International Symposium on Antennas and Propagation
Millimeter-Wave Band Evaluation of Microstrip-Line Transmission Losses using Effective Conductivity Measured by a Whispering Gallery Mode Resonator
Yuanfeng She, Tran Thi Huong, Jiro Hirokawa, Makoto Ando,
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Printed circuit boards (PCBs) are inexpensive and highly reliable. Small via holes can be realized by milling technique. This technique makes PCBs be used to fabricate different kinds of microwave and millimetre-wave circuits, such as antennas, filters, phase-shifters and so on. The roughness of the boundary surface between copper and dielectric for the contact in PCBs, which will degrade the conductivity of the copper equivalently, results in increase of loss. In the millimeter waveband, an open type Whispering Gallery (WG) mode dielectric resonator has been proposed to measure complex permittivity of a dielectric substrate and to get effective conductivity with the consideration of the roughness between the electrodeposited copper foil and substrate . We measure the losses of a microstrip line in order to confirm the effective conductivity results by the WG mode resonator. It can support the reliability of other conductivity results of structure surface situations as post-wall by the same way. A post-wall waveguide antenna consists of densely arrayed metalized via-holes . The waveguide is a closed wave guiding structure free of radiation loss and the losses mainly come from both dielectric and conductor losses. A precise permittivity and conductivity are needed in the design of antennas in the millimeter waveband, although a company usually provides these parameters at certain frequency, 10GHz for mircowave band as an example. The lack of precise estimation of the electric properties of lossy material limits the antenna performance of the so far. We should get more precise data to ensure the accuracy of antenna design, not only the surface between the electrodeposited copper foil and substrate, but also the post-wall surface. A microstrip line as shown in Fig.1 is one of the most widely used planar transmission lines on a PCB in microwave and millimeter-wave engineering, where h is the thickness of the substrate, t and W are the thickness and the width of the strip respectively. A very thin substrate with a thin strip can be fabricated in monolithic microwave integrated circuits (MMICs). By given the dimension of the microstrip line, the characteristic impedance is almost a constant, which makes it convenient to confirm the reliability of the WG mode resonator results in a large frequency band. In this paper, the authors measure the transmission losses of microstrip lines of different lengths on very thin PCBs to evaluate the complex permittivity and conductivity in the millimeter waveband. These losses are estimated by different closed formulas which are compared with the measurement results. In summery, by analyzing the high-frequency-characteristics of PCBs, including the rough surface effect, the edge effect on the current and the dielectric loss, the effective conductivities by the WG mode resonator method in the millimeter waveband have been confirmed. It is a beginning of the future work about post-wall waveguide transmission loss evaluation.