9.1 INTRODUCTION

Geometric simplicity, efficiency, polarization purity, conformal installation, and ability to radiate broadside beams and vertically polarized E-plane beams at very near grazing angle above a ground plane make slot-antenna arrays ideal solutions for many radar, communications, and navigation applications. Especially today with the desire to make antennas for aircraft as low profile as possible to reduce drag and conserve fuel, slot antenna arrays can be positioned above wings and on top of the fuselage while having the capability to look toward the horizon. Classical slot arrays are depicted as narrow nonconductive slits etched or milled into the host metallic ground plane. They are characterized by the methods used to feed or excite the slots. Without what is behind the slot opening being seen, slot radiators appear similar on the surface of the ground plane. Narrow conformal slots tend to be narrowband (< 5 percent f ₀) and have high cross-polarization isolation when operating near their resonant frequency. Wider slots can exceed an octave bandwidth given a well-matched feed. However, polarization purity is usually not as good as with the narrower slots. Conformal slot arrays are generally limited in bandwidth because the array lattice spacing has to be large enough to accommodate the waveguide and feed structures behind the slots without creating grating lobes.

Conformal slot elements can be fed in a variety of ways: (1) tapping into a transmission line such as a waveguide, (2) coupling to a resonant cavity,...