Characteristics of Antenna

by sogrow team

Characteristics of Antenna

One of the most important properties of antennas is gain. The name of this parameter often leads to the wrong assumption that antennas can amplify the signal. This is not an isotropic radiator, or an isotropic antenna is introduced.

An anisotropic radiator is an ideal point source for electromagnetic waves radiating uniformly in all directions. If you imagine a sphere with a centre that coincides with an isotropic emitter, then the energy density emitted by an isotropic source will be the same at any point on that sphere. Therefore, an isotropic emitter is said to form a spherical field with uniform energy density. There are no isotropic emitters in nature. Each transmitting antenna, even the simplest, radiates energy unevenly in one direction; its radiation is maximum. The anisotropic radiator is used exclusively as a specific reference radiator to which all other antennas can be comfortably compared.

Radiation characteristics of the antenna

The directional characteristics of antennas are normally determined by the dependence of the intensity of the field radiated by the antenna with the direction. The graphical representation of this relationship is called an antenna diagram. The three-dimensional radiation pattern is represented as an area that is described by a radio vector originating from the origin, whose length in one direction or the other is proportional to the energy radiated by the antenna in this direction. In addition to three-dimensional diagrams, two-dimensional diagrams that are created for the horizontal and vertical planes are often considered as well.

In this case, the radiation pattern has the form of a closed line in the polar coordinate system, which is constructed in such a way that the distance from the antenna (centre of the pattern) to any point in the radiation pattern is directly proportional to the energy radiated by the antenna in that direction.

For an isotropic antenna that radiates energy equally in all directions, the radiation pattern is a sphere, the centre of which coincides with the position of the isotropic radiator, and the horizontal and vertical radiation patterns of the isotropic radiator are circular.

In the case of directional antennas, the so-called lobes, that is, the direction of the predominant radiation, can be distinguished from the radiation pattern. The direction of the maximum radiation from the antennas is called the main direction; the petal that corresponds to it is the most important; the rest of the lobes are sided lobes, and the lobe in the direction opposite to the main direction is called the rear lobe of the antenna pattern. The directions in which the antenna neither receives nor radiates are called the zero points of the pattern.

As a general rule, the radiation pattern is also characterized by its width, which is understood as the angle at which the gain does not decrease more than 3 dB from the maximum. Almost always, the gain diagram and the connected width: the greater the gain, that is, in the same diagram and vice versa.

Antenna gain

Now that we have an idea of ​​such important concepts as an ideal isotropic point radiator and antenna radiation pattern, we can formulate the concept of antenna gain.

Antenna gain determines how many decibels the power flux density emitted by the antenna in a given direction is greater than the power flux density that would be recorded if an isotropic antenna were used. The antenna gain is measured in so-called isotropic decibels (dB or dBi).

Remember that power in physics is generally measured in watts (W). However, in communication theory, decibels (dB) are most commonly used to measure signal strength. This unit of measure is logarithmic and can only be used to compare physical quantities with the same name. For example, if two values ​​A and B, of the same physical quantity are compared, the A / B ratio shows how often one quantity is greater than the other. If we look at the decimal logarithm of the same ratio, we obtain a comparison of these values, expressed in Bels (B), and the expression 10lg (A / B) determines the comparison of these values ​​in decibels (dB). For example, if they say that one value is 20 dB greater than another, it means that it is 100 times greater than the other.

Decibels are not only used to compare values but also to express absolute values. To do this, a reference value is used as the comparison value. For example, to express the absolute value of the signal power in decibels, a power of 1 mW is taken as a reference, and the power level in decibels is compared with a power of 1 mW. This unit of measurement is called decibels per milliwatt (dBm) and indicates by how many decibels the power of the measured signal is greater than the power of 1 mW.

It is easy to calculate that a power of 100 mW corresponds to a power of 20 dBm and a power of 50 mW corresponds to a power of 17 dBm.

So, if the antenna gain in a certain direction is 5 dBi, it means that the radiated power in that direction is 5 dB (3.16 times) greater than the radiated power of an ideal isotropic antenna. Of course, increasing the signal strength in one direction will decrease the strength in other directions.

