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A Method for Measuring the Maximum Measurable Gain of a Passive Intermodulation Chamber

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A Method for Measuring the Maximum Measurable Gain of a Passive Intermodulation Chamber electronics Article A Method for Measuring the Maximum Measurable Gain of a Passive Intermodulation Chamber Zhanghua Cai 1, Yantao Zhou 1, Lie Liu 2, Francesco de Paulis 3 , Yihong Qi 4 and Antonio Orlandi 3,* 1 Department of Electrical and Information Engineering, Hunan University, Changsha 410082, China; [email protected] (Z.C.); [email protected] (Y.Z.) 2 General Test Systems, Shenzhen 518102, China; [email protected] 3 UAq EMC Laboratory, Department of Industrial and Information Engineering and Economics, University of L’Aquila, 64100 L’Aquila, Italy; [email protected] 4 Peng Cheng Laboratory, Shenzhen 518066, China; [email protected] * Correspondence: [email protected]; Tel.: +39-0862-434432 Abstract: This paper presents an approximate method that allows the calculation of the maximum measurable gain (MMG) in an anechoic chamber. This method is realized by using a low passive intermodulation (PIM) medium-gain directional antenna. By reducing the distance between the antenna and the wall of the chamber to reduce path loss, the purpose of replacing a high-gain antenna with a medium-gain antenna is achieved. The specific relationship between distance and equivalent gain is given in this paper. The measurement interval is determined by the 3 dB beamwidth of the measurement antenna to scan the whole chamber. A set of corresponding data for the residual PIM level and the MMG of the chamber can be obtained by the method of measurement outlined herein. The feasibility of this method was verified by measurements in two PIM measurement chambers. Keywords: anechoic chamber; antenna measurement; passive intermodulation measurement Citation: Cai, Z.; Zhou, Y.; Liu, L.; de Paulis, F.; Qi, Y.; Orlandi, A. A Method for Measuring the Maximum Measurable Gain of a Passive 1. Introduction Intermodulation Chamber. Electronics Passive intermodulation (PIM) has an important impact on wireless communication 2021, 10, 770. https://doi.org/ systems [1–3]. The PIM source is generated by non-linear factors in radio frequency (RF) 10.3390/electronics10070770 systems. It mainly includes contact non-linearity [4,5] and material non-linearity [6,7]. When signals of multiple frequencies are mixed into non-linear passive components, cor- Academic Editor: Rocco Guerriero responding PIM signals are generated. If the frequencies of the PIM signals are in the operating frequency band of the receiver, it will create interference in the system. The Received: 27 February 2021 sensitivity and channel capacity of the system will thereby be reduced. Therefore, for Accepted: 20 March 2021 Published: 24 March 2021 satellite [8], base station [9], indoor-distributed antennas, and other wireless systems, PIM is an important technical indicator. With the development of 5G and autonomous vehicles, Publisher’s Note: MDPI stays neutral achieving the highest possible sensitivity has become extremely important. Hence, the with regard to jurisdictional claims in importance of PIM cannot be ignored. published maps and institutional affil- The antenna is one of the most important components in wireless communication iations. systems, and is also one of the components that could most easily be a source of PIM [10–13]. The design of antennas is highly related to the structure, and therefore structural design and processing technology are the key to controlling the PIM levels of antennas. The purpose of structural design is to minimize PIM levels, such as avoiding direct metal-metal contact and reducing solder joints. The actual PIM level is directly related to the processing Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. technology. The same products may have different PIM levels owing to small differences in This article is an open access article processing. For example, the tightness of contact between metals [5] or a lack of uniformity distributed under the terms and in solder joints [14] will result in different PIM levels. Therefore, it is difficult to predict conditions of the Creative Commons whether a product is qualified or not from a theoretical perspective. For antennas with Attribution (CC BY) license (https:// tight PIM specifications, measurement is the most effective means of evaluation. creativecommons.org/licenses/by/ As PIM levels are very low compared to the power of carrier, the test results are prone 4.0/). to interferences from external signal sources. Therefore, it is necessary to ensure that there Electronics 2021, 10, 770. https://doi.org/10.3390/electronics10070770 https://www.mdpi.com/journal/electronics Electronics 2021, 10, 770 2 of 15 are no interfering signals in the environment during measurement. In reality, it is difficult to guarantee a completely interference-free environment. Hence, the device under test (DUT) is usually measured in a shielded anechoic chamber. The anechoic chamber