A woodpile Electromagnetic Bandgap (EBG) material has been designed, by using an in-house code that implements the Fourier Modal Method (FMM). A couple of alumina-woodpile samples have been fabricated. Several results have been collected for the transmission behaviour of the woodpile and of resonators with woodpile mirrors, in a shielded anechoic chamber, by using a vector network analyzer, in the 8–12？GHz range. These new experimental data highlight interesting properties of 3D EBG resonators and suggest possible innovative applications. Comparisons of the collected results with FMM show a satisfactory agreement. An application of the EBG resonator has been considered, for gain enhancement of a microstrip antenna: an increase of about 10？dB in the broadside gain has been measured; experimental data and numerical results obtained with the commercial software HFSS show a good agreement. A comparison is presented between EBG resonator antennas and two-dimensional uniform arrays. Finally, HFSS results are provided for EBG resonator antennas working at higher frequencies or with a more selective superstrate: a gain enhancement of more than 18？dB is achieved by such antennas. 1. Introduction Electromagnetic Bandgap (EBG) [1–3] structures, also called electromagnetic crystals, are subject to an increasing interest for their desirable properties that cannot be observed in natural materials. In this regard, they are classified as metamaterials [4]. The woodpile is an EBG that may present complete three-dimensional (3D) stop bands [5, 6]. It is a stack of dielectric rods with alternating orthogonal orientations and its main advantage, compared to other geometries promoting the existence of complete bandgaps, is that it can be quite easily fabricated even for high-frequency applications. A woodpile can be realized as a sequence of layers, deposited and patterned by lithographic techniques developed for the semiconductor electronics industry [7], or by the extrusion freeforming technique [8]. Woodpile crystals with a complete band-gap centred at 2.35？ m [9] and 1.55？ m [10] have been realized through silicon double inversion of polymer templates and direct laser writing. This kind of EBG has been employed to realize waveguides [11, 12], waveguide bends [11–14], power dividers [13, 14], and as substrate [15] or superstrate [16] for the improvement of planar antenna performances. The introduction of a defect in a crystal interrupts its periodicity and may cause the occurrence of a transmission peak in a stop band: for this reason, EBGs with defects are employed in
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