Microwave Tech - Basic knowledge of wifi antenna

1. Antenna

1.1 Function and Position of Antenna

The radio frequency signal power output by the radio transmitter is transmitted to the antenna through a feed line (cable), which is radiated by the antenna in an electromagnetic waveform. When the electromagnetic wave reaches the receiving location, it is followed by the antenna (which receives only a small fraction of the power) and fed to the radio receiver. Obviously, antenna is an important radio device for transmitting and receiving electromagnetic waves. Without antenna, there would be no radio communication. There are many kinds of antennas for different frequencies, different uses, different occasions, different requirements and so on. For many kinds of antennas, it is necessary to classify them appropriately: they can be classified into communication antenna, TV antenna, radar antenna, and so on. It can be divided into the short-wave antenna, ultra-short-wave antenna, microwave antenna and so on. It can be divided into omnidirectional antenna, directional antenna, etc. It can be divided into the linear antenna, planar antenna, and so on. And so on.

*Radiation from electromagnetic waves

When there is alternating current flow on the conductor, the radiation of electromagnetic waves can occur. The ability of radiation depends on the length and shape of the conductor. As shown in Fig. 1.1a, if the two wires are very close together, the electric field is bound between them and the radiation is weak. By opening the two wires, as shown in Figure 1.1b, the electric field spreads around the surrounding space, thereby increasing radiation. It must be noted that when the length L of the traverse is much smaller than the wavelength λ When the radiation is very weak; When the length of the wire L increases to a length comparable to that of the wavelength, the current on the wire increases considerably, thus creating strong radiation.

1.2 Symmetric Oscillator

A symmetric oscillator is a classical antenna that has been used most extensively so far. A single half-wave symmetric oscillator can be simply used alone or as a feed for a parabolic antenna, or an antenna array can be composed of multiple half-wave symmetric oscillators. An oscillator with equal arms length is called a symmetric oscillator. An oscillator with a length of one-fourth and a full length of one-half of the wavelength per arm is called a half-wave symmetric oscillator. See Fig. 1.2A. In addition, there is a special type of half-wave symmetric oscillator, which can be seen as folding a full-wave symmetric oscillator into a narrow rectangular frame and overlapping the two ends of the full-wave symmetric oscillator. This narrow rectangular frame is called a folded oscillator. Note that the length of the folded oscillator is also half of the wavelength, so it is called a half-wave folded oscillator. See Figure 1.2b.

1.3 discussion on antenna directivity

1.3.1 antenna directivity

One of the basic functions of the transmitting antenna is to radiate the energy obtained from the feeder to the surrounding space. The other is to radiate most of the energy in the required direction. The vertically placed half wave symmetric oscillator has a flat "doughnut" shaped three-dimensional pattern (Fig. 1.3.1 a). Although the three-dimensional pattern has strong three-dimensional sense, it is difficult to draw. Figure 1.3.1 B and figure 1.3.1 C show its two main plane patterns. The plane pattern describes the directivity of the antenna on a specified plane. As can be seen from Fig. 1.3.1 B, the radiation is zero in the axis direction of the vibrator, and the maximum radiation direction is on the horizontal plane; As can be seen from figure 1.3.1 C, the radiation in all directions on the horizontal plane is the same.

1.3.2 Antenna Directional Enhancement

Several symmetrical oscillator arrays can control radiation to produce "flat bread rings" that further concentrate the signal in the horizontal direction.

The following are the stereo and vertical plane patterns of four half-wave oscillators arranged in a vertical quaternion array up and down the vertical line.

The reflector can also be used to control the radiation energy to a single direction, and the planar reflector can be placed on one side of the array to form a sector covering antenna. The following horizontal pattern illustrates the role of the reflector, which reflects power to one side and improves gain.

