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36 DESIGN007 MAGAZINE I APRIL 2023 tion performance can be controlled by three main parameters (Lt, Wt and W edge ) which are adjusted to optimize the return loss and inser- tion loss properties. e drawback of the coplanar-to-SIW tran- sition is the limitation of bandwidth due to higher-order mode propagation if the via holes are not placed in close vicinity to the CPW slots. Also, if the via pitch is increased while the via hole diameter is unchanged, the elec- tromagnetic field starts to radiate outside the via hole arrays. is gives rise to leakage loss in addition to the dielectric and conductor loss of the waveguide. e energy escaping the SIW channel is shown graphically in Figure 5 where field propagation outside the functional region of the waveguide is observed. us, it is imper- ative to select SIW parameters such that the leakage loss is maintained within an acceptable range. e via pitch length, the hole diameter, and their ratio prove to play a key role in con- fining the fields in an optimal manner. Substrate-integrated waveguides are simi- lar to traditional waveguides in terms of their ability to support the propagation of electro- magnetic waves with low loss and dispersion. However, SIWs achieve this performance in a planar structure by using a substrate material with a low Dk and a low Df. SIWs can be used in a wide range of µWave applications, such as filters, couplers, power dividers, and anten- nas. e combination of microstrip, CPW, and SIW technologies provides designers with a high degree of flexibility in the design of complex µWave/mmWave systems. Overall, the design of RF circuits requires specialized knowledge and techniques that are tailored to the unique challenges of high-frequency cir- cuit design. Key Points • Wireless technologies require more stringent control of electromagnetic fields as they tend to radiate more—particularly on microstrip (surface) layers. • An RF circuit is a special type of analog circuit operating at very high frequencies suitable for wireless transmission. • e most effective board stackup for RF design is to have a ground reference plane immediately adjacent to the surface layers and to keep the RF traces on the surface as much as possible. • To prevent unwanted coupling, use blind microvias from both sides, effectively making two separate back-to-back boards. • At high frequencies, microstrip lines can suffer from significant signal loss due to radiation and dielectric losses. • CPWs offer lower radiation loss and are becoming a popular alternative for high-frequency digital circuits. Figure 5: Electric field comparison for two SIWs with different via spacing. (Source: Taringou 4 )