How to measure the performance of an antenna and whether it can meet the application requirements requires a series of specification parameters and their corresponding index values. Generally, the specification parameters of the antenna are divided into three parts: passive antenna index, active circuit index, structure and environmental requirements.
oneOperating frequency and bandwidth
a The operating frequency determines what kind of signal the antenna can receive. Antenna is a frequency selection device, which receives (or transmits) signals of some frequencies and filters out other unwanted frequency signals. Theoretically, any antenna can only work in a limited frequency band. In addition to the dominant GPS of the United States, China's Beidou system, Russia's GLONASS system, Europe's Galileo system, as well as the regional systems of Japan, India and other countries, each system has its own working center frequency and its covered frequency band resources. Therefore, when selecting an antenna, you must first determine which frequencies of which systems you want to receive. In order to adapt to the development of navigation and positioning technology, the working frequency of antenna has developed from supporting single system to supporting multiple systems, and from single frequency to multiple frequency compatibility.
b Antenna bandwidth is to ensure that the antenna is in the whole required frequency range, and its gain value, return loss, axial ratio and other characteristics must meet certain requirements. Therefore, for some antennas that can support multiple systems and multiple frequency bands, it is necessary to confirm whether the corresponding specific indicators in each frequency band meet the standard. For example, if the gain of a certain frequency band is too low, even if the signal of this frequency band can be received, its strength will be very weak, which will greatly reduce its meaning in practice.
cGain is also a very important indicator, which was specifically mentioned in the previous article. There are many factors affecting the gain. Improving the gain of the antenna also involves various aspects such as scheme, material, technology and so on. How to mention the gain index depends on the use environment. For example, the positioning antenna used on the ship requires the low elevation gain to be as high as possible, because the ship shakes heavily on the water. The low elevation defined when the antenna is placed horizontally may become a high elevation in practice. If the gain is too low, it will not be able to stably capture and track the satellite.
threeAntenna axial ratio
d One of the differences between satellite navigation antenna and ordinary communication antenna is that the polarization mode of the former is mostly circular polarization, and the index to measure the polarization characteristics is the axis ratio. As for what is the polarization mode of antenna and what is circular polarization, if you are interested, you can refer to relevant data, which will not be expanded here.
ehereSay a few points about antenna polarization.
first, the signals received by the ground terminal are transmitted by satellites in the sky, and the signals transmitted by satellites are right-handed circular polarization. In order to minimize the polarization loss caused by the mismatch between transmitting and receiving polarization, the right-handed circular polarization characteristics of the receiving antenna should be as good as possible, that is, the antenna should have a low axis ratio as possible, which is the so-called "door-to-door pair".
secondGenerally, the high elevation axis ratio of the same antenna is better than the low elevation axis ratio. If the antenna axis ratio is poor, it will also reduce the right-hand gain of the antenna, which will "add insult to injury" to the low elevation gain itself. Therefore, reducing the low elevation axis ratio as much as possible is also an important consideration in antenna design.
thirdIn addition to the right-handed circular polarization, the antenna with linear polarization can also receive satellite signals as long as the frequency is right. However, compared with the circular polarization antenna, it is "congenitally deficient" and will lose 3dB. In many places, due to limited conditions, the linear polarization scheme has to be used, which is acceptable when the signal strength requirements are not high, But remember not to use left-handed circular polarization to receive satellite signals. The polarization mode of transmitting and receiving signals is completely orthogonal without intersection, so you can only stare.
fourAntenna impedance and standing wave
f The impedance and standing wave of the antenna measure the ability of the antenna to transmit signals to the back end. The antenna feed network is directly connected to the RF front-end circuit. If the impedance of the antenna matches the input impedance of the circuit, in theory, 100% of the energy flows to the back end. If there is a mismatch, part of the energy will be stuck here and backflow, resulting in energy loss. Just like two sections of water pipes together, only when the water flow is matched can it flow smoothly to the back end. If the water is blocked, it will have to flow back. The value of standing wave is a measure of impedance matching. The smaller the standing wave, the better the matching. Therefore, the value of antenna standing wave is also a very important index.
fiveAntenna out of roundness
g The reason why out of roundness should be put forward separately is that for satellite antennas, the gain is required to be as uniform as possible. If the gain in one direction is significantly higher than that in other directions, the satellite receiving capacity will be greatly reduced, which is specifically reflected in the reduction of the number of satellite receiving. Take the antenna center as the rotation center axis, rotate 360 degrees in the horizontal plane for one cycle, and the gain and axis ratio of each angle are different. The out of roundness value of the antenna with good performance will be relatively small, that is to say, the receiving capacity in all directions is similar. The out of roundness is related to the design of the antenna, the installation structure of the antenna, and whether there are other physical structures around the antenna, especially when designing the built-in antenna.
sixAntenna polarization gain ratio
h The front to back ratio is defined as the antenna normal polarization gain and back & plusmn; 30° The difference between the maximum polarization gain in the antenna indicates whether the antenna suppresses the back lobe. Especially in high-precision antennas, this index is particularly important, because high-precision antennas need very low back lobe gain in order to receive useful signals and block useless interference signals as much as possible. For broadband antennas, the front to back ratio of heart frequency points will be better than that of sideband frequency points. Therefore, pay special attention to the front to back ratio of all supported working frequency points of the antenna.
sevenAntenna phase center
I The biggest difference between high-precision antenna and ordinary navigation and positioning antenna is that the former must have a stable phase center, because the final calculated position information is the position of the antenna phase center. If the phase center fluctuates too much, it will introduce errors, resulting in inaccurate final positioning results and failure to achieve high-precision results. There are many kinds of definition, calculation and measurement methods of phase center, which will be discussed later. In particular, the stability of the phase center is more important than the signal-to-noise ratio of satellite search. The measured signal-to-noise ratio of foreign antennas is rarely higher than that of domestic antennas, and some will even be much lower. However, the stability of the phase center is really excellent, not only in design, but also in technology, materials and many other aspects.