The use of a parabolic reflector enables antenna radiation to be concentrated in a small stereo angle, like a searchlight in optics, resulting in a high gain. It is self-evident that a parabolic antenna consists of two basic elements: a parabolic reflector and a radiation source placed in the focus of the paraboloid.

1.3.3 Gain of antenna

Gain is the ratio of the power density of the signal generated by an actual antenna to an ideal radiation unit at the same point in space, provided that the input power is equal. It quantitatively describes the degree to which an antenna concentrates input power. Obviously, the gain is closely related to the antenna pattern. The narrower the main lobe, the smaller the secondary lobe and the higher the gain. The physical meaning of gain can be understood in this way--to produce a signal of a certain size at a certain distance, if an ideal undirected point source is used as the transmitting antenna, it requires 100W input power, whereas when a directional antenna with gain G = 13 dB = 20 is used as the transmitting antenna, only 100/20 = 5W is needed. In other words, the gain of an antenna, in terms of the radiation effect in its maximum radiation direction, multiplies the input power compared to an ideal point source without direction.

The gain of the half-wave symmetric oscillator is G=2.15dBi.

Four half-wave symmetric oscillators are arranged up and down along the vertical line to form a vertical quaternion array with a gain of about G=8.15dBi (dBi is the unit that indicates that the comparison object is an ideal point source for uniform radiation in all directions).

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

The gain of a half-wave symmetric oscillator is G=0dBd (because it is a ratio of 1 to itself and a logarithm of zero). Vertical quaternion array with gain of about G=8.15-2.15=6dBd.

1.3.4 Lobe Width

Directional patterns usually have two or more valves, of which the one with the highest radiation intensity is called the main valve and the other is called the side or side valve. See Figure 1.3.4a, where the angle between two points with a 3 dB reduction in radiation intensity (half of the power density) on either side of the maximum radiation direction of the main valve is defined as the width of the lobe (also known as the beam width or the width of the main lobe or the half-power angle). The narrower the lobe width, the better the directionality and the farther the action distance are, the stronger the anti-jamming ability is.

There is also a lobe width, the 10dB lobe width, which, as the name implies, is the angle between two points in the pattern where the radiation intensity decreases by 10dB (the power density decreases by one tenth), as shown in Figure 1.3.4b.

1.3.5 Front to Back Ratio

In the pattern, the ratio of the maximum anterior to posterior valves is called the anterior to posterior ratio and is recorded as F/B. The larger the front-to-back ratio, the smaller the backward radiation (or reception) of the antenna. The calculation of front and back is simpler than that of F/B-----

F / B = 10 Lg {(forward power density)/(backward power density)}

Typical values for antennas with antenna front-to-back ratios of F/B are (18 ~30) dB, and (35 ~ 40) dB in special cases.

1.3.6 Several Approximate Calculations of Antenna Gain

1) The narrower the main lobe width, the higher the gain. For general antennas, the gain can be estimated as follows:

G(dBi) = 10 Lg {32000 /(2) θ 3dB, E * 2 θ 3dB, H)}

Formula, 2 θ 3dB, E and 2 θ 3dB, H are the lobe widths of the antenna on the two main planes, respectively.

32000 is statistical empirical data.

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

G(dB i) =10 Lg {4.5 * (D/ λ 0)2}

In formula D is the parabolic diameter;

λ 0 is the center wavelength;

4.5 is statistical empirical data.

3) For vertical omnidirectional antennas, there is an approximate formula

G(dBi) = 10 Lg {2 L / λ 0}

L is the length of the antenna.

λ 0 is the center wavelength;

1.3.7 Superior Side Valve Inhibition

For a base station antenna, it is often required that the first side lobe above the main lobe be be be as weak as possible in its vertical (i.e., pitch) pattern. This is called superior sidelobe suppression. The base station serves mobile phone users on the ground, and radiation pointing to the sky is meaningless.

1.3.8 Downward tilt of the antenna

In order for the main lobe to point to the ground, the antenna needs to be properly tilted down during placement.

