Photonic bandgap (PBG) technology for antennae

Photonic band gap (PBG) technology for antennasL. J. Jasper and G. T. TranArmy Research Laboratory2800 Powder Mill Rd., Adeiphi, MD 20783-1197ABSTRACTUsing the new and powerful technology of photonic band gap (PBG) engineering to produce planar format an-tennas allows designers to shape antenna characteristics and improve their functional capabilities. In this paper,we describe both analytical and experimental work performed to develop ultra-wideband (> octave) PBG anten-nas, and to characterize frequency/bandwidth-agile PBG substrates. We also highlight new and interesting PBGconcepts that should offer additional capabilities and applications, once they have matured and are commer-cially viable.Keywords: photonic band gap, lattice, phased-array, dielectric substrate, photoconductors, antennas1. INTRODUCTIONPhotonic band gap (PBG) structures have pass and stop bands with respect to electromagnetic (EM) radiation.A one-dimensional (1-D) PBG structure has dielectric layers with an index of refraction that alternates betweena high and low value. PBG structures are designed to have overlapping (forbidden range of frequencies) bandgaps in each direction of propagation. A two-dimensional (2-D) PBG structure forbids EM propagation at fre-quencies within the band gap for all directions in a 2-D lattice. A three-dimensional (3-D) PBG forbids EMpropagation at frequencies within the band gap in all directions in a 3-D lattice. The first experimental validationof this concept was published by Yablonovitch and Gmitter [1], and the first correct theoretical model was de-veloped by Leung [2.Planar antennas fabricated on conventional dielectric substrates have evoked tremendous interest because oftheir compatibility with integrated-circuit technology, and because they can be attached flush with the outside ofa vehicle or ordnance. However, it is well known that such antennas are highly inefficient radiators. This ineffi-ciency is due primarily to the fact that the antennas radiate much more efficiently into the dielectric substratethan into air. Energy radiated into the substrate is lost to surface-wave and leaky-wave excitation, which causescoupling to other components of the circuit and generates cross-talk and noise. A PBG substrate is an excellentplanar antenna substrate, since radiation is not allowed to couple into the substrate over the band gap, leading toa significant enhancement in radiation efficiency. This concept has been demonstrated previously for time-harmonic operation using a bow-tie antenna on a 3-D PBG substrate [3].In this paper, we describe our investigative techniques for producing PBG structures with ultra-wideband(UWB) characteristics and frequency/bandwidth agility. We examine several new types of PBG structures withdistinctive band-gap properties that make them well-suited for UWB and high-intense radiation antennas. Weintroduce a new technique for producing a quasi 3-D PBG structure using the property of a Luneberg lens fordirecting (channeling) EM propagation inside a PBG structure. (We suggest a Luneberg PBG structure in con-junction with internal emitters as a means for microwave generation, power enhancement, and radiation direc-tivity.) Finally, we examine several phased-array schemes in which PBG technology will play a major role.1.1. Design and characterization of a 2-D PBG structure and a PBG-backed antennaThe Army Research Laboratory (ARL) sponsored a program with Polytechnic University with the objective ofdemonstrating a UWB 2-D PBG structure for use as a substrate for a planar antenna. For this study, we took EMmeasurements in order to characterize PBG structures, and to examine their utility for producing highly efficientUWB planar antennas.