ISSN 1064-2269, Journal of Communications Technology and Electronics, 2015, Vol. 60, No. 8, pp. 847–851. © Pleiades Publishing, Inc., 2015. 847 1 1. INTRODUCTION Slotted waveguide array antennas are increasingly used in numerous areas of applications mainly in the field of military communications as radar antennas, remote sensing antenna systems, aerospace antennas, as well as in wireless internet distribution systems since they have various advantageous over other competitive antennas such as high power handling capability, low losses and good control over the side lobes. In addi- tion, slotted waveguide antennas are light, compact, easy to fabricate and capable of withstanding radia- tion, thermal and vibration requirements of aircraft applications. Electromagnetic numerical techniques provide the possibility of modeling slotted waveguide antennas as well as optimizing their electrical charac- teristics for further advancements. The aim of this work is to simply analyze and design a high gain antenna with omnidirectional characteris- tics for the use as a distributing antenna for wireless internet communication systems. For such a purpose, many antenna systems have been used in the recent years and many antennas have been recently devel- oped to satisfy the market needs [1–3]. Waveguides have been first used as guided struc- tures with low losses and later have been applied for the use as radiating elements such as open end waveguide radiators, horn antennas and slotted waveguide antenna systems. Due to the fact that the waves are considered as travelling waves in a guide, radiating slots have been placed on the broad wall of 1 The article is published in the original. the waveguide at the points of maximum currents and arranged in a chess order for it to operate as a broad- side array with a radiation pattern having a maximum normal to the plane containing the slots [4] as shown in Fig. 1. It is important to notify here that the radia- tion pattern of the slotted waveguide antenna depends on the number of slots, the position of the slots, the spacing between adjacent slots, and the length of the slots as well as the progressive phase difference between the slots of the array. From Fig. 1, it is obvious that for providing omni- directionality, using regular slotted waveguide anten- nas, it is need to put several such antennas on a tower spaced by an angle in order for each one to cover a cer- tain sector of space as developed and proposed by P. Mondal and A. Chakrabarty [3, 5]. This technique is not economically competitive since the cost of the antenna system will increase. In addition, this will also lead to the appearance of some nulls in the horizontal pattern, so that the omnidirectionality aimed is nega- tively affected. The nulls appearing in the x–y pattern in wireless internet distribution is a major drawback since some of the randomly distributed subscribers might suffer from weak signals or coverage. In this paper a novel approach has been developed for the purpose of obtaining the desired directional characteristics. So, a folded rectangular slotted waveguide antenna made out of 30 resonant slots on the broad wall of the guide in the form of a ring has provided a perfect omnidirectional pattern with a directivity that reaches 15 dB. The maximum size of this antenna does not exceed 60 cm in diameter and Ring-Shaped Omnidirectional Rectangular Slotted Waveguide Antenna with Enhanced Characteristics 1 A. Harmouch and H. Haddad Doctoral School of Lebanese University Tripoli, 1300 Lebanon e-mail: harmush_ah@hotmail.com, hassan.haddad110@gmail.com Received February 5, 2015 Abstract—Antennas are becoming crucial in an increasingly growing number of applications namely with the emerging of the wireless internet communications where efficiency, compactness and cost play a decisive role. Nonetheless a major drawback of such solutions is that the aforementioned criteria are hard to simulta- neously optimize. In this paper an innovative approach relying on using circularly bent rectangular slotted waveguide antenna as an ultimate alternative is presented that allows drastically enhancing the ratio of per- formance versus cost and size. Simulation results were carried out at 2.4 GHz frequency and indubitably dem- onstrated high effectiveness in terms of directivity and high gain given a reduced cost, shape and size. Keywords: Slot, waveguide, omnidirectional, radiation efficiency, compactness, pattern, directivity DOI: 10.1134/S1064226915080082 ANTENNA AND FEEDER SYSTEMS