1.4 Polarization of antenna

The antenna radiates electromagnetic waves into the surrounding space. Electromagnetic waves consist of an electric field and a magnetic field. It is stipulated that the direction of the electric field is the direction of the antenna polarization. The antenna used in general is unipolar. The following illustration illustrates two basic unipolarizations: vertical polarization, which is the most common one; Horizontal polarization - also to be used.

The following illustration illustrates two other unipolarizations: +45 and -45, which are used only for special occasions. In this way, there are four unipolarizations, as shown in the following figure. A new antenna, bipolar antenna, is formed by combining vertically polarized antenna with horizontally polarized antenna, or by combining +45 degree polarized antenna with -45 degree polarized antenna.

The following illustration shows two single polarized antennas mounted together to form a pair of dual polarized antennas. Note that the dual polarized antenna has two connectors.

A bipolar antenna radiates (or receives) waves with two polarizations that are orthogonal (vertical) to each other in space.

1.4.2 Polarization Loss

Vertical polarization wave is received by an antenna with vertical polarization characteristics, and horizontal polarization wave is received by an antenna with horizontal polarization characteristics. Right-handed circular polarized wave is received by an antenna with right-handed circular polarization, while left-handed circular polarized wave is received by an antenna with left-handed circular polarization.

When the polarization direction of the incoming wave is inconsistent with that of the receiving antenna, the received signal will be smaller, that is, the polarization loss will occur. For example, when a vertically or horizontally polarized wave is received with a +45 degree polarized antenna, or when a +45 degree or -45 degree polarized wave is received with a vertically polarized antenna, the polarization loss will occur. When a circularly polarized antenna receives any linear polarized wave, or a linear polarized antenna receives any circularly polarized wave, and so on, the polarization loss must also occur - only half the energy of the received wave can be received.

When the polarization direction of the receiving antenna is completely orthogonal to the polarization direction of the incoming wave, for example, when the receiving antenna with horizontal polarization receives a vertically polarized incoming wave, or when the receiving antenna with right circular polarization receives an incoming wave with left circular polarization, the antenna will not receive the energy of the incoming wave at all. In this case, the polarization loss is maximum, which is called polarization complete isolation.

1.4.3 Polarization Isolation

There is no ideal complete polarization isolation. The signals fed to one polarized antenna always appear a little bit in another polarized antenna. For example, in the bipolar antenna shown in the figure below, the input vertical polarized antenna has a power of 10W, and the output power measured at the output of the horizontal polarized antenna is 10mW.

1.5 input impedance Zin of antenna

Definition: the ratio of signal voltage and signal current at the antenna input is called the input impedance of the antenna. The input impedance has a resistance component RIN and a reactance component Xin, that is, Zin = Rin + J Xin. The existence of reactance component will reduce the extraction of signal power from the feeder by the antenna. Therefore, the reactance component must be zero as far as possible, that is, the input impedance of the antenna should be pure resistance as far as possible. In fact, even if the antenna is well designed and debugged, its input impedance always contains a small reactance component.

The input impedance is related to the structure, size and working wavelength of the antenna. The half wave symmetrical oscillator is the most important basic antenna, and its input impedance is Zin = 73.1 + j42.5 (Ω). When the length of the antenna is shortened by (3 ~ 5)%, the reactance component can be eliminated and the input impedance of the antenna is pure resistance. At this time, the input impedance is Zin = 73.1 (Ω), (nominal 75 Ω). Note that strictly speaking, the purely resistive antenna input impedance is only for the point frequency.

Incidentally, the input impedance of the half wave reduced oscillator is four times that of the half wave symmetrical oscillator, that is, Zin = 280 (Ω), (nominal 300 Ω).

Interestingly, for any antenna, people can always adjust the antenna impedance to make the imaginary part of the input impedance very small and the real part quite close to 50 Ω within the required operating frequency range, so that the input impedance of the antenna is Zin = Rin = 50 Ω - which is necessary for the antenna to be in good impedance matching with the feeder.

