## 1. Introduction

[2] Planar transmission lines have been widely investigated in the last four decades by many researchers throughout the world [*Gupta et al.*, 1979]. Among these lines, the slotline has been one of the most studied structures. In particular, much effort has been devoted to studying the properties of its dispersion characteristics and characteristic impedance [*Itoh and Mitra*, 1971]. Most of the analyses have dealt with eigen waves, i.e., waves that can propagate along the line and that are described without taking any source into consideration. These studies have mainly focused on the propagation characteristics of the bound mode, as it represents the required line working regime. Like other open planar transmission lines, however, the slotline is capable of exciting leaky waves [*Zehentner et al.*, 1998a]. These waves usually affect the expected propagation behavior of the line, causing strong attenuation of the transmitted signal and other spurious effects.

[3] A qualitatively higher form of analysis takes into account a source that excites waves on the transmission line [*Mesa et al.*, 1999; *Jackson et al.*, 2000]. This analysis makes it possible to compute the actual amplitudes of the corresponding waves excited by a particular source, or at least to know the ratio between bound and leaky modes of various kinds.

[4] Most work related to the excitation of planar transmission lines has focused on microstrip lines, one of the most widely used lines. Waves excited by a voltage source connected into a gap in the strip of this line were studied by *Mesa et al.* [1999], *Jackson et al.* [2000], and *Mesa et al.* [2001]. The model of the wave excitation presented in the above papers is applicable for a number of planar transmission lines after employing the appropriate Green functions [*Neto and Maci*, 2003]. It is known that the bound mode propagates along the microstrip line in a wide frequency band, theoretically at all frequencies, and the leaky modes propagate simultaneously from a certain frequency. However, the slotline has a different behavior, which has been shown, e.g., by the eigenmode analysis [*Zehentner et al.*, 1998a]. The bound mode propagates from zero up to a certain cutoff frequency. At higher frequencies only the leaky and residual waves propagate. An investigation of the fields excited under these specific conditions in the slotline is one of the main goals of the present work.

[5] Thus, this paper presents the results of a study of the waves propagating along a slotline fed by a current source connected across the slot (some preliminary results on this topic were reported by *Kotlan et al.* [2009]). The distribution of the voltage across the slot of the wave excited on the line is calculated using the spectral domain method [*Mesa et al.*, 2001]. This method applied to a slotline is briefly presented, and a code based on it has been implemented. The analytical and numerical studies are complemented and validated with the measurements of the voltage distribution along the line. This experimental study has confirmed our theoretical/numerical predictions that the excited voltage wave is composed of the bound and leaky modes together with the residual wave [*Jackson et al.*, 2000]. All the above results are consistent with the dispersion characteristics of the slotline calculated using the spectral domain [*Zehentner et al.*, 1998a, 2004].