higher than 300 GHz, the LTCC fabrication becomes quite challenging because of its process limit such as via‐hole pitch.
1.6.2 Commonly Used Transmission Line Systems for Antennas
Like any antenna systems, their feeding structures will be a critical issue in the implementation of the antenna design. In particular, the losses caused in the feeding networks greatly degrade the performance of the antenna arrays at mmW bands. Unlike the antennas at microwave bands, the losses can be caused by not only the dielectric substrate but also metals used as conductors for transmission and radiation. Like any dielectric at microwave bands, the loss of a dielectric substrate is measured by its loss tangent. Different from the designs at microwave bands, the metal loss that may be caused by the conductivity and surface roughness of the conductors can't be ignored.
Figure 1.5 Simplified descriptions of PCB and LTCC processes. (a) PCB and (b) LTCC.
Besides 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].
Moreover, the substrate integrated gap waveguide (SIGW) or printed ridge gap waveguide (PRGW) is proposed for the transmission line systems at mmW bands. The SIGW or PRGW is the combination of the microstrip‐line and gap‐waveguide technology based on the PCB or LTCC process [42–44]. The invertedprinted strip line is arranged on or above the periodic mushroom structures where the unwanted surface waves are suppressed and only the quasi‐TEM mode is permitted over the operating band. Unlike SIW or SICL, the top and bottom grounds of a SIGW are unconnected. Therefore, the processing complexity is greatly reduced. The SIGW/PRGW technology has been widely used in the antennas and arrays at mmW bands [44–51].
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].
1.7 Note on Losses in Microstrip‐Lines and Substrate Integrated Waveguides
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.
Figure 1.6 shows a 10‐mm long bent MS transmission line on an LTCC board. The 50‐Ω MSL is with two ports in the simulation. Figure 1.7 compares the insertion losses for varying thickness of the LTCC board over a frequency range of 0–70 GHz. It is seen that when the thickness increases, the insertion at higher frequency edge quickly increases. For instance, the loss per centimeter reaches up to 13 dB when the thickness is larger than 0.7 mm.
Figure 1.8 clearly shows the causes of the insertion losses at higher frequencies or mmW bands. The losses caused by the dielectric substrate and conductor in the system are just a small percentage of the total losses. It is believed that at 60 GHz, the higher‐order modes excited by the discontinuity of the MS cause large surface wave (SW)/leaky losses as previously discussed. This issue is even severer for the thicker substrate. So the SW of MS at mmW is definitely a big problem for practical antenna design.
Figure 1.9 shows a 10‐mm long bent SIW in an LTCC board. Figure 1.10 shows the main losses at 60 GHz of a bent SIW in an LTCC board with varying thickness. It is clear that on the contrary, the SIW system does not suffer from such a dilemma, with the highest loss less than 1 dB per centimeter at smaller thicknesses and total losses lower than 0.6 dB for a thickness larger than 0.3 mm. The low‐loss feature is quite stable for all the thicknesses. But actually for the very thin thickness of 0.1 mm, the conductor loss is high for the SIW. Fortunately, the thickness of 0.5 mm is usually selected for SIW at 60 GHz. In particular, the majority of losses are caused by both the dielectric and conductors, which is different from the MS lines.
Figure 1.6 A bent MS transmission‐line on a LTCC board.