1.6 operating frequency range of antenna (bandwidth)

Whether transmitting antenna or receiving antenna, they always work within a certain frequency range (bandwidth). The bandwidth of antenna has two different definitions------

One refers to the working bandwidth of the antenna when the standing wave ratio SWR ≤ 1.5;

One refers to the bandwidth within the range of 3 dB of antenna gain reduction.

In the mobile communication system, it is usually defined according to the former one. Specifically, the bandwidth of the antenna is the working frequency range of the antenna when the standing wave ratio SWR of the antenna does not exceed 1.5.

Generally speaking, the antenna performance is different at each frequency point within the working band width, but the performance degradation caused by this difference is acceptable.

1.7 base station antenna, repeater antenna and indoor antenna commonly used in mobile communication

1.7.1 plate antenna

Whether GSM or CDMA, plate antenna is the most widely used and very important base station antenna. The antenna has the advantages of high gain, good sector pattern, small back lobe, convenient depression angle control of vertical pattern, reliable sealing performance and long service life.

Plate antenna is also often used as the user antenna of repeater. According to the range of action sector, the corresponding antenna model should be selected.

1.7.1 formation of high gain of plate antenna

A. Multiple half wave oscillators are arranged into a vertically placed linear array

B. Add a reflecting plate on one side of the linear array (take the vertical array of two-and-a-half wave oscillators with reflecting plate as an example)

C. In order to improve the gain of plate antenna, eight half wave oscillator arrays can be further used

As pointed out earlier, the gain of four half wave oscillators arranged in a vertical linear array is about 8 DBI; A four element linear array with a reflector on one side, that is, a conventional plate antenna, has a gain of about 14 ~ 17 DBI.

An eight element linear array with a reflector on one side, i.e. an extended plate antenna, has a gain of about 16 ~ 19 DBI. It goes without saying that the length of the extended plate antenna is twice that of the conventional plate antenna, up to about 2.4 M.

1.7.2 high gain grid parabolic antenna

From the perspective of performance price ratio, grid paraboloid antenna is often used as the donor antenna of repeater. Due to the good focusing effect of the paraboloid surface, the paraboloid antenna has strong collection ability. For the grid paraboloid antenna with a diameter of 1.5 m, its gain can reach g = 20dbi in the 900 megaband. It is especially suitable for point-to-point communication. For example, it is often used as the donor antenna of repeater.

The paraboloid adopts grid structure, one is to reduce the weight of the antenna, the other is to reduce the wind resistance.

Paraboloid antenna can generally give a front to back ratio of no less than 30 dB, which is the technical index that the repeater system must meet for the receiving antenna to prevent self excitation.

1.7.3 Yagi directional antenna

Yagi directional antenna has the advantages of high gain, light structure, convenient erection and low price. Therefore, it is particularly suitable for point-to-point communication. For example, it is the preferred antenna type for outdoor receiving antenna of indoor distribution system.

The more units of Yagi directional antenna, the higher its gain. Generally, Yagi directional antenna with 6 - 12 units is used, and its gain can reach 10-15dbi.

1.7.4 indoor ceiling antenna

Indoor ceiling antenna must have the advantages of light structure, beautiful appearance and convenient installation.

Nowadays, the indoor ceiling antenna seen in the market has many shapes and colors, but the purchase and manufacture of its inner core are almost the same. Although the internal structure of this ceiling antenna is very small, it can well meet the requirements of standing wave ratio in a very wide working frequency band because it is based on the antenna broadband theory, with the help of computer-aided design and debugging with network analyzer. According to the national standard, the standing wave ratio index of antenna working in a very wide frequency band is VSWR ≤ 2. Of course, it is better to achieve VSWR ≤ 1.5. Incidentally, the indoor ceiling antenna is a low gain antenna, generally g = 2 DBI.

