What is Antenna 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. It quantitatively describes the degree to which an antenna concentrates input power to radiate. The gain is obviously closely related to the antenna pattern. The narrower the main lobe of the pattern, the smaller the side lobe, and the higher the gain. Antenna gain is used to measure the antenna's ability to send and receive signals in a specific direction. It is one of the most important parameters for selecting a base station antenna. Generally speaking, the increase of the gain mainly depends on reducing the lobe width of the radiation in the vertical direction, while maintaining the omnidirectional radiation performance in the horizontal plane. Antenna gain is extremely important for the quality of mobile communication systems because it determines the signal level at the edge of the cell. Increasing the gain can increase the coverage of the network in a certain direction, or increase the gain margin within a certain range. Any cellular system is a two-way process. Increasing the gain of the antenna can simultaneously reduce the gain budget margin of the two-way system. In addition, the parameters representing the antenna gain are dBd and dBi. dBi is the gain relative to the point source antenna, and the radiation is uniform in all directions; dBd relative to the gain of the symmetrical array antenna is dBi = dBd + 2.15. Under the same conditions, the higher the gain, the longer the distance the radio wave travels. Generally, the antenna gain of a GSM directional base station is 18dBi, and the omnidirectional is 0dBi.

Antenna gain

When a certain radiation source radiates energy into space, ideally the energy is scattered by a sphere, both research and practice have found that if this energy radiation is emitted in a certain direction, the distance the energy reaches and the The range covered in the direction will be greatly improved. The application of such research results to wireless communication is the origin of antennas. The antenna gain is used to quantitatively describe the degree to which the antenna concentrates the input power (energy) to radiate. From the perspective of communication, it is the capacity of generating signals in a certain direction and range.

In practical applications, even if a certain direction is concentrated, the antenna will still have different gains in all directions in space. The antenna gain usually refers to the gain in the direction that produces the maximum gain. Mathematically use the formula

The unit is dBi or dBd. The difference between the two is that the reference reference is different. The former reference is an omnidirectional antenna (antennas with the same radiation characteristics in all directions in space), and the latter is a dipole antenna. Is a two-way antenna). To better understand the antenna gain, use this formula as an example: To generate a signal of a certain size at a certain point in the space, an ideal radiation source can be obtained if the input power is 126 W, assuming that the gain of an antenna is 18 dBd, Then use this antenna to input the power level using the above calculation formula.

From this formula,

According to the definition of logarithm,

You can calculate the power of the antenna.

It can be seen that the antenna gain actually represents the effect of amplifying the ideal radiant energy at a certain point in space. In this example, it is equivalent to the effect of amplifying 2 W radiant energy to 126 W radiant energy. From this point From the point of view, there is an essential difference between the antenna gain and the gain of the active circuit: the gain of the active circuit is a quantitative description of how much a certain device can actually amplify the power of a certain value.

The physical meaning of gain can be understood as follows: A signal of a certain size is generated at a certain point at a certain distance. If an ideal non-directional point source is used as the transmitting antenna, 100W input power is required, and the gain is G = 13 When a directional antenna with dB = 20 is used as a transmitting antenna, the input power only needs 100/20 = 5W. In other words, the gain of an antenna, in terms of its radiating effect in the direction of maximum radiation, is a multiple that multiplies the input power compared to an ideal point source with no directivity.

The gain of the half-wave symmetric oscillator is G = 2.15dBi. The four half-wave symmetrical oscillators are arranged up and down along the vertical line to form a vertical quaternary array with a gain of approximately G = 8.15dBi (the unit of dBi indicates that the comparison object is an ideal point source with uniform radiation in all directions).

If a half-wave symmetrical oscillator is used as a comparison object, the unit of the gain is dBd.

The gain of the half-wave symmetric oscillator is G = 0dBd (because it is compared with itself, the ratio is 1, and the logarithm is zero.) The vertical quaternary array has a gain of about G = 8.15 -2.15 = 6dBd.

In order to compare the pros and cons of the antenna's ability to receive signals. The sensitivity of the non-directional half-wave dipole antenna (the direction of which is two circles) is set to 0db. In contrast, the antenna with high sensitivity and good directivity has gain.

The ideal omnidirectional antenna gain is defined as 1. In fact, the so-called ideal omnidirectional antenna does not exist in the real world, but under this ideal condition, the microwave power distribution in space can be easily calculated. The gain of the antenna can be obtained by comparing it with the measured power of the maximum radiated pointing position of an actual antenna with the same transmitting power. By the way, the previous statement that a half-wave oscillator is an omni-directional antenna does not mean that the gain is 1. It must be proper, it is omnidirectional on the H plane, but on the E plane, the main lobe half power width is 90 degrees, and the antenna gain is greater than 1.

There is a fundamental difference between the gain of an antenna and the gain of an active circuit.

Measurement of antenna gain

The test equipment is signal source, spectrum analyzer or other signal receiving equipment and point source radiator.

1. First use an ideal (of course approximately ideal) point source to radiate the antenna and add a power; then, at a certain distance from the antenna, use a spectrum analyzer or receiving device to test the received power. The measured received power is P1

2. Switch to the antenna under test, add the same power, repeat the above test at the same position, and measure the received power as P2;

3 Calculating gain: G = 10Lg (P2 / P1)

In this way, the gain of the antenna is obtained.

1) The narrower the antenna main lobe width, the higher the gain. For a general antenna, the gain can be estimated using the following formula:

G (dBi) = 10Lg {32000 / (23dB, E × 23dB, H)}

In the formula, 23dB, E and 23dB, H are the lobe widths of the antenna on the two main planes;

32000 is statistical experience data.

2) For a parabolic antenna, the gain can be calculated approximately using the following formula:

G (dBi) = 10Lg {4.5 × (D / 0) 2}

Where D is the diameter of the paraboloid;

0 is the central working wavelength;

4.5 is statistical experience data.

3) For upright omnidirectional antennas, there are approximate calculation formulas

G (dBi) = 10Lg {2L / 0}

Where L is the length of the antenna;

0 is the central working wavelength;

Antenna gain is not only one of the most important parameters of the antenna, but also very important to the operating quality of the wireless communication system. Increasing the antenna gain can increase the signal coverage in a certain direction, or the range is unchanged, but within this range The signal strength increases. For a single antenna, the simplest way to increase the antenna's gain is to further narrow the transmission direction of the antenna, which is called narrowing the lobe width. After all, this method has limited improvement in system performance in practical applications. Generally, the method of directly increasing the bandwidth and spectrum is also restricted by various conditions, and it cannot be increased indefinitely. Under the premise of constant bandwidth and frequency spectrum, in order to improve the system's user capacity, data throughput and coverage distance and range, smart antenna technology and MIMO technology came into being. Among them, the smart antenna technology uses multiple antennas to form an antenna array. By using the positional relationship between the antennas, the same data is sent to the user, which is equivalent to concentrating radiant energy in a certain direction, thereby improving the antenna gain. They all use multiple antenna systems and utilize the characteristics of electromagnetic waves such as multipath propagation to send and receive different data, while improving transmission efficiency, and achieving spatial multiplexing. From the perspective of antenna gain, it can also be considered that instead of increasing the gain of a single antenna, the number of antennas is increased to obtain the effect of the gain product of the transmitting and receiving antennas. In addition, regardless of the antenna array or MIMO technology, the diversity technology is used when transmitting signals, and this technology can reduce the chance of signal fading and reduce the fluctuation of the signal-to-noise ratio, thereby obtaining a part of additional gain, which is called diversity gain. MIMO technology has been widely used in 4G networks based on LTE technology.