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EMC Antenna selection for electromagnetic compatibility test system


  Selection of Antenna for EMC Electromagnetic Compatibility Test System

  Choosing an antenna may be a difficult task that every EMC test engineer must face. Very few electrical engineers, whether they are developers of digital circuits, EMC test experts or engineers in other fields, can understand the manufacture of antennas. The trade-offs that manufacturers make to create useful, affordable antenna products. However, EMC electromagnetic compatibility test engineers do not need to be embarrassed by the lack of knowledge in a certain aspect, because although most antenna experts have good theoretical knowledge of antennas, they are often used to the requirements of antennas based on communication requirements or other RF application fields. To consider the problem, they often do not understand the unique requirements of the antenna used in the EMC electromagnetic compatibility test system.

  basic concept

  For the antenna of the EMC electromagnetic compatibility test system, receiving refers to the measurement of undesired radiated emissions, and "emission" refers to the generation of electromagnetic fields for susceptibility testing. Let's review several other key terms.

  • Gain Gain is used to measure the ability of the antenna to receive and transmit signals from a certain direction. When compared with other directions, the gain is usually expressed in dB compared with the omnidirectional antenna. The gain can be positive or negative. When the signal is Gain is positive when focused in one direction, like the beam of a flashlight, and is negative when there are losses in the antenna system or when the length of the antenna used is smaller than the wavelength of the signal being measured.

  • Antenna Factor (AF) The antenna factor represents the relationship between the radiation field of the antenna and the input voltage of the antenna. When the AF is in dB, the AF has the following relationship with the gain:

  G(dBi)20lg(F)-29.79-AF(dB)

  Among them, F is the frequency, the unit is MHz.

  • Beam width The beam width refers to the angle between two half-power points in the direction of the peak response of the antenna. The beam width has two components, the E plane and the H plane, and the two are not necessarily completely equal. If the gain is designed to be positive, its beam width and gain are often just opposite.

  • Bandwidth Bandwidth refers to the frequency coverage of the antenna. If the bandwidth is expressed as a part of the rated frequency range of the antenna, the bandwidth of the non-resonant antenna is greater than the bandwidth of the resonant antenna, and the bandwidth of the low-gain antenna is greater than the bandwidth of the high-gain antenna. For wideband, balun or matching network antennas, the bandwidth is more affected than the antenna factor.

  • Impedance The impedance of the antenna is usually considered little, because the load impedance of all EMC test equipment is designed to be 50Ω, and the impedance of the EMC antenna is usually designed to be close to 50Ω within its frequency range, but the tester should also be aware Possible problems caused by impedance mismatch, especially for low-frequency magnetic field loop antennas. The impedance of the antenna often changes with frequency, but many low-frequency loop antennas do not have a matching network to compensate for this change.

  • Standing Wave Ratio (VSWR) VSWR is an indirect parameter to measure the impedance matching of two RF devices. VSWR is very important to most users. There are several complicated reasons for this. Simply put, in general, the impedance shown by the feeder is the sum of the rated impedance of the feeder and the load impedance. Therefore, at both ends of the feeder Impedance mismatches may occur, so that most of the signal will be reflected at the load, and then reflected again at the source along the feed line, when accurate measurements are required, or when the signal source is sensitive to impedance mismatch VSWR will become a problem when the loss of the feeder is very high.

  •Size is a very important antenna characteristic. The actual size of the antenna is limited by the need for control and movement of the antenna. The use of the antenna in a shielded room also limits the maximum size of the antenna. Some coupling will also affect the size, but on the contrary, if you want the antenna to have a good low frequency response, high gain or wide bandwidth, you need to increase the size of the antenna.

  selection process

  According to the basic concepts about antennas mentioned earlier, EMC electromagnetic compatibility test engineers can start to choose antennas according to actual uses. Strictly speaking, any transmitting antenna can be used for receiving, but in many receiving occasions, active antennas The extra sensitivity turns nearly all electronic circuits into open circuits.

  A very important issue is the frequency range. In most EMC electromagnetic compatibility test system measurements, the required frequency range is much larger than the frequency range of a single antenna. In fact, due to the limitation of bandwidth and antenna size, in a test It is not uncommon to use 5 antennas to cover the entire test frequency band.

  In many cases the low frequency end is low enough to require two antennas - one to measure the electric field component and the other to measure the magnetic field component. This usage can be explained as follows, when the distance is measured in absolute units and When specified by the measurement standard, the distribution of the electromagnetic field is a function of the source impedance (possibly unknown) and the distance expressed in wavelength. In the "far field" area, the electromagnetic field can be considered to have the characteristics of a plane wave. The E field component and the H field component can be obtained by The wave impedance of free space is 377Ω. Thus, the third variable can be obtained from any two of the three variables of field power, E field and H field. In the "near field" region, there is no such fixed relationship, and any point in the near field region The wave impedance is between the unknown source impedance and the known free space wave impedance. Since the wave impedance is unknown, it is necessary to measure the E field and the H field separately, and then calculate the power, (It is also possible to calculate the wave impedance at this point and deduce the source impedance, this calculation is very useful for diagnosing the actual source of harmful radiation), some testers may ask, why can't one or two antennas cover the entire frequency range? Because, when choosing an antenna, you also need There are many other trade-offs to consider, which are detailed in the following sections.

  The importance of gains

  Each antenna has a positive gain and a negative gain. Positive gain is good, but negative gain is necessary, though undesirable. Ideally, each antenna used in the EMC electromagnetic compatibility test system measurement All should have positive gain as much as possible and reduce negative gain. Considering the limitations of actual antenna design, the cost of antenna gain is often lower than the sensitivity or amplifier power of the receiving antenna, because it is hoped to detect the smallest radiated emission or produce The field required for the sensitivity test is the least expensive, so it is necessary to choose the best antenna gain.