1.7.5 indoor wall mounted antenna

Indoor wall mounted antenna must also have the advantages of light structure, beautiful appearance and convenient installation.

Nowadays, the indoor wall mounted antenna seen in the market has many shapes and colors, but the purchase and manufacture of its inner core are almost the same. The internal structure of the wall mounted antenna belongs to an air dielectric microstrip antenna. Due to the auxiliary structure of widening antenna bandwidth, computer-aided design and debugging with network analyzer, it can better meet the requirements of working broadband. Incidentally, the indoor wall mounted antenna has a certain gain, about g = 7 DBI.

Some basic concepts of radio wave propagation

At present, the frequency bands used in GSM and CDMA mobile communications are:

GSM:890 - 960 MHz, 1710 - 1880 MHz

CDMA: 806 - 896 MHz

The frequency range of 806 - 960MHz belongs to the ultrashort wave range; The frequency range of 1710 ~ 1880 MHz belongs to the microwave range.

The propagation characteristics of radio waves with different frequencies or wavelengths are not exactly the same, or even very different.

2.1 free space communication distance equation

Set the transmitting power as Pt, the transmitting antenna gain as GT and the working frequency as f If the receiving power is PR, the receiving antenna gain is GR, and the distance between the receiving and transmitting antennas is r, then the radio wave loss l0 during the propagation of the radio wave without environmental interference has the following expression:

L0 (dB) = 10 Lg( PT / PR )

= 32.45 + 20 Lg f ( MHz ) + 20 Lg R ( km ) - GT (dB) - GR (dB)

[example] set: Pt = 10 W = 40dbmw; GR = GT = 7 (dBi) ; f = 1910MHz

Q: when r = 500 m, PR =?

Answer: (1) calculation of l0 (DB)

L0 (dB) = 32.45 + 20 Lg 1910( MHz ) + 20 Lg 0.5 ( km ) - GR (dB) - GT (dB)

= 32.45 + 65.62 - 6 - 7 - 7 = 78.07 (dB)

(2) Calculation of PR

PR = PT / ( 10 7.807 ) = 10 ( W ) / ( 10 7.807 ) = 1 ( μ W ) / ( 10 0.807 )

= 1 ( μ W ) / 6.412 = 0.156 ( μ W ) = 156 ( m μ W )

Incidentally, when the 1.9GHz radio wave penetrates a brick wall, it loses about (10 ~ 15) dB

2.2 propagation sight distance of ultrashort wave and microwave

2.2.1 limit direct viewing distance

Ultrashort wave, especially microwave, has high frequency and short wavelength, and its surface wave decays rapidly. Therefore, it can not rely on surface wave for long-distance propagation. Ultrashort wave, especially microwave, is mainly transmitted by space wave. In short, space wave is a wave propagating along a straight line in space. Obviously, due to the curvature of the earth, there is a limit direct viewing distance Rmax for space wave propagation. The area within the farthest direct viewing distance is traditionally called lighting area; The area beyond the limit direct viewing distance Rmax is called the shadow area. Needless to say, when using ultrashort wave and microwave for communication, the receiving point shall fall within the limit direct viewing distance Rmax of the transmitting antenna. Affected by the curvature radius of the earth, the relationship between the limit direct viewing distance Rmax and the height HT and HR of transmitting antenna and receiving antenna is: Rmax = 3.57 {√ HT (m) + √ HR (m)} (km)

Considering the refraction of radio waves by the atmosphere, the limit direct viewing distance should be corrected to

Rmax = 4.12 { √HT (m) +√HR (m) } (km)

Since the frequency of electromagnetic wave is much lower than that of light wave, the effective direct viewing distance re of radio wave propagation is about 70% of the limit direct viewing distance Rmax, i.e. re = 0.7 Rmax

For example, if HT and HR are 49 m and 1.7 m respectively, the effective direct viewing distance is re = 24 km.