eightAntenna multipath effect value
J Multipath effect is the interference delay effect caused by the multipath transmission of signal in the propagation channel. The suppression degree of antenna to multipath signal is generally measured by the value of multipath effect. Generally, high-precision antennas are required to have certain anti multipath ability, especially the reference station antenna as the signal source of the reference station. If a strong multipath signal enters the back end to participate in the signal solution, it will lead to a large deviation between the calculated position information and the actual position information, and will also affect the efficiency of the whole data. At present, there are many design methods to suppress multipath signals, such as choke structure and EBG structure, which have their own advantages and environmental limitations. The poor anti multipath ability will also affect the final phase center solution of the antenna.
nineBasic link of active circuit
kThe antenna active circuit is composed of a series of dual port devices cascaded in turn, mainly the combination of low-noise amplifier and filter. According to different antenna types, there will be different schemes such as multi-level filtering and multi-level amplification. If it is a multi system multi band signal, there will also be a signal combiner. Of course, the antenna also includes power management module, DC bias circuit, etc. The shortest path is used to connect the passive antenna signal to the active circuit, which can reduce the loss of passive gain. This is why most antennas now make the passive feed network and the active circuit board on the same circuit board.
tenActive gain of antenna
L The active gain represents the amplification factor of the low noise amplifier to the small signal. The required gain value is different according to the different positioning modules of the antenna. The gain is too small, the signal is too weak, and the back-end cannot extract useful signals. Too high gain is easy to cause the back-end devices to saturate and enter the abnormal working state. Matching is the key.
M Low noise amplifier is a very mature component, but not all models are suitable for navigation antenna,firstA sufficiently low noise figure is required, which will be described in detail later.secondIt is necessary to have good linearity to ensure that the antenna can work normally within a certain strength signal range.thirdIt is the stability of low noise amplifier, including the stability in high and low temperature environment, which is very important for the reliability of antenna.
elevenNoise figure of active antenna
N The noise of antenna active circuit will be added to the noise of the whole receiver system. The increase of noise will reduce the signal-to-noise ratio of the system, and then affect the receiving sensitivity. How to reduce the noise figure of the circuit?first, it is important to choose a low-noise discharge pipe with low noise figure, although this is nonsense.second, all low-noise amplifiers, filters or other devices in the circuit will increase the overall noise of the link. According to the noise coefficient cascade formula, the gain of the first stage amplifier should be as high as possible, which can reduce the contribution of the following devices to the overall noise.third, the insertion loss of the filter will also increase the noise of the circuit. Sometimes, in order to improve the anti-interference ability of the antenna, the pre filter will be designed. At this time, the filter with smaller insertion loss should be selected.
twelveOut of band suppression of antenna
O When the satellite signal reaches the ground receiving end, it is already very weak and mixed with various interference signals. Therefore, it is generally necessary to filter the received signal first and only let the required signal enter the back end for signal processing. The selection of filter is very important. In order to improve the anti-interference ability of antenna, multi-stage filter is generally used. It is worth noting that the filter can only filter the interference signal outside the working frequency band, and the interference signal in the band can only be weakened or eliminated by other means. When the navigation antenna is used close to other high-power transmission modules, there is often a significant decline in performance. At this time, you can customize the filter to filter specific frequency points, or keep the antenna away from the high-power signal transmission source as far as possible.
thirteenCarrier noise ratio of antenna
P Unlike the passive antenna, its performance can be measured and evaluated by microwave anechoic chamber and other methods. Due to its integrated and integrated design, the active antenna cannot be measured one by one after disassembly. How to evaluate the performance of an active antenna, there is the index of carrier noise ratio. The gain, axial ratio and standing wave of the passive part of the antenna, and the gain, noise figure and out of band suppression of the active part will be reflected in the final carrier noise ratio data. How to measure the carrier noise ratio data of an antenna?first, look at the number of search stars.second, see the carrier noise ratio of available satellites.third, look at the search speed.fourth, see the stability of carrier noise ratio data. Generally, the antenna with a large number of satellite search, high antenna carrier noise ratio, fast satellite search speed and small carrier noise ratio jump is better. However, the cut record noise ratio is not the only standard to measure the performance of the antenna. Important performances such as phase center, anti multipath ability and antenna stability need to be comprehensively considered.
fourteenWorking voltage and current of antenna
Q The low-noise amplifier of the antenna needs to be fed and matched through the bias circuit. DC is generally fed through the antenna RF connector. The antenna generally needs a separate power module, which is mainly used to stabilize voltage and remove some clutter interference signals. The working current of the antenna varies according to the selected tubes. It is used on some handheld navigation devices or UAV navigation terminals. Try to select tubes with low current to reduce power consumption and prolong the working time of the whole machine.
fifteen antennaAnti static, anti burning and lightning protection design of circuit
R As an electronic product, conventional protection design is still necessary. Anti static treatment prevents the components in the antenna from being damaged by static electricity. The anti burn processing is to prevent the antenna from being pressed by the sudden high-power signal to damage the components, protect the circuit from burning out, and the antenna can return to normal operation after the high-power signal disappears. Because some external antennas are placed outdoors for a long time, it is inevitable to encounter lightning weather, which makes the antenna have a certain lightning protection ability, which is also an important content of its reliability design.
All indicators of the antenna are interrelated and affect each other. In order to improve one indicator, it will generally have a certain impact on another indicator. It is very important to understand and recognize this. Antenna design is an art of balance and trade-offs. A good antenna designer will know how to trade-offs to achieve the overall optimization of the system. It's interesting to study antenna as an art.
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