points into the strength of different current, and then use wired or radio channel to each other. Each other in fax receiver will receive the signal current use of different methods, copy the original photos, charts, or text.
Fax advantage is not needed to decipher procedures, you can directly transfer photos, graphics, files, scripts, etc., so as to improve the communication efficiency and reduce the error rate. The disadvantage is consuming bandwidth wider, but also more complex.
【 Authentic Telegraph 】 fax. Transmitted by fax to the text or chart-like (but not including photos). You can transfer to or cannot use the General content of the telex, such as handwritten documents, charts, etc.
【 Simplex communication 】 Bulletin during the same line on both sides of the transmitters and sink must turn to the side release is not sink, your time is not release. This way is called Simplex communication.
【 Duplex communication 】 both sides can send and receive at the same time for briefing mode is called the duplex communication pattern.
Implement a duplex communication is needed to satisfy the conditions of: 1, sink is ready to receive to reported that 2, transmitters work should not affect this side of the sink unit.
【 Half duplex communication 】 also known as a "quasi-duplex" communication is SSB communication in a special way. Is the use of the other "fat" clearance signals to each other, is a "restricted duplex". As usual the phone is to use "half duplex" way, someone is speaking, the other party can only listen. Such as eager to answer, you can only use each other's speech of clearance, slipped stresses.
【 Exclusively on speech machine 】 commonly referred to as intercom. Each phone is a set of frequency and frequency intervals when used for 25KHz, may be based on the need to configure one or more frequency. Phone 5W output power range (typically divided into 0.5W, 2W, 3W, 5W several levels). This kind of phone generally limited use native with small whip antenna for use on an exclusive Talkback, forbid high antenna for long distance communication, to control the effects of distance and reduce interference.
【 Single frequency network phone 2002】the commonly referred to as vehicle-mounted or base stations. Each phone is a set of frequency and frequency intervals when used for 25KHz, may be based on the need to configure one or more frequency. Its output power range 5 ~ 25W (General-10W, 5W, 15W, 25W). Can also be used in accordance with a request, press the volume production of telephone, 5W following as networking. Center or base station transmitter output power shall not exceed 25W. This type of telephone used primarily for single-frequency Simplex remote networking, few exclusively to communication between can also be used.
【 Dual band network phone 2002】the each phone is a set of frequency and frequency intervals as 25KHz, transceiver frequency difference as 5.7MHz (D band) and 10MHz (E band), use as needed, each phone configuration a certain amount of frequency. This phone is mainly used for dual Simplex (or duplex) networking, few distance between an exclusive communication can also be used. The scope of their power levels and single frequency network phone.
【 Wireless phone 2002】the each wireless microphone from a number of Pocket transmitter (can be installed in the Pocket, the output power is about 0.01W) and composed of a central receiver, each Pocket transmitter have a disparate operating frequency, set the receiver can receive Pocket transmitter emits the ministries of different frequency voice signals. It responds to the stage from the rostrum, etc.
【 Cordless phone 2002】the is a stand-alone automatic telephone. This phone stand-alone pay machine by the host and made up of two parts. Use the host access telephone network, the user may leave the host dozens of meters away, using the pay machine listening and dial phone. This phone stand-alone hosts and pay for the machines are connected via radio, call content will be exposed to the air, such as accidentally, causing the air leakage. So when you full attention is used.
【 Pocket Bell phone 2002】the each Pocket Bell phone consists of a transmitter and a number of Pocket receiver, transmitter as a center or base units, the output power in General as 25W, larger work area, can be controlled using 50W or 100W following of power. But power must not exceed 100W. Some of its ancillary Department Pocket Bell receivers only have one and the same frequency, the consumer can through central station to the distributed job point (or staff) do one-way Messenger or other signals. Such as phone numbers or characters, etc.
【 Wireless paging system 】 wireless paging system, is not the voice of one-way select calling System. The receiver is more accessible by users of the high sensitivity of the recipient machine (commonly known as Pocket Bell). On the recipient machine receives the call, it will automatically ring, display digital or Chinese characters, to pass specific information to the user.
