A novel self-packaged microstrip line

A new low-loss quasi-transverse electromagnetic (TEM) microstrip line, called as a self-packaged microstrip line, is proposed for microwave and millimeter-wave applications. It is fully based on the low-cost PCB process, but without the issues of radiation and surface waves by using a top layer perfect magnetic conductor (PMC) substrate. The self-packaged microstrip line consists of three layer substrates: a bottom layer to place the metal strip, a top layer to build the PMC shielding with the periodic plated vias, and a middle layer to separate the PMC layer from the metal strip and base ground plane. The ground plane works as an ideal PEC creating a PEC-PMC structure. It is evident that, within the band of interest, the space radiation, surface waves, and cavity resonances appearing in the standard microstrip line (without/with a bulky metallic shielding box) can be efficiently suppressed by the top-layer PMC shielding when a proper gap height is implemented [1]-[4]. As a result, a thicker bottom layer substrate can be applied in the packaged microstrip line to reduce the conductor losses; and no issues of interference or crosstalk will arise between the adjacent circuits/components [5], [6]. Therefore, this self-packaged microstrip line is very helpful to improve the microstrip circuit performance, such as the gain, bandwidth and radiation efficiency of the microstrip antenna, and the passband/stopband characteristics and compactness of the microstrip filter, in particular at high frequencies, due to the reduced attenuation and the absence of surface waves (and/or radiation) [7]-[9]. Indeed, the suppression of the surface waves in the substrate can also be realized by electromagnetic bandgap (EBG) or defect ground structure (DGS), because of the high-impedance surface [10], [11]. However, an additional metal shielding that surrounds a microstrip-EBG/DGS circuit is still required to prevent the space radiation. Otherwise, significant radiation will increase the insertion loss and additionally cause the unwanted RF interference among the adjacent circuit elements. Moreover, such packaging is very complicated, since a recessed region is required in the metal bearing carrier under the defected ground slots, and the resonance frequency of the DGS depends on the dimensions of this region [12], [13]. So, it can be seen that the proposed novel self-packaged microstrip line perfectly merges both the merits of suppressing the radiation in space and the surface waves in the substrate, which makes it very attractive for the high cost-effective circuit design in both performance improvement and size reduction.