Comic: The Principle of Antenna

eric_wang

Comic: The Principle of Antenna [Copy link]

When I was a child, the radios and televisions at home all had poles that could be flexibly rotated and stretched. Everyone must still remember this pole that could rotate back and forth, and perhaps they were also curious to find that there was some mysterious connection between the length and direction of this pole and the reception effect of the radio and television.

This rod is actually a telescopic antenna (also called a whip antenna or a rod antenna), which is a type of antenna. Antennas are a relatively common device that is widely used in broadcasting, television, radio communications, radar, navigation, electronic countermeasures, remote sensing, radio astronomy and other fields. All devices that transmit signals through electromagnetic waves must have antennas.

Our mobile phones also need antennas to make calls. So some friends are curious, why is my mobile phone not attached to a pole? In fact, most of the original old-fashioned mobile phones had an antenna on top. Later, with the evolution of technology, the antenna was hidden inside the mobile phone.

The "little stick" we often see on the tail of a car is the car antenna!

We often see devices like the following on building roofs or towers. These are antennas used in radio communications.

Some of the antennas in front are a pole, some are a board, and there are also antennas in the shape shown below.

We can't help but wonder, are they really all antennas???

◆◆What is an antenna◆◆

An antenna is a device that can effectively radiate electromagnetic waves in a specific direction in space or effectively receive electromagnetic waves from a specific direction in space.

We use the communication between people as an analogy. The antenna is our ears and mouth. We convert sound into sound waves through our mouth and send them out. The sound waves propagate in the air and are finally heard by our ears. In the communication system, the antenna plays the role of the mouth and the ear. The difference is that the antenna can both send and receive electromagnetic waves.

◆◆Principle of the antenna◆◆

The antenna transforms the guided wave propagating on the transmission line into an electromagnetic wave propagating in free space, or vice versa. A guided wave is an electromagnetic wave in which all or most of the electromagnetic energy is confined within a limited cross section and transmitted in a certain direction.

We use the analogy of train travel, where the passengers are like electromagnetic waves and the transmission line is like the train.

After getting on the train, passengers can only move inside the train. Passengers move in the direction of the train's movement, which is like a guided wave, constrained to transmit in a certain direction within a finite cross-section.

After exiting the station, passengers can move freely, which is like the propagation of electromagnetic waves in free space. Here, the train door is like an antenna.

Train doors can be used for both boarding and disembarking of passengers.

Similarly, antennas can be used to convert guided waves into free-space electromagnetic waves, and can also convert free-space electromagnetic waves into guided waves. This is the reciprocity principle of antennas.

How do antennas convert guided waves into electromagnetic waves in free space?

In 1894, scientist Popov discovered in an experiment that the distance at which the receiver could detect radio waves was significantly longer than usual. After some investigation, Popov found that a wire touched the metal shavings detector. It was this wire that greatly increased the experimental distance. This wire is considered the world's first antenna.

In Popov's experiment, the wire accidentally hit the metal chip detector, which invisibly changed the shape of the transmission line.

Continuing research based on Popov's ideas from this experiment, scientists discovered that as the angle of the transmission line increases, the radiated electromagnetic waves become stronger. Later, the symmetrical oscillator antenna theory was proposed, and various 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. The speed of light = frequency × wavelength (). From 1G to 5G, the frequencies used are getting higher and higher. The smaller the wavelength, the smaller the corresponding antenna size. This is why the antenna of a mobile phone can be hidden inside the phone.

Since antenna theory is relatively complicated, I will not explain it in detail here. If you are interested, you can learn it on your own.

◆◆Classification of antennas◆◆

In order to meet various practical needs, engineers have invented various antennas. The various devices mentioned above are really antennas. A general classification diagram of antennas is given below.

◆◆Antenna indicators◆◆

Previously, friends have discovered that there is a certain relationship between the length direction of the antenna and the reception effect of the radio and television. In fact, we intentionally or unintentionally rotate and stretch the radio or TV antenna, which changes the antenna parameters and affects the reception of electromagnetic waves. The quality of the antenna's transmission and reception effect is closely related to the antenna parameters. Below we introduce some basic parameters of the antenna.

1. Operating frequency band

Antennas always work within a certain frequency range (bandwidth), which depends on the requirements of the index. The frequency range that meets the index requirements is the working frequency of the antenna. Different wireless standards have different frequency bands used by operators, so you need to choose an antenna with a suitable frequency band.

2. Polarization

The polarization of an antenna refers to the direction of the electric field strength formed when the antenna radiates. When the electric field strength direction is perpendicular to the ground, the wave is called a vertically polarized wave; when the electric field strength direction is parallel to the ground, the wave is called a horizontally polarized wave.

The dual-polarized antenna is composed of two antennas with orthogonal polarizations enclosed in the same radome. For performance reasons, the two antennas use ±45 degrees polarization.

3. Impedance

For a wire antenna, the ratio of the voltage to the current at the antenna input is called the antenna's input impedance.

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

Select a suitable feeder and impedance matcher to ensure that the input impedance of the antenna matches the characteristic impedance of the feeder to maximize the power input to or output from the antenna.

4. Antenna Directivity

The directivity of an antenna refers to its ability to radiate electromagnetic waves in a certain direction. For a receiving antenna, directivity refers to the antenna's ability to receive radio waves from different directions. The characteristic curve of an antenna's directivity is usually represented by a radiation pattern.

(The directional pattern can be used to illustrate the antenna's ability to transmit or receive electromagnetic waves in all directions in space. There are usually two or more lobes in the diagram, the largest of which is called the main lobe, and the remaining lobes are called side lobes.)

5. Beam width (also known as beam width or main lobe width or half-power angle)

The lobe width refers to the angle between two points on both sides of the maximum radiation direction of the main lobe where the radiation intensity decreases by 3 dB (the power density is reduced by half). The narrower the lobe width, the better the directivity, the longer the effective distance, and the stronger the anti-interference ability.

6. Front-to-back comparison

The front-to-back ratio refers to the ratio of the maximum value of the main lobe to the maximum value of the back lobe, which indicates how well the antenna suppresses the back lobe.

7. Gain

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

8. Inclination

The tilt angle of the antenna refers to the tilt angle of the radio wave, not the mechanical tilt angle of the antenna vibrator itself. The tilt angle reflects the altitude angle at which the antenna receives the strongest radio wave.

9. Isolation

Antenna isolation refers to the uncorrelation between two antennas or a dual-polarized antenna. A qualified isolation parameter ensures the performance of antenna diversity reception in the same sector.

10. Standing Wave Ratio

Antenna standing wave ratio is an indicator of the matching degree between the antenna feed line and the base station. It is caused by the fact that the incident wave energy is not completely radiated after being transmitted to the antenna input end, resulting in reflected waves and superposition.

In addition, antenna indicators also need to pay attention to the antenna's downtilt method, as well as the number of ports, connectors, feeders, etc.

What shapes of antennas have you seen?

Source: ZTE Documents

火辣西米秀

The earliest knowledge of antennas was the beginning of radio

btty038

This is good. The first one I played was made by myself with iron wire. The instructions are exposed.