Abstract
Gap-waveguides made of bed-of-nails are revisited with special interest given to the maximum practically achievable gap height. In the cases where the gap height is to be maximized, it is important to have excitation techniques that are also conforming to the same gap height. To achieve this, various forms of gap-waveguides are investigated. The most suitable form is found to be the groove-gap-waveguide with top/bottom excitation. This type is called here separated-gap-waveguide (SGW). The maximum theoretical gap height between the top surface of the nails and the cover plate is a quarter wavelength. However, this is achieved at the expense of usable bandwidth. A practical experiment is presented to validate this feature. To increase this theoretical limit, the SGW made of bed-of-helical wires (springs) is investigated. In this case, the frequency bandgap is found to be in the vicinity of the first resonance of the helical wires. The benefits of using bed-of-springs instead of bed-of-nails are exposed in this article. The theoretical analysis needed to justify the use of the springs despite their difficult implementation is discussed. The benefits include minimizing the height of the bed structure itself while adding the saved height to the gap height. Furthermore, the use of bed-of-springs allows the frequency of operation to be much lower compared to the use of bed-of-nails. A comparison between the two structures is presented based on experimental results.