Wireless paging systems can be divided into private and public systems. Dedicated systems to take manual mode. General in console side of a fixed line telephone. When the operator to receive cable users call a Pocket Bell, i.e. for continuation, encoding, then the encoder to the wireless transmitter for call; Pocket Bell receives the call and then automatically ring. Public system uses artificial and automatically in two ways.
【 Wireless paging techniques】 since 1984, China began to establish the paging network, paging business, basic coding system adopted, POCSAG, formerly a large amount of paging station adopts 1200bit/s for low speed, and even lower 512bit/s encoding scheme. POCSAG encoder standard transmission rate is low, and you want to add a new user can only be through increasing frequency, is opening a new one. In paging industry rapid development stages, the amount you want to allow users to continue to increase, particularly in the application of new technology, there are many problems. FLFX speed dial page encoding is a full synchronization, multiple rate and timeshare transfer encoding format. FLFX encoding format to every 4 minutes for one cycle, each cycle is divided into 128 frames and adopt 1600bit/s basic rate shipping in each frame structure by 1/2/4 basic frames multiplex will group call user information into a single frame, send realizing high rate.
Paging technique and related control technology, can produce telemetry, remote control, etc. For example, by paging technique, can be sent to the remote device control signals, such as a telephone, using paging control signal, before work, you can remote control to open the air conditioner at home. Similarly, some telemetry information also by paging techniques, regular route to the specified location. In short, the paging technology and control technology, can produce many new applications.
【 Mobile radio communication 】 i.e. mobile (car, train, ship, aircraft) equipment of radio communications equipment and fixed (or move) locations of the radio communication between devices. Mobile radio communication including land mobile communications, maritime mobile communications, air mobile communication and public mobile communication, etc.
Mobile radio communication system usually consists of a number of base stations and mobile units. Its brief communication process is this: when making a person on the base table microphone speech, base stations will speaker voice with the microphone, the converter into electrical signals, then by a transmitter launch out; vehicle mobile station of the receiver receives a signal, after converter, speakers transform into original acoustic, then listener will hear the speaker's voice.
Mobile radio communication with a wide band, long wave and long wave, wave, HF, UHF and microwave.
Mobile radio communication can conduct business content is very wide, it can pass the Telegraph, telephone, fax, data transfer and transfer images. According to the different systems transmitting signal, analog mobile communication and digital mobile communication, etc.
【 Microwave communication 】 microwave often frequency in megahertz (MHz) 1000 (wavelength in 30 cm) of electromagnetic waves, the use of microwave propagation communication known as microwave communication.
Microwave propagation characteristics similar to the spread of light, usually a straight transmission, diffraction capacity is weak, general communication within a range, for long distance communication can relay for microwave radio relay communication, or microwave relay communication can also communicate using tropospheric propagation, called tropospheric scatter communications; or the use of satellites for forwarding, namely satellite communications.
Microwave communication characteristics: 1, band range, communications capacity, microwave communication generally multi-channel communication; 2, spread relatively stable.
【Quality millimeter-wave communication 】 wavelength from 10 mm to 1 mm, from 30 GHz frequency (GHz) to 300 GHz (GHz) radio wave called a millimeter-wave, the use of millimeter-wave communication method called millimeter-wave communications. Millimeter-wave communications sub millimeter wave waveguide communications and millimeter-wave radio communication.
Millimeter-wave communication advantages are: 1, available in very wide frequency band. Mm-wave band width 270 GHz bands (GHz), for the entire ten thousand times of short-wave band; 2, direction, privacy is good; 3, interference is very small, almost not subject to atmospheric disturbances, cosmic noise and industrial noise impact, thus communication stability.
【 Scatter communications 】 scatter communications is a horizon of means of communication, which utilizes air media electromagnetic wave scattering from a role in the communication between the two places. The troposphere and ionosphere, Meteor, artificial scattering objects, and so have the scattering properties of electromagnetic waves. If the transmitter emits electromagnetic radiation to such places, are scattered in all directions, including radiation out towards the oblique direction to go in front of electromagnetic waves can reach very far away. Far from the receiver, if there is a sufficiently high sensitivity, the scattering to receive weak electromagnetic waves, enabling communication.
