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When I was a child, the radios and televisions at home all had poles that could flexibly rotate and stretch. Everyone must still remember this pole that could be turned around. Perhaps we are curious to find that there is a mysterious connection between the length and direction of this pole and the reception effect of radio and television.

This rod is actually a telescopic antenna (also known as a whip antenna or pull rod antenna), which belongs to a kind of antenna. Antenna is a kind of common equipment, which is widely used in radio, television, radio communication, radar, navigation, electronic countermeasures, remote sensing, radio astronomy and other fields. All equipment that transmits signals through electromagnetic waves must carry antennas.

Our mobile phones also need an antenna in order to receive and make phone calls. So some friends wondered why my phone didn't have a pole with it. In fact, most of the original old-fashioned mobile phones had an antenna. Later, with the evolution of technology, the antenna was hidden in the mobile phone.

The "stick" we often see on the car tail is the car antenna.

We often see the following equipment on the roof or iron tower, which is the antenna in radio communication.

Some antennas in front are a pole, some antennas are a board, in fact, there are antennas in the shape of the following.

We can't help but wonder if they are really antennas.

What is ◆◆ antenna? ◆◆ antenna is a device that can effectively radiate electromagnetic waves to a specific direction in space or can effectively receive electromagnetic waves from a specific direction in space.

We use the analogy of communication between people. Antennas are our ears and mouths. Through our mouths, we convert sound into sound waves, which travel through the air and are finally heard by our ears. In the communication system, the antenna plays the role of mouth and ear, the difference is that the antenna can not only send electromagnetic waves but also receive electromagnetic waves.

The principle of ◆◆ antennas ◆◆ antennas transform guided waves propagating on a transmission line into electromagnetic waves propagating in free space, or vice versa. Guided waves are electromagnetic waves in which all or most of the electromagnetic energy is constrained to travel in a certain direction in a finite cross section.

We use train travel as an analogy, passengers are like electromagnetic waves, and transmission lines are like trains.

After getting on the bus, the passengers can only move in the car, and the passengers move with the direction of the train, which is like a guided wave, which is constrained in a finite cross section to transmit in a certain direction.

After leaving the station, passengers can move freely, which is like electromagnetic waves propagating in free space. Here the train door is similar to the antenna.

The train door can be used not only for passengers to get on the bus, but also for passengers to get off.

Similarly, antennas can be used not only to convert guided traveling waves into free space electromagnetic waves, but also to convert free space electromagnetic waves into guided traveling waves, which is the reciprocity principle of antennas.

How does an antenna convert guided waves into electromagnetic waves in free space?

In 1894, the scientist Popov found in an experiment that the distance of radio waves detected by the receiver was significantly longer than usual. After some inquiry, Popov found that a wire touched the metal chip geophone. It is this wire that greatly increases the distance of the experiment. This wire is considered to be the world's first antenna.

In Popov's experiment, the wire accidentally met the metal chip geophone, which virtually changed the shape of the transmission line.

According to the idea of Popov's experiment, scientists found that with the increase of the angle of the transmission line, the radiated electromagnetic wave became stronger. Later, the theory of symmetrical dipole antenna was proposed, and then a variety of antennas were developed.

It can be seen that the size of the antenna matches the wavelength.

There is a close relationship between wavelength and frequency, speed of light = frequency × wavelength (

From 1G to 5G, the frequency is getting higher and higher, and the smaller the wavelength, the smaller the size of the corresponding antenna, which is why the antenna of the phone can be hidden inside the phone.

As the antenna theory is more complex, I will not elaborate on it here. If you are interested, you can learn it by yourself.

Classification of ◆◆ antennas ◆◆ in order to meet a variety of practical needs, engineers have invented a variety of antennas. The devices mentioned above are really antennas. A general classification diagram of antennas is given below.

Before the ◆◆ antenna index ◆◆, friends found that the length and direction of the antenna had something to do with the reception effect of radio and television. In fact, we intentionally or unintentionally changed the parameters of the radio or television antenna by rotating and stretching it, thus affecting the reception of electromagnetic waves. The transmitting and receiving effect of the antenna is closely related to the parameters of the antenna. Here we introduce some basic parameters of the antenna.

1. The working band antenna always works in a certain frequency range (band width), which depends on the requirements of the index. The frequency range that meets the requirements of the index is the working frequency of the antenna. Various wireless systems are different, and operators use different frequency bands, so it is necessary to choose antennas with appropriate frequency bands.

two。 The polarization of the polarization mode antenna refers to the direction of the electric field intensity formed by the antenna radiation. When the direction of electric field intensity is perpendicular to the ground, this wave is called vertical polarization wave; when the direction of electric field intensity is parallel to the ground, this wave is called horizontal polarization wave.

A dual-polarized antenna is composed of two antennas polarized orthogonal to each other and encapsulated in the same radome. Due to the performance reasons, the two antennas adopt the polarization mode of ±45 degrees.

3. Impedance for wire antennas, the ratio of voltage to current at the input of the antenna is called the input impedance of the antenna.

For the surface antenna, the voltage standing wave ratio on the feeder is often used to represent the impedance characteristics of the antenna.

The appropriate feeder and impedance matcher are selected to ensure that the input impedance of the antenna matches the characteristic impedance of the feeder, so that the input antenna or the output power from the antenna is maximum.

4. The directivity of the antenna the directivity of the antenna refers to the ability of the antenna to radiate electromagnetic waves in a certain direction. For receiving antennas, directivity represents the receiving ability of antennas to radio waves coming from different directions. The characteristic curve of the directivity of the antenna is usually represented by the pattern.

The pattern can be used to illustrate the ability of the antenna to transmit or receive electromagnetic waves in all directions of space.

There are usually two or more valves in the picture, of which the largest valve is called the main valve and the rest is called the side valve. )

5. Lobe width (also known as beam width or main lobe width or half power angle) lobe width refers to the angle between two points where the radiation intensity is reduced by 3 dB (power density is reduced by half) on both sides of the maximum radiation direction of the main lobe. The narrower the lobe width is, the better the directivity is, and the farther the operating distance is, the stronger the anti-jamming ability is.

6. The front-to-back ratio refers to the ratio of the maximum value of the main lobe to the maximum value of the dorsal lobe. It shows whether the antenna suppresses the back lobe well or not.

7. Gain antenna gain refers to the ratio of the power density of the signal generated by the actual antenna and the ideal radiation unit at the same point in space under the condition that the input power is equal. The gain is closely related to the antenna pattern. The narrower the main lobe of the pattern, the smaller the sidelobe, the higher the gain. Antenna gain is used to measure the ability of an antenna to radiate electromagnetic waves in a particular direction. It should be noted that the antenna itself does not increase the energy of the radiated signal, it only concentrates the energy to a certain direction through the combination of antenna vibrators and changing its feeding mode.

8. The inclination angle of the antenna refers to the inclination of the radio wave, not the mechanical inclination of the antenna oscillator itself. The inclination angle reflects which altitude angle the antenna receives the strongest radio waves.

9. The isolation of the isolation antenna refers to the irrelevance of two antennas or one dual-polarized antenna. The isolation parameters are qualified to ensure the diversity reception performance of the same sector antenna.

10. The standing wave ratio of the antenna is an index indicating the matching degree between the antenna feeder and the base station. It is generated by the superposition of reflected waves after the incident wave energy is transmitted to the input of the antenna and not all of it is radiated out.

In addition, the antenna index also needs to pay attention to the downward tilt mode of the antenna, as well as the number of ports, connectors, feeders and so on.

This article comes from the official account of Wechat: ZTE documents (ID:ztedoc)

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