1032 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 46, NO. 7, JULY 1998 A Versatile Leaky-Wave Antenna Based on Stub-Loaded Rectangular Waveguide: Part I—Theory Paolo Lampariello, Fellow, IEEE, Fabrizio Frezza, Senior Member, IEEE, Hiroshi Shigesawa, Fellow, IEEE, Mikio Tsuji, Member, IEEE, and Arthur A. Oliner, Life Fellow, IEEE Abstract— A new leaky-wave antenna is presented here that possesses many desirable features and is suitable for application to both the millimeter-wave and microwave ranges. These desir- able features, some of which are unusual, include a simple config- uration, a wide flexibility in the range of available beamwidths, the ability to control the beamwidth and the direction of the beam essentially independently, and negligible cross polarization at all scan angles. The antenna structure consists of a parallel-plate stub guide of small height, less than a half wavelength, located off center on the top of rectangular waveguide. The beamwidth is easily controlled from very wide to very narrow by adjusting the stub width or location. Part I presents the principle of operation and the theory, employing a new transverse equivalent network that is accurate, but also simple, so that it permits rapid and inexpensive numerical calculations. Part II describes and derives the modifications required in the theory to account for flanges and/or finite stub length, and Part III is devoted to measurements at -band and from 40 to 60 GHz on both the propagation properties and the radiation patterns; excellent agreement with the theory is demonstrated. Index Terms— Leaky-wave antennas. I. INTRODUCTION A leaky-wave line-source antenna is basically an open waveguide possessing a mechanism that permits a slow leakage of power along the length of the waveguide. This length then constitutes the radiating aperture of the line source and the radiation occurs in the form of a conical fan beam that is narrow in the plane of scan. When the aperture is horizontal and faces upward, the radiated beam is customarily scanned in elevation by varying the frequency. Most of the early leaky-wave antennas were based on closed rectangular waveguide where leakage was produced by intro- ducing a physical cut in the waveguide top or side wall in the Manuscript received April 30, 1996; revised December 2, 1997. This work was supported by the Ministero dell’Universit` a e della Ricerca Scientifica e Tecnologica and the Consiglio Nazionale delle Ricerche, Italy, by Doshisha University’s Research Promotion Fund, Japan, and by the United States Air Force RADC Contract F19 628-84-K-0025. P. Lampariello and F. Frezza are with the Department of Electronic Engineering, “La Sapienza” University of Rome, Rome, 00184 Italy. H. Shigesawa and M. Tsuji are with the Department of Electronics, Doshisha University, Kyoto, 61003 Japan. A. A. Oliner is with the Department of Electrical Engineering, Polytechnic University, Brooklyn, NY 11201 USA. Publisher Item Identifier S 0018-926X(98)05781-0. form of a long slit or a series of closely spaced holes. Excellent summaries are presented in [1] and [2]. Within the past two decades, interest has shifted to the millimeter-wave range, where the smaller wavelengths and higher waveguide losses introduced new problems and, therefore, new challenges. To overcome these new problems, various novel leaky-wave antennas were proposed, analyzed, and measured based on some low-loss open waveguide designed specifically for use at millimeter wavelengths. Since these waveguides are already open, a physical cut to produce leakage is not meaningful and new mechanisms were tried for permitting the initially bound dominant mode to leak, the most common of which was the introduction of asymmetry. The most widely used of these open waveguides for antenna purposes are the groove guide, the NRD guide, and the dielectric image guide. Summaries of the most successful of these new antennas together with their principles of operation are presented in [3] and [4]. A desirable leaky-wave line-source antenna should have the following properties. It should involve a simple structure, be easily fed by a standard source, have low metal loss, radiate in a single polarization (with negligible cross polarization at all scan angles), possess flexibility in beamwidth (narrow or wide, as we wish), and permit one to change the beamwidth without affecting the angle of maximum radiation. Most of the previous antennas satisfied most, but not all, of these desirable characteristics, but the antenna described in this paper seems to be the best so far in attempting to satisfy all of these features. The actual antenna structure is certainly simple in form (as seen in Fig. 1) consisting of a rectangular waveguide with an asymmetrically located stub guide. The antenna can be fed directly from an ordinary rectangular waveguide source, per- haps with a small modification at the feed junction between the source and the antenna. This question is considered further in Part III, when measurements are discussed. It is interesting that this antenna structure was not conceived directly by us from a rectangular guide, but actually evolved as a modification of groove guide and was discussed in much less complete form at two conferences [5], [6]. It was referred to then as the “offset groove-guide” antenna, where the groove-guide top and bottom stubs were shifted over, or offset, to introduce the required asymmetry so that the initially bound dominant mode would become leaky. It was only later, when we considered 0018–926X/98$10.00  1998 IEEE