The following figure shows that two single-polarized antennas are installed together to form a dual-polarized antenna. Please note that the dual polarization antenna has two connectors.
A dual-polarized antenna radiates (or receives) two kinds of waves, whose polarizations are orthogonal to each other in space. Vertically polarized waves should be received by antennas with vertical polarization characteristics, and horizontally polarized waves should be received by antennas with horizontal polarization characteristics. Right-handed circularly polarized waves should be received by antennas with right-handed circularly polarized characteristics, while left-handed circularly polarized waves should be received by antennas with left-handed circularly polarized characteristics.
When the polarization direction of the incident wave is different from that of the receiving antenna, the received signal will become smaller, that is, polarization loss will occur. For example, when the+45 polarization antenna receives vertical polarization wave or horizontal polarization wave, or when the vertical polarization antenna receives+45 polarization wave or-45 polarization wave, and so on, polarization loss will occur. When receiving any linearly polarized wave with a circularly polarized antenna, or receiving any circularly polarized wave with a linearly polarized antenna, etc., polarization loss will inevitably occur-only half of the incident wave energy can be received.
When the polarization direction of the receiving antenna is completely orthogonal to that of the incident wave, for example, when the receiving antenna with horizontal polarization receives the incident wave with vertical polarization, or when the receiving antenna with right-handed circular polarization receives the incident wave with left-handed circular polarization, the antenna cannot receive the energy of the incident wave at all. In this case, the polarization loss is the largest, which is called complete polarization isolation. "Resonance frequency" and "electrical resonance" are related to the electrical length of the antenna. The electrical length is usually the physical length of the wire divided by the ratio of the wave propagation velocity in free space to the velocity in the wire. The electrical length of an antenna is usually expressed in terms of wavelength. Generally speaking, an antenna is tuned to a certain frequency, and it is effective in a frequency band centered on the resonant frequency. However, other antenna parameters (especially radiation pattern and impedance) vary with frequency, so the resonant frequency of the antenna may only be close to the center frequency of these more important parameters.
The antenna can resonate at a frequency corresponding to the relationship length of the target wavelength component number. Some antenna designs have multiple resonant frequencies, while others are relatively efficient over a wide frequency band. The most common broadband antenna is log-periodic antenna, but its gain is much smaller than that of narrow-band antenna. "Gain" means that the ratio of the intensity of the antenna radiation pattern in the strongest radiation direction of the antenna to the intensity of the reference antenna is logarithmic. If the reference antenna is an omnidirectional antenna, the unit of gain is dBi. For example, the gain of dipole antenna is 2. 14dBi. A dipole antenna is also commonly used as a reference antenna (because a perfect omnidirectional reference antenna cannot be manufactured), in which case the gain of the antenna is in dBd.
Antenna gain is a passive phenomenon. The antenna does not increase the excitation, but redistributes it so that it radiates more energy in a certain direction than the omnidirectional antenna. If the gain of the antenna is positive in some directions, it is negative in other directions due to the conservation of energy. Therefore, the gain that an antenna can achieve should be balanced between its coverage and its gain. For example, the dish antenna on the spacecraft has a large gain, but its coverage is very narrow, so it must be pointed to the earth accurately; But the gain of broadcast transmitting antenna is very small, because it needs to radiate in all directions.
The gain of the dish antenna is proportional to the aperture (reflection area), the surface accuracy of the antenna reflection surface and the transmitting/receiving frequency. Generally speaking, the larger the aperture, the greater the gain, and the higher the frequency, the greater the gain. However, at higher frequencies, the surface accuracy error will greatly reduce the gain.
"Aperture" and "radiation pattern" are closely related to gain. Aperture refers to the cross-sectional shape of the "beam" in the direction of the highest gain, which is two-dimensional (sometimes the aperture is expressed as the radius of the circle or the angle of the beam cone that approximates the cross-section). Radiation pattern is a three-dimensional graph showing gain, but usually only the horizontal and vertical two-dimensional sections of radiation pattern are considered. The radiation pattern of high gain antenna is often accompanied by "sidelobe". Sidelobe refers to the beam other than the main lobe (the "beam" with the highest gain). When radar and other systems need to determine the signal direction, the sidelobe will affect the antenna quality. Because of power distribution, sidelobes also reduce the gain of the main lobe.
Gain refers to the ratio of the power density of the signal generated by the actual antenna and the ideal radiating element at the same point in space under the condition of equal input power. It quantitatively describes the degree of concentrated input power of the antenna. The gain is obviously closely related to the antenna pattern. The narrower the main lobe, the smaller the sidelobe and the higher the gain. We can understand the physical meaning of gain as follows: in order to generate a signal with a certain size at a certain distance, if an ideal non-directional point source is used as the transmitting antenna, the input power is 100W, while if a directional antenna with a gain of G = 13 dB = 20 is used as the transmitting antenna, the input power is only 100/20. In other words, in terms of its radiation effect in the maximum radiation direction, the gain of the antenna is a multiple of the input power compared with the non-directional ideal point source.
The gain of half-wave dipole is G=2. 15dBi.
Four half-wave dipoles are arranged up and down along the vertical line to form a vertical four-element array, and its gain is about G = G = 8. 15 dBi(DBI dBi unit means that the comparison object is an ideal point source with uniform radiation in all directions).
If the half-wave symmetric oscillator is taken as the comparison object, the unit of its gain is dBd.
The gain of the half-wave dipole is G=0dBd (because it is compared with itself, the ratio is 1 and the logarithm is zero. ) The gain of the vertical quaternary array is about g = 8.15–2.15 = 6 DBD.
