Analysis of Radial Line Slot Antennas with Dielectric Layers Over the Aperture Kensuke Miyachi, 1 Jiro Hirokawa, 2 and Makoto Ando 2 1 NTT DoCoMo, Inc., Yokosuka, 239-8536 Japan 2 Department of Electrical and Electronic Engineering, Tokyo Institute of Technology, Tokyo, 152-8552 Japan SUMMARY The slot coupling characteristics are analyzed and the aperture distributions are evaluated for several dielectric layers loaded on the upper surface outside the radial line slot antenna. First, for the design of a uniform aperture distribution, mutual coupling is expressed approximately by an infinite array of two-dimensional slot pairs with equal- amplitude excitation in a model with periodic bound- ary walls in the two directions of an external region. Based on this approximation, the slot length and the pair spacing in the radial direction are controlled so as to design an array with a uniform aperture distribution. Next, as a model effective for an arbitrary distribution over a finite aperture, a slot array that is finite in one dimension is analyzed by the spectral domain approach. Also, if the situation with dielec- tric loading through a gap such as a radome is considered, it is shown that the effect on the aperture distribution can be neglected if a dielectric layer with a thickness reducing the Fresnel reflection coefficient is placed about 1/8 wave- length away. The validity of the analysis and design is confirmed by trial fabrication and measurement at 38 GHz. © 2003 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 87(3): 19–27, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.10169 Key words: radial line slot antenna; dielectric layer; spectral domain approach. 1. Introduction The radial line slot antenna (RLSA) [1] can realize arbitrary excitation distributions by controlling the slot length and the spacing in the radial direction of the slot pairs that are the radiating elements for circular polarization. Its low profile, high power handling, large diameter, low loss, and high efficiency are likely to result in new applications of this antenna as a microwave source for plasma generation [2, 3]. In plasma generation, a glass vacuum window and a plasma layer exist on the antenna aperture. On the other hand, when the use of a radome is considered for commu- nications applications, a dielectric plate in a gap is used. These examples are modeled as shown in Fig. 1, where several dielectric layers exist on or near the RLSA aperture. In an application with a radome, the first layer is an air layer. It is therefore necessary to perform design and evaluation of the antenna while taking into account the effect of the dielectric layers on the slot coupling. In the present paper the slot coupling is analyzed for an RLSA loaded with two dielectric layers on the upper surface of the exterior, as shown in Fig. 1. According to the conventional design of a large-aperture RLSA, the circular array is approximated by a rectangular array. First, as a simple design model for a large-diameter uniform aperture, which is used for many applications, the effect of the dielectric on the coupling in a single slot pair is evaluated by an analytical model with periodic boundary conditions for the exterior region. The external mutual coupling is approximated by the value in a two-dimensional slot pair array with infinite size. Based on this model, the slot length © 2003 Wiley Periodicals, Inc. Electronics and Communications in Japan, Part 1, Vol. 87, No. 3, 2004 Translated from Denshi Joho Tsushin Gakkai Ronbunshi, Vol. J86-B, No. 3, March 2003, pp. 511–518 19