Broadband Suspended Plate Antennas
Many techniques have been developed to broaden the impedance bandwidth of single-element single-layer microstrip patch antennas as discussed in Chapter 2. The use of thick dielectric substrates is a simple but effective method to enhance the impedance bandwidth of a microstrip patch antenna by reducing its unloaded Q. However, it is important to note that as the impedance bandwidth increases, surface wave losses also increase owing to the thick dielectric substrate which reduces the radiation efficiency. A practical method to suppress the surface waves is to lower the permittivity of the substrate. The medium between the planar radiator and ground plane may be a low-permittivity or air-filled substrate such as foam (∈r ≈ 1.07), or even air. As described in Chapter 1, so-called suspended plate antennas (SPAs) with thicknesses ranging from 0.03λ1 to 0.12λ1 (λ1 is the wavelength corresponding to the lower edge of the well-matched impedance bandwidth) and a low relative dielectric constant of ~1 have a broad impedance bandwidth and unique radiation performance. SPAs give rise to special design considerations, although they are essentially a variation of microstrip patch antennas defined by the IEEE Standard 145-1993: a thin metallic conductor bonded to a thin grounded dielectric substrate.
SPAs are not difficult to fabricate. The necessary elements include a ground plane, a radiating plate, a feeding structure, and low-permittivity ...