Suppression of Stopbands in Periodic Leaky-Wave Antennas

Abstract

This paper discusses the suppression of the stopbands in periodic leaky-wave antennas. Periodic leaky-wave antennas (LWAs) usually suffer from a series of stopbands, in which the reflection coefficient is increased and the gain drops drastically, because of the coupling between the space harmonics of almost identical phase constant. The open stopband occurs as the main beam scans through the broadside, which is usually caused by the coupling between the forward traveling and the backward traveling n = -1 space harmonics. Besides, another stopband, which is caused by the coupling between the forward traveling <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$n$</tex> = -1 space harmonic and the backward traveling n = -2 space harmonic, may occur and limits the scanning range in the forward quadrant. This paper presents four methods to completely suppress the open stopband in periodic LWAs, including the unit cell of dual non-identical element, the shorting-via matching, the tapered half-wavelength transmission line, and the profile modulation method. With the open stopband suppressed, the main beam scans continuously from the backward endfire to the forward direction. Besides, the profile modulation method can be developed to suppress the n = -2 space harmonic. With the suppression of the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$n$</tex> = -2 space harmonic, the related stopband and the grating lobe are suppressed accordingly, and therefore the scanning range in the forward quadrant is enlarged.