Periodic surfaces made of patches with metallized via holes or grounding pins (ldquomushroomsrdquo) have been shown to have bandgaps in their dispersion diagrams, inside which surface waves cannot propagate. These bandgaps are defined in all directions in the surface structure. The purpose of this work is to show how by changing the position of the metallic via of the mushroom from its center to its edge, the bandgap moves towards lower frequencies. This allows an easy structure size reduction which is critical in many applications. Results prove a size reduction of the unit cell size by approximately 20% by only moving the via position. The conclusions have been validated with measurements.

Multifrequency microstrip patch antennas partially filled with left-handed (LH) structures are presented. The presence of LH structures also enables miniaturized antennas. Thus, multifrequency, dual-mode, and miniaturized antennas based on single patches partially filled with LH structures have been obtained. First, multiple operation over several band frequencies is proven from the transmission-line theory. Then, an implementation based on microstrip patches partially filled with mushroom-like structures is proposed. Two multifrequency patch antennas have been designed, built, and measured. The first one shows two dipolar (patch like) modes at 1.06 GHz and 2.16 GHz and a monopolar mode (null at broadside) at 1.45 GHz. The ratio between the resonant frequencies of both dipolar modes can be arbitrarily chosen (by means of a proper design of the patch and the LH structures) and, in this case, is 2.04. The second antenna is designed to work at closer frequencies. In this case, the frequency ratio is dramatically reduced to produce a dual dipolar antenna working at 1.81 GHz and 2.20 GHz (giving a ratio of 1.21).

A novel approach to design dual-frequency printed dipoles is presented. This approach is based on an antipodal printed dipole loaded with split ring resonators (SRRs). This technique allows the choice of any pair of working frequencies. Two prototypes, the first one working at 1.32 and 2.83 GHz and the second one working at 1.2 and 2.05 GHz, have been manufactured and measured. The experimental results show reasonable values for the efficiency at both working frequencies. Moreover, the obtained radiation pattern is dipolar at both frequencies with low cross polarization levels.

The results of a parametric study and design of an ultrawideband dual-polarized array of differentially-fed tapered slot antenna elements are presented. We examine arrays of bunny-ear antennas and discuss the capabilities and limitations of differential antenna technology. As we focus on radio astronomical applications, the absence of a balancing-feed circuit not only reduces the first-stage noise contribution associated to losses in the feed, but also leads to a cost reduction. Common-modes are supported by the antenna structure when a third conductor is present, such as a ground plane. We demonstrate that anomalies may occur in the differential-mode scan impedance. Knowledge of both types of scan impedances, differential and common mode, is required to properly design differential LNAs and to achieve optimal receiver sensitivity. A compromise solution is proposed based on the partial suppression of the undesired common-mode currents through a (low loss) balancing-dissipation technique. A fully steerable design up to 45° in both principal planes is achieved.

We propose a dielectric rod waveguide antenna (DRW) integrated with a photomixer as a THz emitter. This represents a different approach as opposed to the classical solution of a substrate lens. Main goals are an inexpensive alternative to substrate lenses, reduction of both reflections on the semiconductor-air interface and scattering of terahertz-generated power into the substrate. A radiation pattern measured at 137 GHz is shown as a proof-of-concept. In order to increase radiated power, the improvement of the rod antenna is discussed. Finally, as an application example, evanescent coupling of the DRW into a high index whispering gallery mode resonator is shown.

Conventional ultra-high sensitivity detectors in the millimeter-wave range are usually cooled as their own thermal noise at room temperature would mask the weak received radiation. The need for cryogenic systems increases the cost and complexity of the instruments, hindering the development of, among others, airborne and space applications. In this work, the nonlinear parametric upconversion of millimeter-wave radiation to the optical domain inside high-quality ($Q$) lithium niobate whispering-gallery mode (WGM) resonators is proposed for ultra-low noise detection. We experimentally demonstrate coherent upconversion of millimeter-wave signals to a 1550 nm telecom carrier, with a photon conversion efficiency surpassing the state-of-the-art by 2 orders of magnitude. Moreover, a theoretical model shows that the thermal equilibrium of counterpropagating WGMs is broken by overcoupling the millimeter-wave WGM, effectively cooling the upconverted mode and allowing ultra-low noise detection. By theoretically estimating the sensitivity of a correlation radiometer based on the presented scheme, it is found that room-temperature radiometers with better sensitivity than state-of-the-art high-electron-mobility transistor (HEMT)-based radiometers can be designed. This detection paradigm can be used to develop room-temperature instrumentation for radio astronomy, earth observation, planetary missions, and imaging systems.