2.3 propagation characteristics of radio wave on plane ground

The radio wave directly emitted from the transmitting antenna to the receiving point is called direct wave; The radio wave directed to the ground emitted by the transmitting antenna is reflected by the ground and reaches the receiving point, which is called the reflected wave. Obviously, the signal at the receiving point should be the combination of direct wave and reflected wave. The synthesis of radio waves will not be simply algebraic addition like 1 + 1 = 2, and the synthesis results will vary with the difference of wave path between direct wave and reflected wave. When the wave path difference is an odd multiple of half a wavelength, the direct wave and reflected wave signals are added to form the maximum; When the wave path difference is a multiple of one wavelength, the direct wave and reflected wave signals are subtracted and synthesized to the minimum. It can be seen that the existence of ground reflection makes the spatial distribution of signal intensity very complex.

The actual measurement shows that within a certain distance RI, the signal strength will fluctuate with the increase of distance or antenna height; Beyond a certain distance RI, the signal strength will increase with the increase of distance or the decrease of antenna height. Monotonic decline. The theoretical calculation gives the relationship between RI, antenna height HT and HR:

RI = (4 HT HR) / L, l is the wavelength.

It goes without saying that RI must be less than the limit viewing distance Rmax.

2.4 multipath propagation of radio waves

In the ultrashort wave and microwave band, the radio wave will also encounter obstacles (such as buildings, tall buildings or hills) to reflect the radio wave. Therefore, a variety of reflected waves (broadly speaking, ground reflected waves should also be included) arrive at the receiving antenna. This phenomenon is called multipath propagation.

Due to multipath transmission, the spatial distribution of signal field strength becomes quite complex and fluctuates greatly. In some places, the signal field strength increases and in some places, the signal field strength decreases; Also due to the influence of multipath transmission, the polarization direction of radio waves will change. In addition, the reflection ability of different obstacles to radio waves is also different. For example, the reflection ability of reinforced concrete buildings to ultrashort wave and microwave is stronger than that of brick walls. We should try our best to overcome the negative impact of multipath transmission effect, which is the reason why people often use spatial diversity technology or polarization diversity technology in communication networks with high communication quality requirements.

2.5 diffraction propagation of radio waves

When a large obstacle is encountered in the transmission path, the radio wave will bypass the obstacle and propagate forward. This phenomenon is called radio wave diffraction. Ultrashort wave and microwave have high frequency, short wavelength and weak diffraction ability. The signal intensity behind tall buildings is small, forming the so-called "shadow area". The degree to which the signal quality is affected is related not only to the height of the building, the distance between the receiving antenna and the building, but also to the frequency. For example, there is a building with a height of 10 meters. At a distance of 200 meters behind the building, the received signal quality is hardly affected, but at 100 meters, the received signal field strength is significantly weaker than that without buildings. Note that as mentioned above, the attenuation degree is also related to the signal frequency. For 216 ~ 223 MHz RF signals, the received signal field strength is 16 dB lower than that without buildings, and for 670 MHz RF signals, the received signal field strength is 20 dB lower than that without buildings If the height of the building increases to 50m, the field strength of the received signal will be affected and weakened within 1000m from the building. That is, the higher the frequency, the higher the building, and the closer the receiving antenna is to the building, the greater the impact on the signal strength and communication quality; On the contrary, the lower the frequency, the shorter the building, the farther the receiving antenna is from the building, and the smaller the impact.

Therefore, when selecting the base station site and erecting the antenna, we must consider various possible adverse effects of diffraction propagation and pay attention to various factors affecting diffraction propagation.

3 some basic concepts of transmission line

The cable connecting the antenna and the transmitter output (or receiver input) is called a transmission line or feeder. The main task of the transmission line is to effectively transmit signal energy. Therefore, it should be able to transmit the signal power sent by the transmitter to the input of the transmitting antenna with the minimum loss, or the signal received by the antenna to the input of the receiver with the minimum loss. At the same time, it should not pick up or generate stray interference signals. Therefore, the transmission line must be shielded.