shields the interference from external signals and reduces the space reflection, which is widely used in microwave measurements [15–20]. The use of an anechoic chamber can also greatly improve the accuracy of the PIM measurement, especially for radiating components such as antennas. In addition, the shielding enclosure and absorber of the chamber could also generate PIM signals [7]. The shielding enclosure of the PIM measurement chamber is made of several metal plates, and the connections of the metal plates can be fixed by welding, screws, etc. However, no matter which method is used, contact non-linearity will inevitably occur at the connections and will become a potential source of PIM. The absorber generally consists of polyurethane foam (commonly referred to as sponge) and carbon black or graphite powder. The shaped absorber is obtained by mixing, soaking, and drying the materials, which can lead to the uneven distribution of materials. In addition, there may be incomplete contact between large-sized carbon particles, which form loose clusters. Therefore, the absorber is another potential source of PIM, while the part in contact with the shielding enclosure may also create PIM products. The antenna under test (AUT) radiates electromagnetic waves during measurement. When electromagnetic waves are transmitted to the absorbers, the absorbers may generate PIM. At the same time, a portion of the energy will produce another PIM product on the shielding enclosure. These PIM products can be received by the AUT and increase with increases in the gain of the antenna, which could artificially inflate the measured PIM of the AUT. In this situation, some antenna products that originally meet the PIM performance indicators will be mistakenly regarded as substandard products, resulting in a reduction in the efficiency of industrial production and a waste of resources. Currently, the manufacturers of PIM measurement chambers only provide a single residual PIM index. This indicator is not a problem in the measurement of non-radiating devices; however, it is not appropriate for antennas, as the increase in the gain of the AUT could cause an increase in the residual PIM level of the chamber. Therefore, for antennas, a reasonable residual PIM level of the chamber should be accompanied by some sort of index related to AUT gain. We thus propose an index denoted the maximum measurable gain (MMG). The AUT should not produce a PIM value higher than the residual PIM index in the PIM test chamber. The MMG can then be defined as the maximum antenna gain that satisfies this condition. The novel concept of MMG proposed in this paper is critical for the evaluation of the PIM performance of a chamber for antenna PIM measurements. However, according to our knowledge, the recent research on PIM is mainly focused on analyses of PIM models [21–23] and PIM test devices [24–27]. At present, no one has proposed a similar idea of defining the MMG of the PIM measurement chamber, and thus there is no corresponding method of estimating the MMG. The direct method of evaluating the MMG of a PIM measurement chamber is to measure corresponding high-gain, low-PIM antennas. However, this is not a simple task. First, designing high-gain antennas with low PIM is difficult, as high-gain antennas often have complex metal structures and feeding networks. Solder joints, metal connections, and substrates can cause non-linearity, and generate more PIM products. Secondly, high- gain antennas typically have a more narrowly radiated main lobe. It would be extremely laborious to scan the entire chamber with a high-gain antenna. Finally, the size of a high-gain antenna is large, which is not convenient for measurement. Hence, we present here a method for evaluating the MMG of a shielded anechoic chamber with a single low-PIM medium-gain directional antenna. The medium-gain directional antenna has three advantages. First, the gain of a medium-gain antenna is generally around 10 dBi, which can be achieved by a smaller unit. Low PIM levels can be achieved owing to its simple structure. Secondly, the beam of a medium-gain antenna has a certain directivity, and its side lobes and back lobes are small, which can clearly detect the differences in Electronics 2021, 10, x FOR PEER REVIEW 3 of 15 Electronics 2021, 10, 770 3 of 15 be achieved by a smaller unit. Low PIM levels can be achieved owing to its simple struc- ture. Secondly, the beam of a medium-gain antenna has a certain directivity, and its side lobes and back lobes are small, which can clearly detect the differences in performance in differentperformance positions in different within the positions anechoic within chamber. the anechoic Thirdly, chamber.the results Thirdly, of the measurements the results of ofthe the measurements medium-gain of antenna the medium-gain can be directly antenna used can to judge be directly whether used the to chamber judge whether can be usedthe chamber to measure can an be antenna used to measurewith a smaller an antenna gain, which with a reduces smaller gain,the workload.
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