Miniaturized Hard Waveguides in Multifunction Array Antennas

Doktorsavhandling, 2005

Recent advances in technologies of wireless communications systems have led to an increasing desire for several antennas performing diverse functions at different frequencies to be integrated into a common radiating aperture. This gives rise to the so-called multifunction antenna, which affords the saving of scarce premium real estate and can alleviate problems arising from excessive antenna protrusions in aspects such as aerodynamics, aesthetics, and stealth capability in military applications. Where rapid and simultaneous operations are required, electronically phased-steered array antennas using waveguide elements offer an effective and viable solution. This is in virtue of the excellent isolation which waveguides can provide when operated below the fundamental modal cutoff. Miniaturized waveguide elements are thus needed for realizing multi-frequency interlaced arrays. Although to entirely fill waveguides with dielectric material to realize compact waveguides is feasible, the increased attenuation losses and weight may cause problems. An alternative approach is the partially-filled sidewall loaded rectangular waveguide. The advantages of this waveguide include having lower dielectric losses and being lighter than its entirely filled counterpart. These advantages, however, are still trivial. The prominent property which this alternative waveguide possess is that when operated at the so-called TEM frequency, the fundamental mode of this structure displays TEM plane wave properties, i.e. uniform aperture field distribution over the empty region with clean linear polarization, and exhibiting a modal propagation constant that is equal to that of a uniform plane wave in freespace. Therefore, these are called quasi-TEM hard waveguides. Intuitively, these attributes may provide good radiation performance when the waveguides are terminated as open-ended radiating elemental apertures of a planar array antenna. However, they may be advantageous only for broadside array scan and when the radiating aperture entirely fills the unit cell. The latter aspect is certainly violated in interlaced arrays requiring miniaturized waveguide elements. It is then a primary objective of this thesis to investigate the complete and true effects of the TEM modal plane wave property of the hard waveguide elements on the array performance. Results have shown that, even when operated at the TEM condition under broadside array scan, miniaturized apertures of the quasi-TEM hard waveguides lead to poor array transmission. Nevertheless, miniaturization in E-plane is found to be more facilitated than in H-plane. The performance at the TEM condition is optimal for a particular aperture size, this being a crucial finding. When an internal homogeneous dielectric layer is incorporated into each waveguide element to serve as a matching section, very good array transmission can be obtained even for highly miniaturized apertures under a specific set of operating conditions, e.g. matched for broadside array scan and at the TEM frequency. However, the matching is narrowband. Nonetheless, this provides good isolation from other frequencies and allows closely separated bands to be operated simultaneously. Moreover, the matching is fairly well maintained even when the array is scanned off broadside in both principal E and H planes. It is also learnt that the bandwidth of the matching degrades more rapidly when the aperture is miniaturized in H-plane than in E-plane. Although the waveguide elements of the array operating at the lower frequency band are unable to interfere with those of the higher frequency bands, the converse is not true. This may be solved if the waveguides of the lower frequency band have, in addition to a usual lower cutoff frequency, an upper one as well. Novel waveguides with such a feature can be realized by loading the internal walls with metamaterials. This thesis has studied one such structure comprising an array of metallic strip dipoles imprinted on the surfaces of the two dielectric sidewall loadings of a rectangular waveguide. This structure serves as a waveguide filter, displaying a passband within which the lower frequency band can be operated, and an upper stopband in which the higher frequency band may reside. The thesis concludes with a final investigation on the prospect of using an array of open-ended waveguide apertures as a focal plane array feed for reflector antennas. The goal of the study was to ascertain the effects of mutual coupling on radiation efficiency. Although an active uniformly illuminated and linearly phase steered array only has reflection losses but no coupled losses, an array feed whose aperture is synthesized to match the focal plane fields of a reflector antenna has both reflection and coupled losses. Hence, although a desired feed radiation pattern may be synthesized to achieve high feed efficiency, the radiation efficiency has often been neglected. This work elucidates the importance of the latter aspect.