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24 DESIGN007 MAGAZINE I AUGUST 2020 Key Points • Microwave energy is absorbed by water, fats, and sugars. • Electromagnetic energy travels in a wave through a vacuum or air at the speed of light. • The distance between the peaks and troughs of the energy is a half wavelength. • The dielectric constant of the surrounding materials impacts the velocity of propaga- tion of the signal. • The FR-4 material in a stripline configu- ration slows the propagation speed and decreases the wavelength of the electro- magnetic wave down by about half. • In a microstrip (outer layer), the electro- magnetic energy travels in a combination of prepreg, solder mask, and air, which reduces the effective Dk. • In a stripline, there is a combination of prepreg and core. • The propagation speed of the electromag- netic energy is always faster on the outer microstrip layers than the inner stripline layers. • At high frequencies, short traces (particularly stubs or unterminated traces) on a PCB can act as a monopole or loop antenna. • Trace antennas form a monopole with a quarter wavelength (l/4) at the resonant frequency. • High-frequency components of the funda- mental radiate more readily because their shorter wavelengths are comparable to trace lengths. • Outer microstrip layers will radiate; hence critical, high-speed traces should be avoided on these layers. • A low-Dk material is preferred for high- speed designs. Figure 4: Plot of dielectric constants of high-speed materials (iCD Materials Planner).

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