What is antenna gain in simple terms?

Radio communication, cellular communication, television, wireless Wi-Fi and 3G Internet are carried out at the reception and transmission of radio waves by antennas. Radio waves are electromagnetic radiation. Any radio wave is characterized by three main parameters: wavelength, amplitude and speed of propagation. Light and radio waves propagate at an enormous speed: 300 thousand kilometers per second (that is, a ray of light reaches the Earth from the Moon in less than two seconds). The wavelength is the distance between any two of its crests. Amplitude is the “height” that this crest rises to. The greater the amplitude of the light wave, the brighter the light (in other words, the higher the intensity of the wave). Knowing the wavelength and the speed of its propagation, one can always calculate the frequency of the wave (this is the number of crests formed in one second). Frequency is measured in Hertz.

The radio wave has one more characteristic: polarization, but we will tell about it later.

Anyone has seen radio waves, and more than once. After all, light is also radio emission, but with a very short wavelength (respectively, a very high frequency), a thousand times smaller than a millimeter. To understand how radio waves propagate, it suffices to draw an analogy with the propagation of light.

- light propagates in a straight line;

- if a large obstacle is placed in the path of the light beam, then a shadow is formed;

- if you put obstacles in the path of a beam of light that are less than the wavelength or comparable with it, then the light, having undergone some changes, will pass on;

-glass weakens the brightness of the light, sometimes very much;

- If you put a magnifying glass on the way of sunlight, then in its focus you will get a bright dazzling point that can ignite a tree.

Radio waves have a longer wavelength than light, but this does not change the laws of their propagation. The technique uses radio waves of different frequencies (wavelengths):

TV: 50-600 MHz (6-0,5 m)

- GSM900 mobile communication: 900 MHz (33 cm);

- GSM1800 mobile communication: 1800 MHz (17 cm);

- 3G Internet: 2000 MHz (15 cm);

- Wi-Fi: 2450 MHz (12 cm) and 5750 MHz (5 cm).

Radio waves propagate in a straight line, as well as light.

If the path of the radio waves presented in the table, put an obstacle the size of the order of one meter, the wave does not weaken. Here you can draw an analogy with the waves at sea: a big wave will not weaken due to a person in the water, and a large ship will not let the waves pass.

If there is a big obstacle on the way of the radio waves, for example, a multi-storey building, then it will significantly reduce the signal, up to its complete weakening.

Window glass also attenuates radio waves.

A satellite dish acts like a magnifying glass: it collects a signal from a large area and concentrates at one point. Conversely, the signal comes from a single point, and the plate collects it and converts it into a narrow directional beam.

A radio wave, entering the antenna, causes electromagnetic oscillations in it, and an electric current begins to flow through the conductive parts of the antenna. This current is lowered by cable into the receiving device, where information is extracted from it (sound, images, data, ...). And vice versa, if an electric current of a certain frequency is applied to the antenna, the antenna will emit radio waves of the same frequency into space.

Any antenna will work equally well both at reception and at signal transmission within its operating frequency range. Therefore, for the sake of simplicity, in the future we will only speak about reception or only about transmission.

Antenna gain characterizes the ability of an antenna to concentrate a signal in any particular direction. Let us give an analogy: imagine that in a dark room you have a weak 1 W light bulb. You can only see the contours of the objects in this room, and the far corners will remain dark. Now you have a small mirror in your hands. It reflects part of the light from the light bulb, and one half of the room is twice as bright, but the other half is hidden in the shadow of the mirror. In the third case, we place this light bulb in the reflector from the flashlight: you will get a spot of bright light the size of a palm. With the help of this lamp you can light the farthest corner of the room. But nothing but this spot of light you will not see. Thus, in all cases, the light bulb remained the same. We used different reflectors, changing the concentration of the light beam in a certain direction.

Absolutely the same thing happens with antennas. In fact, antennas do not amplify, but concentrate the signal in one or several directions, and the term “gain” should not mislead you.

Gain is measured in decibels (dB). This is a logarithmic quantity and was introduced to simplify mathematical calculations. The gain compares the power of an isotropic radiator (a single light bulb without mirrors in the example) and the power of this antenna. To convert the power ratio to decibels, you need to use the following table.

For example, if one antenna has gain = 10 dB, the second has gain = 13 dB, then the second antenna is two times more powerful than the first antenna.

Of the two antennas with the same gain and similar design, the antenna will be smaller, which is designed to receive shorter wavelengths. For example, WiFi antenna with a gain of 20 dB at a frequency of 5500 MHz has a size of 18x18 cm, and the antenna gain is also 20 dB, but at a frequency of 1800 MHz, has dimensions of 60x60 cm.

Polarization of a radio wave is the phenomenon of directional oscillation of the electric and magnetic field strength vectors. Polarization can be linear (in the direction perpendicular to the direction of wave propagation), circular (right or left, depending on the direction of rotation of the induction vector) or elliptical (intermediate case between circular and linear polarizations). In ground communications, only linear polarization is mainly used.

Polarization of the wave corresponds to a rough analogy with the waves running along the rope. If you firmly fix one end of the rope and start to move the other end in a vertical direction, then vertical waves will run along the rope - they are said to be vertically polarized. If the end of the rope is moved in the horizontal direction, then horizontally polarized waves will run along the rope.

Any antenna with linear polarization from the entire spectrum of the waves incident on it will receive only those waves whose polarization coincides with the polarization of the antenna. The direction of its polarization is applied to each antenna at the factory (usually it is an arrow). When installing the antenna can be installed so that the arrow was located either vertically or horizontally. Accordingly, the antenna will receive either vertically or horizontally polarized waves. Therefore, the choice of polarization is a very important point when building a wireless network. A misplaced antenna will receive a signal, but with very significant attenuation, sometimes unacceptable for good communication.