Incidentally, when the physical length of the transmission line is equal to or greater than the wavelength of the transmitted signal, the transmission line is also called a long line.

3.1 types of transmission lines

There are generally two kinds of transmission lines in ultrashort band: parallel two-wire transmission line and coaxial cable transmission line; The transmission lines in microwave band include coaxial cable transmission line, waveguide and microstrip. Parallel two-wire transmission line is composed of two parallel conductors. It is a symmetrical or balanced transmission line. This feeder has large loss and can not be used in UHF frequency band. The two conductors of coaxial cable transmission line are core wire and shielded copper net respectively. Because the copper net is grounded and the two conductors are asymmetric to the ground, it is called asymmetric or unbalanced transmission line. Coaxial cable has wide working frequency range and small loss, which can shield electrostatic coupling, but it can't do anything to interfere with magnetic field. When using, do not run in parallel with the line with strong current, nor close to the low-frequency signal line.

3.2 characteristic impedance of transmission line

The ratio of voltage to current on an infinite transmission line is defined as the characteristic impedance of the transmission line, expressed by Z0. The calculation formula of characteristic impedance of coaxial cable is

Z.=〔 60/√ ε r〕 * Log (D / D) [Euro].

Where, D is the inner diameter of copper mesh of outer conductor of coaxial cable; D is the outer diameter of coaxial cable core;

ε R is the relative dielectric constant of the insulating medium between conductors.

Usually Z0 = 50 ohms, but also Z0 = 75 ohms.

It is not difficult to see from the above formula that the characteristic impedance of the feeder is only related to the conductor diameters D and D and the dielectric constant of the medium between the conductors ε R, but independent of feeder length, working frequency and load impedance connected to feeder terminal.

3.3 attenuation coefficient of feeder

When signals are transmitted in feeders, there are not only the resistive loss of conductors, but also the dielectric loss of insulating materials. These two losses increase with the increase of feeder length and working frequency. Therefore, the feeder length shall be shortened as far as possible.

The attenuation coefficient is used to calculate the loss per unit length β Indicates that the unit is dB / M (dB / M), and the unit on the cable technical specification is mostly dB / 100 m (dB / 100M)

Let the power input to the feeder be P1, the power output from the feeder with length L (m) be P2, and the transmission loss TL can be expressed as:

TL = 10 * Lg ( P1 /P2 ) ( dB )

The attenuation coefficient is

β = TL / L ( dB / m )

For example, Nokia 7 / 8-inch low consumption cable has an attenuation coefficient of 900 MHz β= 4.1 dB / 100 m, which can also be written as β= 3 dB / 73 m, that is, the signal power with a frequency of 900 MHz is half less when passing through a 73 m long cable.

For ordinary non low consumption cables, for example, when syv-9-50-1900mhz, the attenuation coefficient is β = 20.1 dB / 100 m, which can also be written as β= 3dB / 15m, that is, the signal power with frequency of 900MHz will be reduced by half every 15m long cable!

3.4 matching concept

What is matching? Simply put, when the load impedance ZL connected to the feeder terminal is equal to the feeder characteristic impedance Z0, it is called that the feeder terminal is matched and connected. During matching, there is only incident wave transmitted to the terminal load on the feeder, but no reflected wave generated by the terminal load. Therefore, when the antenna is used as the terminal load, matching can ensure that the antenna can obtain all signal power. As shown in the figure below, when the antenna impedance is 50 Ω, it matches the 50 Ω cable, while when the antenna impedance is 80 Ω, it does not match the 50 Ω cable.

If the diameter of the antenna oscillator is large, the change of the antenna input impedance with frequency is small, which is easy to match with the feeder. At this time, the working frequency range of the antenna is wide. On the contrary, it is narrower.