Due to the scattering of communication electromagnetic wave transmission loss is high, reaching the receiver of the signal is very weak, in order to achieve reliable communication, typically uses high-power transmitter, high sensitivity receivers and high-gain, narrow-beam antenna.
【 Satellite communication 】 satellite communication is the use of artificial Earth satellites as the relay station for forwarding or reflection of radio signals, in two or more Earth stations which communicate between.
Advantages of satellite communications are: 1, communication distance farther. Earth satellite over the Earth in General for thousands to tens of kilometres run, using it as a relay station for international communication and intercontinental traffic. Combine several satellite launched into the air, as long as the height and location correctly, you can deliver global communications. 2. communications capacity, satellite communication in GeneralUse 1 ~ 10 GHz bands (GHz) microwave band, has a very wide bandwidth, you can transfer multi-channel TV and large capacity of the phone. 3, satellite communication is not affected by the impact of atmospheric disturbances, communication and reliability. 4, the most prominent advantage is a large area coverage capabilities for CDMA and channels on demand, communication flexibility.
Satellite communication is in recent decades, there's a new means of communication. It can transmit telephone, Telegraph, data, TV, large promising.
【 Meteor communications 】 use Meteor reflection radio waves of long-distance communication called Meteor communications. It features communication can only be used in the Meteor occur within a short time, so it is a fast means of communication. Meteor communications common band of 30 ~ 100 megahertz (MHz).
Meteor communications major advantages are: 1, communication distance farther. Experiments show that the use of power 500W-few kW transmitter and ordinary Yagi antenna, communication distance is up to 1500 km, maximum communication distance of about 2300 km; 2, strong. Because the radio waves reflection with a very clear direction, not tapped, and easy to prevent jamming station; 3. communication of good stability, less subject to ionospheric disturbances and Aurora. Its shortcomings is: because the sending State was intermittent, information is delayed, sometimes up to several minutes, which do not meet the transfer during this time. Printing Telegraph transmission of information, a larger percentage of error; the terminal device is more complex.
【 Single sideband communication 】 general communication system, carrier via audio signal modulation carrier frequency, contains and the upper, down volumn) (in two side-band, the two side-band can be used to transmit information. Usually passed signal, only need a single sideband is sufficient, but in general communication system, often on the carrier frequency and, following band together and send out so that the carrier and the other side with a transmitter power consumed in the majority of power, but also occupy a wide band of communication. In order to improve the communication efficiency and economical communication band, in communications, carrier and on the other side of the tape removed, just send a sideband, such communication is called sideband communication.
SSB communication advantages are: 1, save power; 2, saving bandwidth; 3, because SSB transmitter does not send a carrier frequency, improves confidentiality. Its disadvantage is that the device is more complex.
SSB communication can be used on cable carrier telephone, mobile telephone, fax, TV and data transfer, etc., by far the most commonly used in the cable carrier telephone and long distance point-to-point short-wave radio communication.
【 Independent side-band communications 】 all produce sideband signals method, almost always first create bilateral with signal, therefore, can be bilateral with amplitude modulation signal of two side-band respectively, so that they contain different information. Such signals are called independent side-band signal, the signal of communication is called independent side-band communication.
Independent side-band communications has the advantage that you can also transfer multi-channel information, reduce mutual interference. The drawback is the amplifier in the design and manufacture more difficult.
【 Suppressed carrier SSB 】 in SSB technologies, with various forms of band-edge. If a carrier and a side strip completely removed (Suppression), known as suppressed carrier SSB. Sometimes in order to simplify the receiving system, preserve part of the carrier, is called a carrier SSB. Sometimes only suppressed carrier and leave the two side-band as a multi-channel communications, called suppressed carrier bilateral band, also known as synchronous communication.