If the antenna gain is 10 dBi, this naturally means the direction in which the maximum radiated power is reached (the main lobe of the radiation pattern).

Knowing the antenna gain and transmit power, it is easy to calculate the signal power in the direction of the main lobe of the radiation pattern. When using a wireless access point with a transmit power of 20 dBm (100 mW) and a directional antenna with a gain of 10 dBi, the signal power in the direction of maximum gain is 20 dBm + 10 dBi = 30 dBm (1000 mW), that is, 10 times more than with an isotropic antenna.

Types of antennas for WLAN devices

All antennas for WLAN devices can be roughly divided into two broad classes concerning their use: antennas for outdoor use and indoor use. These antennas differ mainly by their dimensions and their gain. The antennas for outdoor use are, of course, larger and can be placed on the wall of the house or a vertical pole. High gain with such antennas is achieved by the small width of the radiation pattern (main lobe). External antennas are generally used to connect two radio networks that are widely separated from each other. Two of these antennas are installed in the line-of-sight area, and in this case, each must be in the main lobe of the other antenna. Indoor antennas are smaller and have lower gain. These antennas can be mounted on the table, on the wall or directly on the access point. The antennas can be connected to the access point itself directly or with a cable. At the same time, a special miniature SMA plug is used to connect an antenna or cable to the access point. Access points use a male connector, and the antenna itself or the antenna cable uses a female connector.

Other types of RF connectors can also be used to connect an outdoor antenna to a cable, most often an N connector.

Whip antenna

All 802.11b / g access points are equipped with standard miniature whip antennas, which can be detachable or stationary. A whip antenna is the simplest antenna option. It is also known as an asymmetric vibrator.

If the whip antenna is placed vertically, it radiates energy uniformly in all directions in the horizontal plane; therefore, such an antenna is omnidirectional in the horizontal plane, and, of course, it is not necessary to speak of preferential radiation in a given direction. At the same time, such an antenna radiates unevenly in the vertical plane. In particular, there is no radiation along the axis of the antenna. Therefore, even with the simplest whip antenna, the directions corresponding to the maximum gain can be distinguished. With whip antennas, the maximum gain is achieved in a plane that runs perpendicular to the antenna and passes through its centre.

If you disassemble the standard whip antenna, in most cases, it turns out that the length of its active part is only 31 mm. Of course, this length was not chosen by chance. The fact is that the frequency range for WLAN devices is 2400 to 2473 MHz. Therefore, the wavelength of radiation varies from 12.12 to 12.49 cm, and the quarter wavelength is about 31 mm. That is, in most cases, the length of the whip antenna is chosen to be equal to a quarter of the wavelength of the radiation.

Note that due to the isotropic nature of whip antenna radiation in the horizontal plane, it is optimal to install an access point with the whip antenna in the centre of an office or apartment to cover a maximum of the entire area of ​​an apartment or office with a wireless network.

Vertical Reflector Whip Antenna

The design of a whip antenna can be slightly improved by using a reflector perpendicular to the antenna with a metal surface (screen) that serves as an ideal grounding surface. Such antennas are not manufactured by industry (they are not commercially available in any case), but such an antenna is easy to make yourself.

For an antenna length of 1/4 l, in the case of an ideal infinite reflector, the maximum gain is 5.18 dBi, while for the same antenna without a reflector, the maximum gain is only 1.73 dBi.

As with a conventional whip antenna, it is recommended to install a whip antenna with a vertical reflector in the middle of a room (apartment or office).

Parallel reflector whip antenna

Another way to modify a whip antenna is to use a reflector parallel to the antenna rather than perpendicular to the antenna. In this case, its radiation pattern changes significantly and, in the horizontal plane, said antenna is no longer isotropic.

The type of radiation pattern in the horizontal plane (in the plane perpendicular to the antenna) depends on both the size of the antenna and the distance between the antenna and the reflector.

With a 1/4 L antenna length, the maximum gain with an ideal infinity reflector at a 1/4 L distance from the antenna is 7.17 dBi. It is advisable to place such an antenna near a wall.