In practice, the input impedance of the antenna will also be affected by the surrounding objects. In order to make the feeder and antenna match well, it is also necessary to properly adjust the local structure of the antenna or install matching devices through measurement when erecting the antenna.

3.5 reflection loss

It has been pointed out earlier that when the feeder is matched with the antenna, there is no reflected wave on the feeder, only incident wave, that is, the wave transmitted on the feeder is only moving towards the antenna. At this time, the voltage amplitude and current amplitude on the feeder are equal, and the impedance at any point on the feeder is equal to its characteristic impedance.

When the antenna and the feeder do not match, that is, when the antenna impedance is not equal to the characteristic impedance of the feeder, the load can only absorb part of the high-frequency energy transmitted on the feeder, but not all, and the unabsorbed part of the energy will be reflected back to form a reflected wave.

For example, in the right figure, because the impedance of the antenna and the feeder is different, one is 75 ohms and the other is 50 ohms, the impedance does not match, and the result is

3.6 VSWR

In the case of mismatch, there are both incident wave and reflected wave on the feeder. Where the phases of the incident wave and the reflected wave are the same, the voltage amplitude is added to the maximum voltage amplitude Vmax to form the antinode; Where the phases of the incident wave and the reflected wave are opposite, the voltage amplitude is subtracted to the minimum voltage amplitude Vmin to form a wave node. The amplitude values of other points are between antinodes and nodes. This synthetic wave is called a traveling standing wave.

The ratio of the amplitude of the reflected wave voltage to the incident wave voltage is called the reflection coefficient and is recorded as R

Reflected wave amplitude (ZL - Z0)

R =───── = ───────

Incident wave amplitude (ZL + Z0)

The ratio of antinode voltage to node voltage amplitude is called standing wave coefficient, also known as voltage standing wave ratio, and is recorded as VSWR

Antinode voltage amplitude Vmax (1 + R)

VSWR = ────────────── = ────

Node voltage radian Vmin (1 - R)

The closer the terminal load impedance ZL is to the characteristic impedance Z0, the smaller the reflection coefficient r is, and the closer the standing wave ratio VSWR is to 1, the better the matching is.

3.7 balancing device

Signal source or load or transmission line can be divided into balanced and unbalanced according to their relationship to ground.

If the voltage between the two ends of the signal source and the ground is equal and the polarity is opposite, it is called a balanced signal source, otherwise it is called an unbalanced signal source; If the voltage between the two ends of the load and the ground is equal and the polarity is opposite, it is called balanced load, otherwise it is called unbalanced load; If the impedance between the two conductors of the transmission line and the ground is the same, it is called a balanced transmission line, otherwise it is an unbalanced transmission line.

Coaxial cable shall be used to connect the unbalanced signal source and unbalanced load, and parallel two-wire transmission line shall be used to connect the balanced signal source and balanced load, so as to effectively transmit signal power, otherwise their balance or imbalance will be damaged and cannot work normally. If the unbalanced transmission line is to be connected with the balanced load, the usual way is to install a "balanced unbalanced" conversion device between the grain producers, which is generally called the balanced converter.

3.7.1 half wavelength balanced converter

Also known as "U" tube balanced converter, it is used for the connection between unbalanced feeder coaxial cable and balanced load half wave symmetrical vibrator. The "U" tube balance converter also has the function of 1:4 impedance conversion. The characteristic impedance of coaxial cable used in mobile communication system is usually 50 Ω. Therefore, in Yagi antenna, a reduced half wave oscillator is used to adjust its impedance to about 200 Ω, so as to finally match the impedance of 50 Ω coaxial cable of main feeder.

3.7.2 quarter wavelength balance unbalance

The balanced unbalanced transformation between the balanced input port of the antenna and the unbalanced output port of the coaxial feeder is realized by using the property that the terminal of one quarter wave long and short transmission line is a high-frequency open circuit.

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