## 1 Introduction  
The development of sub-THz antennas (above 100 GHz) offering the crucial characteristics of narrow-beam, high efficiency and wide-beam scanning capabilities has attracted much attention in recent years [1, 2]. They have the potential to be used in recent emerging applications including high data-rate communications [3], speech recognition radar systems [4], remote sensing of terrestrials [5] and molecular spectroscopy [6]. In this regard, a leaky wave antenna (LWA) as a frequency scanning radiating system is a promising candidate due to its simple feeding structure along with a high gain and a wide band performance [7, 8]. There are two forms of LWAs based on geometry, termed uniform and periodic. For the uniform LWA, the continuous perturbation exists along the guiding structure, and for the periodic LWA, periodic perturbation of the waveguide is required to create a leaky-wave mode radiation [9]. One advantage of the periodic LWA is its backward to forward scanning capabilities. In contrast, a uniform LWA can only scan in the forward quadrant [9]. To date, various perturbed guiding structures to realize periodic LWAs have been reported in the literature [10, 11, 12, 13, 14, 15, 16, 17]. Transverse slotted metallic waveguides including rectangular waveguides [10], substrate integrated waveguides (SIW) [11, 12, 13], a microstrip line (ML) [14, 15] or coplanar waveguides (CPW) [16] as a simple structure of leaky-wave antennas were investigated and realized for their microwave and millimeter-wave applications. However, at high frequencies above 100 GHz, LWAs based on such metallic waveguides are not a popular solution owing to extreme conductor losses and the generation of surface waves [18]. Moreover, for a single mode operation, their sectional dimensions must be in the order of one-tenth of a wavelength, which makes their fabrication challenging [18]. To overcome these weaknesses at sub-THz frequencies, a dielectric waveguide with superior loss characteristics is effectively used to develop LWAs [19; 20; 21; 22; 23; 24; 25; 26; 27; 28]. Dielectric waveguides are a different class of transmission lines that include one or more insulating dielectrics to guide the electromagnetic wave. Multiple dielectric layers are generally used for more sized-reduction purposes. The transmission mechanisms of dielectric waveguides are based on reflections at the dielectric and air interfaces without requiring any conductors; hence, it inherently eliminates conductor loss [29; 30; 31]. However, some dielectric waveguides use a metal ground plane for the purpose of high levels of integration. A dielectric image line (DIL), as an example of such a transmission line, is more compatible with a planar circuit integration and has been successfully used in millimeter-wave (mm-w) and sub-THz integrated circuits. It significantly reduces the electric current on the ground plane by concentrating the electromagnetic energy along the dielectric interface, thus allowing the transmission with an extremely reduced conductor loss [32]. By introducing a periodic perturbation along the DIL as a semi-open structure, power leakages will happen, and radiation can be produced in a controlled manner to satisfy specific applications. To date, various LWAs have been proposed in the literature based on the periodic perturbations of the DIL either by dielectric gratings or metallic loadings [19; 20; 21; 22; 23; 24; 25; 26; 27]. Dielectric grating LWAs usually do not radiate in the exact broadside frequency direction as the internal resonance prevents radiation there, which is called the open-stop-band (OSB) phenomenon. These LWAs also suffer from fabrication complexity owing to different types of dielectric grating [19; 20; 21; 22]. In [23], the coupling of sinusoidal metallic gratings with the narrow face of a dielectric line was studied to avoid the OSB problem. The metallic perturbation on the top of a DIL is a second method for leaky wave generation in a DIL environment [24; 25; 26; 27].