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Substrate-Integrated Millimeter-Wave Antennas for Next-Generation Communication and Radar Systems


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microstrip transmission lines, the waveguide‐type transmission line systems are popularly used because they may enjoy the lower losses caused by dielectric and metals at mmW bands [23–27, 31]. Accordingly, for instance, the loss analyses have been conducted for microstrip lines, solid‐metal‐wall waveguides, and post‐wall or laminated waveguide or SIW [32]. The study shows that in general the solid‐metal‐wall waveguides without dielectric loss enjoy less metal loss while microstrip lines suffer from several dielectric losses. The post‐wall waveguides or SIWs feature acceptable total losses caused by both dielectric and metal losses at mmW bands. However, it should be noted that the causes of losses of transmission line systems can be complicated because they will be determined by the materials such as dielectric and metals as well as the types or configurations of transmission lines.

      The transmission line systems can be in the form of microstrip lines and coaxial lines. Compared with conventional cylindrical versions, the substrate integrated coaxial line (SICL) is a type of planar rectangular coaxial lines. The lines comprise a strip sandwiched between two grounded dielectric layers and laterally shielded by the arrays of metallized vias [33]. Similar to the conventional coaxial line, the propagation of SICL is still in the dominant mode of transverse electromagnetic (TEM).

      The SICLs can be realized using a traditional multilayer PCB or LTCC process. Therefore, SICLs feature the combined advantages of the coaxial lines and the planar transmission lines, including the wideband unimodal operation, low cost, non‐dispersive performance, good electromagnetic compatibility, and easy integration with other planar circuits. It has been used for high‐speed data transmission [34] and various other applications such as antennas, couplers, baluns, and filters at mmW bands [35–41].

      It should be noted that the selection of the materials and transmission line systems significantly affects the antenna efficiency. The loss analyses of antennas including their feeding structures are strongly suggested to understand the main causes of the losses in order to control the overall loss by properly selecting the materials and the types of transmission systems, as well as optimizing the design configurations [52].

      As previously mentioned, to compensate for the path‐loss at higher frequencies, usually very large‐scale antenna arrays are required in mmW systems. In such large‐scale antenna arrays the feeding network inevitably becomes complicated with a labyrinth of feeding network. The long current or power paths in the network are the critical causes for transmission losses. The additional unignorable transmission losses may be the stopper to limit the achievement of high gain of larger‐scale antenna arrays when the insertion loss cancels the increase in the gain by increasing the number of the elements of arrays. For example, if the insertion loss caused by the increase of the power path of the feeding network reaches nearly 3 dB, the antenna array with doubled number of elements will achieve very little gain enhancement. Therefore, it is important to check the transmission line systems in terms of insertion loss before the design of the arrays at mmW bands.

      Next, the insertion losses in microstrip‐lines (MSLs) and SIWs in LTCC at 60 GHz are compared as an example. The LTCC is Ferro A6‐M with relative dielectric constant εr = 5.9 ± 0.20, loss tangent tanδ = 0.002 at 100 GHz. The conductor used for metallization and vias is Au, whose conductivity is 4.56 × 107 S·m−1.

Graphs depict the comparison of |S11| and |S21| of a bent MS transmission-line on a LTCC board. Graph depicts the main losses at 60 GHz of a bent MS transmission-line on a LTCC board with varying thickness. Schematic illustration of a 10-mm long bent SIW in a LTCC board. Graph depicts the main losses at 60 GHz of a bent SIW in a LTCC board with varying thickness.