Microstrip leaky wave strip antennas

During the late 1870's. a paper presented by H. Ermert at the European Microwave Conference stimulated instant controversy. The paper was devoted to the properties of higher modes on microstrip line, and one of its conclusions was that a "radiation" region exists close to the cutoff of those modes. Because the description of this region, made in that talk and in published papers [1,2], was incomplete and therefore unclear to many, confusion persisted and certaln practical consequences remained hidden. Also in this general perlod. a paper by W. Menzel [3] presented a new traveling-wave antenna on microstrip line fed in its first higher mode and operated near t~ the cutoff of that mode. Menzel proposed his structure as a competitor to a microstrip patch antenna, and he therefore made his antenna short in terms of wavelength. He also assumed that the propagation wavenumber of the flrst higher mode was real in the very region where Ermert said no such solutions exist; since his guided wave, with a real wavenumber, was f a s t in that frequency range, Menzel presumed that i t should radiate. His approximate analysis and his physical reasoning were therefore also incomplete, but his proposed antenna was valid and his measurements demonstrated reasonably successful performance. The present paper flrst explains that the "radiation" region of Ermert is directly related to leaky modes, and that such a representation for this region is both convergent and practical. It then shows that Menzel's antenna Ls in reality a leaky wave antenna that was made too short: an accurate analysis of it not only explatns quantitatively the performance features of the antenna, but also indicates how they can be improved. Numerical values are presented to show what performance characteristics can be expected when leaky wave antennas of this class are properly designed.