Gain characteristics:
(1) antenna is a passive device and cannot generate energy. Antenna gain is only the ability to effectively concentrate energy in a certain direction to radiate or receive electromagnetic waves.
⑵ Antenna gain is generated by superposition of oscillators. The higher the gain, the longer the antenna length.
⑶ The higher the antenna gain, the better the directivity, the more concentrated the energy and the narrower the lobe. "Impedance" is similar to the refractive index in optics. When radio waves pass through different parts of the antenna system (radio station, feeder, antenna and free space), they will encounter impedance differences. At each interface, according to impedance matching, part of the energy of radio waves will be reflected back to the source, forming a certain standing wave on the feeder. At this time, the ratio of maximum energy to minimum energy can be measured, which is called standing wave ratio (SWR). The standing wave ratio of 1: 1 is ideal. The standing wave ratio of 1.5: 1 is regarded as a critical value in low-energy applications where energy consumption is critical. Standing wave ratio as high as 6: 1 can also appear in the corresponding equipment. Minimizing the impedance difference between interfaces (impedance matching) will reduce the standing wave ratio and maximize the energy transmission between antenna system components.
The complex impedance of the antenna is related to the electrical length of the antenna when it works. By adjusting the impedance of the feeder, that is, using the feeder as an impedance transformer, the impedance of the antenna can be matched with the feeder and the radio station. It is more common to use antenna tuners, baluns, impedance transformers, matching networks or matching segments including capacitors and inductors, such as gamma matching. The gain (dBi) radiation pattern of the half-wave dipole antenna (as above) is a graphic description of the relative field strength transmitted or received by the antenna. Because the antenna radiates into three-dimensional space, several diagrams are needed to describe it. If the antenna radiation is symmetrical to a certain axis (such as dipole antenna, spiral antenna and some parabolic antennas), only one pattern is needed.
Different antenna suppliers/users have different standards and drawing formats for the pattern. The ratio of voltage to current on an infinite transmission line is defined as the characteristic impedance of the transmission line, which is expressed by Z0. The formula for calculating the characteristic impedance of coaxial cable is
Z .=[60/√εr]×log(d/d)[ Europe].
Where d is the inner diameter of the copper mesh of the outer conductor of the coaxial cable; D is the outer diameter of the coaxial cable core;
εr is the relative dielectric constant of insulating medium between conductors.
Usually Z0 = 50 euros, and some Z0 = 75 euros.
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 εr of the medium between the conductors, but has nothing to do with the length of the feeder, the working frequency and the load impedance connected to the feeder terminal. The signal transmission in the feeder has not only the resistance loss of the conductor, but also the dielectric loss of the insulating material. These two kinds of losses increase with the increase of feeder length and working frequency. Therefore, the feeder length should be shortened as much as possible through reasonable layout.
The loss per unit length is expressed by the attenuation coefficient β, and the unit is dB/m (decibel/meter). The units in cable technical specifications are mostly dB/ 100 m (dB/100 m).
Let the input power of the feeder be P 1 and the output power of the feeder with the length of L(m) be P2, and the transmission loss TL can be expressed as:
TL = 10 ×Lg (P 1 /P2) (decibel)
The attenuation coefficient is
β = TL/L (dB/m)
For example, the attenuation coefficient of Nokia's 7/8-inch low-loss cable at 900MHz is β= 4. 1 dB/ 100 m, which can also be written as β=3 dB/73 m, that is, the signal power at 900MHz is reduced by half every time it passes through this cable with a length of 73 m.
Ordinary non-low loss cables, such as SYV-9-50- 1 0,900MHz, have attenuation coefficient β = 20. 1 dB/ 100 m, which can also be written as β=3dB/ 15 m, that is, signal power with frequency of 900 MHz. Definition: The ratio of signal voltage to signal current at the input end of the antenna is called the input impedance of the antenna. The input impedance has a resistive component Rin and a reactive component Xin, that is, Zin = Rin+j Xin. The existence of reactance component will reduce the extraction of signal power by feeder, so the reactance component must be as zero as possible, that is, the input impedance of antenna should be as pure resistance as possible. In fact, even a well-designed and debugged antenna always contains a small reactance component in its input impedance.
The input impedance is related to the antenna structure, size and working wavelength. Half-wave dipole is the most important basic antenna, and its input impedance is Zin = 73. 1+j42.5 (ohm). When the length 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 (ohm) (nominal 75 ohm). Note that strictly speaking, the input impedance of a purely resistive antenna is only for the point frequency.
By the way, the input impedance of the half-wave hybrid oscillator is four times that of the half-wave symmetric oscillator, that is, Zin = 280 (ohm) (nominal 300 ohm).
Interestingly, for any antenna, people can always debug the antenna impedance, so that the imaginary part of the input impedance is very small, and the real part is quite close to 50 ohms in the required working frequency range, so that the input impedance of the antenna is Zin = Rin = 50 ohms, which is necessary for the antenna and feeder impedance to match well. Whether it is a transmitting antenna or a receiving antenna, they always work in a certain frequency range (bandwidth), and the bandwidth of an antenna has two different definitions:
One is the working band width of the antenna when the standing wave ratio SWR is less than or equal to 1.5;
One refers to the bandwidth within the range where the antenna gain is reduced by 3 decibels.
In a mobile communication system, it is usually defined by the former. 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 in the working bandwidth, but the performance degradation caused by this difference is acceptable.