The demand for small, light-weight and multi-functional wireless devices continues to increase, fueled by growth in the commercial and defense industry. Although the RF back-ends allowed for multi-functionality and miniaturization via improvements in semiconductors, the RF front-ends using conventional materials have not evolved analogously. As such, use of novel and periodic materials has become a new frontier for smaller and multi-functional integrated RF devices.
This dissertation presents how to realize unprecedented propagation and radiation properties using periodic anisotropic material arrangements. This was done by manipulating the well-known K-ω diagrams. Our previous designs with engineered anisotropy were shown to lead to high sensitivity and non-reciprocal RF devices. Motivated by these novel properties, in this dissertation, we realize similar propagation behavior using printed layouts and present two different applications. Such printed layouts are attractive for low-cost antennas and were already shown to lead to miniaturization and improved gain-bandwidth performance.
The first design is based on coupled transmission lines (TLs) on a ferrite substrate for guided-wave applications, where strong nonreciprocity is attained in the guided/slow wave region (with |β|>k0, β is the phase constant in the substrate, and k0 is the free space wavenumber) of the K-ω diagram. Previous work has theoretically demonstrated that nonreciprocal slow group velocity (frozen) mode can be supported on this design. To observe its existence experimentally, we constructed 2 printed prototypes and employ "T-matrix method" to determine the dispersion properties by measuring the S-parameters of these finite periodic prototypes. Through careful measurements, the frozen mode was observed to propagate at a significantly slower speed (286 times slower) than the speed of light.
The second design utilizes the same coupled-line geometry to realize nonreciprocal radiation through leaky/fast waves. This was done by tuning the first fast space harmonic (|β-1|≤k0) of the dominant mode to be fast and to exhibit spectral asymmetry at the operation frequency. For proper operation, ferrite needs be biased with an external bias field. However, the nonuniformities in the bias field alter the leaky wave antenna's (LWA) radiation properties and limit its scanning performance.
To reduce the bias field nonuniformities, a new miniaturized LWA is proposed for wide angle scanning via magnetic tuning. This new design is comprised of coupled composite right left handed (CRLH) TLs, which incorporate series capacitors and shunt inductors and lead to significantly miniaturized LWAs. As the bias field nonuniformity is less over the smaller footprint, we were able to scan the beam in the E-plane by 80° by tuning the bias field (i.e. changing the distance between the permanent magnet and the LWA). Clearly, this approach is not practical. As a future direction, a new class of magnetoelectric (ME)-based LWA is proposed for electrical beam-steering. In this design, the LWA is considered on a ME composite film, comprised of ferrite and piezoelectric layers. The voltage VDC applied to the piezo layer changes the magnetic field inside the ferrite layer due to the magnetostrictive strain at the interface. This tunes the permeability of the ferrite to achieve beam-steering.