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Mutual coupling between two circular window elements has been reported in various papers and the results indicate that mutual coupling level is greater for the window patch antennas than for the normal patches due to cylindrical waves which are trapped by the metal of the low frequency element and these cylindrical waves are added to the radiative waves to increase the mutual coupling level. Since mutual coupling between antenna elements in an array can be problematical, some techniques will be presented in this paper to reduce the mutual coupling between elements in a multi-band window element depending on the phase cancellation between the radiative and the trapped waves. 1INTRODUCTION: Microstrip antenna technology [1] is continuous to be a growth area in modern system design. One advantage of the technology is the opportunity to combine two or more antennas in one device. In addition, microstrip patch antennas have desirable features such as low cost and light weight which make them extremely attractive candidates for use in a variety of applications. Recently, a novel multi-band microstrip antenna element offering a band separation of 6: 3: 1 has been presented [2]. The design is based on the window concept whereby higher frequency elements can be housed within low frequency elements provided that they are isolated by a window gap in the low frequency conductor. This allows substrate trapped waves to propagate in addition to the radiative waves causing an increase in mutual coupling levels. The results [3] indicate that mutual coupling levels between window elements are higher than for normal elements and increase as the window size reduces. Since high coupling levels between elements in an array can be problematical, some techniques will be presented here to reduce the mutual coupling between elements in the multi-band window element. The main idea is to control the amplitude and phase of the trapped waves so that it can be cancelled by the radiative waves for certain substrate type and at certain separation distance between patch elements. The authors are working with the Technical Research Center of the armed forces, Cairo, Egypt. SIXTH ASAT CONFERENCE 2 4 May 1995, CAIRO CO-7 238 2 -PARAMETERS AFFECTED ON WINDOW COUPLING: Mutual coupling between window antennas shown in Fi.g.1 is assumed to be the sum of the radiative and trapped waves coupling. The parameters that affected on radiative wave coupling have been reported in many papers [4, 5]. In general, radiative coupling increases for higher dielectric permittivity E,, lower separation distance w/X, and higher substrate thickness h/X. In this section, we will concentrate on parameters which affect on the trapped waves coupling. Trapped coupling is due to waves which are guided by multiple reflections between the ground plane and the low frequency patch conductor. This coupling is given by [3]:

REDUCTION OF MUTUAL COUPLING IN A MULTI- BAND MICROSTRIP WINDOW ELEMENT