  Except for the tuned dipoles specified by the standard, most antennas below 100 MHz are non-resonant and exhibit negative gain (loss). Antennas commonly used in this frequency range are whip antennas for low frequency E-field measurements (Figure 1 ), the E field generator is also used in the sensitivity test. In such a low frequency band, the shielded loop antenna is used to measure the H field, and the loop antenna is used to generate the H field (Figure 2). When it is above 20-30MHz, the most commonly used The antenna is a non-resonant dipole antenna, often with a biconical structure (Figure 3 and Figure 4). This type of antenna is used because as the frequency decreases, the size of the broadband resonant antenna will become larger, and it will be very large quickly. Unacceptably, the negative gain is due to the small size of this type of antenna compared to the wavelength of the signal being measured, and the antenna needs to be tuned at each frequency point only when test time permits.

Figure 1 Active E-field rod antenna

 

Figure 2 Active Loop Antenna

 

Figure 3 Tuned dipole antenna array

 

Figure 4 Biconical antenna

 

Figure 5 Log Periodic Antenna

  In the frequency range of 100 ~ 200MHz, broadband resonant antenna is the most practical, and used a lot, but because of the large size of this type of antenna, sometimes it can not be used, therefore, in this frequency range, sometimes the use efficiency is not very high When the frequency is higher than 200MHz, the gain antenna is very common, but the size is one of the main limiting factors. Theoretically, a log-periodic antenna can be designed to cover the entire frequency range and provide high gain (Figure 5). However, as the bandwidth increases, the size of the antenna also increases. At higher frequencies, the short The length of the vibrator and its support do not match the mechanical strength required to support the larger low-frequency vibrator. Another type of antenna is often used in the high-frequency band, such as the double-ridged waveguide horn antenna. Due to the mechanical problems involved and High gain, this type of antenna is best used in frequency bands above 1 to 2 GHz.

  size problem

  It has been mentioned above that in the design of the antenna of the EMC electromagnetic compatibility test system, the size is a very heavy constraint factor, and many problems will arise when using a very large antenna. We require that the coupling from the antenna to the ground, the interference of the device under test, the wall of the shielded room and any other surrounding objects must be minimal. If the vibrator of the antenna is kept at a certain distance from the surrounding objects, the coupling problem will be smaller. Obviously, from this point of view, the smaller the antenna, the better.

  Beam width is another factor that must be traded off with gain. The antenna cannot generate power! The function of the antenna is only to concentrate power, just like the function of a reflector placed behind the light. In other words, gain and beam width are a pair Contradictory, when the gain increases, the beam width decreases and vice versa, the practical effect of this phenomenon is to limit the "tested" area at any time, if in the sensitivity test, the radiation range of the test field or in the radiation In the transmission test, the receiving range of the receiving antenna is smaller than the size of the device under test, so it is necessary to repeat the test many times until the device under test is completely tested, but even so, the test results may be invalid.

  In conclusion, gain, bandwidth and size are interrelated, usually as the gain increases, the size also increases, and at the same time, as the bandwidth increases, the size also increases, so the problems surrounding the antenna are obvious, It is impractical to make a very small antenna with a very wide bandwidth; a wideband antenna must be very large by its nature.

  Factors to consider when selecting an antenna

  On the basis of the antenna parameters mentioned above, EMC electromagnetic compatibility test system testers must consider some very professional data when selecting antennas, such as how flat is the antenna gain curve? In many cases, using an antenna with a flat gain ratio It is easy to use an antenna with a very high gain point. For another example, how is the antenna coefficient (or gain) measured? Some manufacturers will provide data at the usual distance from the antenna, while others only provide far-field data. The numbers look better, but not very useful, also, when measuring the antenna factor, are the pads used? If they are used, they will introduce a lot of loss; unless the pads are removed, they will obstruct the antenna Used as a transmitting antenna. Also, are individual calibration curves or calibration charts provided for each antenna? Are testers limited to calibrated data? Does the manufacturer offer recalibration services? Antennas are precision instruments and their precision must be maintained .

  In order to ensure the basic test antenna needs, the EMC electromagnetic compatibility test system test engineer must investigate the antenna manufacturer. Does the manufacturer have professionals who are willing and able to take the time to answer questions? Need antenna corrections? Most importantly, since no one can manufacture a 100% perfect product, what is the pass rate of the product? Practical evaluation of reliability and reputation apply when considering.

  The last thing to consider is the price issue. Since the antenna of the EMC electromagnetic compatibility test system is a precision instrument, there is no cheap EMC electromagnetic compatibility test antenna. However, the antenna is only a small part of the EMC electromagnetic compatibility test equipment. Therefore, it is necessary to purchase the antenna When using antennas, the price should not be the main issue. Of course, the more savings the better, but the price should not be the decisive factor in the purchase of antennas.

  Summarize

  To select the test antenna for the EMC electromagnetic compatibility test system, the test task must first be clearly defined. What are the requirements for the transmitting antenna and the receiving antenna? What is the frequency range? What is the minimum acceptable gain? What is the maximum size of the acceptable antenna? After clarifying these issues, it is necessary to decide how many antennas to use? What is the frequency range and gain of each antenna? Has each antenna been calibrated separately? Does the obtained data meet the needs of the tester? The form indicates the most appropriate? Can you get the necessary help from the manufacturer? How much is the total cost of the test configuration? Is it within the budget? When these key issues are solved, EMC testers can fill in the order with confidence

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