Propagation and narrow cylindrical antennas for non-line-of-sight links

Marconi’s century-old commercialization of wireless has grown to billions of radio links. In cities there can be thousands of cellular base stations, usually mounted on buildings, to link millions of terminals, and there are many WiFi devices in homes and offices. These links are nearly all non-line-of-sight (NLOS), with signal processing at the terminals striving to cope with the degradation from the propagation and the antennas. The signal processing, antenna design, and the propagation, are now separate disciplines as a result of their expansion. The limitations from the propagation channels and the antennas are often blindly accepted by signal processing. Innovations become most likely when there is an in-depth understanding of each discipline, an increasingly difficult prospect. But no matter how powerful or innovative the electronic signal processing, the propagation and antenna performance remain the biting constraint for communications performance. This motivates a hypothesis: improving the understanding of the bottleneck mechanisms - the propagation and antennas - enables innovation for better link performance. The approach is to select topics in propagation and antennas which bottleneck the link performance. For NLOS, diffraction is the critical mechanism. The thesis therefore opens with a look at diffraction, in the context of two applications: classical around the corner propagation, where simple arrangements of passive dipoles are demonstrated to drastically improve a diffraction-limited link; and through-forest propagation, where a new model, combining diffraction across the tree tops and direct transmission, is demonstrated to fit the full range of short- to long-distances established from recent experiments. For the antennas, tubular platforms offer challenges which have not been widely addressed, and yet such platforms are ubiquitous in the form of bicycle frames, drone struts, and masts. Designs are investigated where compactness is a critical requirement: externally-mounted, small narrowband antennas for where the curvature of the cylindrical tube is too small for planar groundplane principles to guide the design; and configurations that deploy the tubular structure as a compact coaxial cylindrical waveguide, to feed slot elements in the cylinder. These are demonstrated to be extremely wideband and have low loss at microwave frequencies.