International Symposium on Antennas and Propagation
Optimization of Microstrip-to-slot Transition for Ultra-wideband Bulk LTSA
Damri Radenamad, Akira Hirose,
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Tapered slot antennas (TSAs) are used in many applications for example Ground penetrating radars (GPRs)  and millimeter-wave (MMW) imaging for concealed weapon detection . To operate the TSAs, it is necessary to feed them via transitions such as coplanar-to-slot  or microstrip-to-slot  to guide unbalanced signal to feed TSA which requires balanced feeding. In the MMW front-end, a bulk linearly tapered slot antenna (Bulk LTSA)  is proposed to be used as the receiving antenna. Its impedance depends on its fin thickness. The bulk LTSA enables the direct connection between the receiving antenna and a detection circuit without the impedance matching circuit by using the optimized fin thickness to avoid the loss. However, to use the bulk LTSA as the transmitting antenna, we need a balun structure to feed the LTSA antenna. We prefer microstrip-to-slot transition to coplanar-to-slot transition in order to avoid the loop height of the wire bonding assembly that influences the antenna properties such as the radiation pattern and return loss. In addition, the measured fractional bandwidth based on S11 of microstrip-to-slot transition in  is less than 0.25 although its transmission loss is low, around 1.7 dB at the center frequency. This transition also needs a via hole which works as the quarter-wavelength short stub to achieve low reflection loss in short frequency rage. In this paper, we investigate the bulk LTSA fed by microstrip-to-slot transition. We find that two via holes in the microstrip-to-slot transition can reduce reflection in wide frequency range and thus we can obtain an ultra-wideband characteristic. Numerical analysis shows a fractional bandwidth of 0.4. We present the effect of microstrip-line width to improve the reflection characteristics. The results indicate that we can obtain an ultra wideband by expanding the microstrip-line width wider than 2.8 mm.