In the circuit theory, the Norton and Thevenin equivalent generators are tools that simplify the solutions of networks involving passive or active components. They have been extensively used in the frequency domain to describe time-harmonic sources. A time-stepped evolution is instead typically used to include transient sources. As a particular case of the latter, the Norton equivalent circuit is extended here to investigate pulsed photoconducting sources, where a dc bias voltage and a pulsed optical laser are combined to generate terahertz (THz) bursts. The proposed derivation relies on the application of the electromagnetic (EM) equivalence theorem. The main conclusion of this derivation is the understanding that, from the three different spectral regions (dc, THz, and optics), only the THz radiation is to be explicitly included in the equivalent circuit. The theory is validated by a campaign of measurements reported in a connected paper.

The number of independent links that can be hosted by an antenna platform for Line-of-Sight (LoS) communications is limited by its physical size and the interference between the beams associated with different users. For large-size platforms, the interference can be reduced by compromising the aperture efficiency, and this trade-off is the metric to quantify the effective use of the platform. This metric fails for antenna platforms that are not electrically large, for which the aperture efficiency is no longer a useful parameter. Here we resort to the concept of the Observable Field, related to the maximum theoretical directivity, to estimate the potential number of independent links supported by moderate-size platforms. This allows the introduction of coupling coefficients between the beams associated with the observable portion of the incident field and the beams associated with the receiving antennas. These coefficients are bounded to unity for any platform dimension, unlike the aperture efficiency, and they are maximized when the antenna pattern is equal to the pattern predicted by the Observable Field. Accordingly, selecting beams dictated by the Observable Field constitutes a benchmark for the effective use of the volume. Any antenna design can be compared to this benchmark to assess its merits.

The observable field is defined as the portion of the incident field that can contribute to the power received by an antenna. Recently, the observable field was estimated for a plane wave incidence. Here, the procedure is extended to a general incident field expressed as a superposition of homogeneous plane waves. The observable field concept provides a methodology to evaluate the maximum power that could be received by an ideal terminal antenna. In particular, it emerges that to maximize the received power, the pattern in transmission of the antenna should be synthesized to reproduce the angular pattern of the observable field. This is specifically relevant in cases of non-line of sight (NLOS) at high frequencies, where the power received can drop by orders of magnitude. As a case study, we consider a communication scenario which involves a base station and distributed receivers embedded in a complex scattering environment.

Future sub millimeter imagers are being developed with large focal plane arrays (FPAs) of lenses to increase the field of view (FoV) and the imaging speed. A full-wave electromagnetic analysis of such arrays is numerically cumbersome and time-consuming. This article presents a spectral technique based on Fourier optics combined with geometrical optics for analyzing, in reception, lens-based FPAs with wide FoVs. The technique provides a numerically efficient methodology to derive the plane wave spectrum (PWS) of a secondary quasi-optical component. This PWS is used to calculate the power received by an antenna or absorber placed at the focal region of a lens. The method is applied to maximize the scanning performance of imagers with monolithically integrated lens feeds without employing an optimization algorithm. The derived PWS can be directly used to define the lens and feed properties. The synthesized FPA achieved scan losses much lower than the ones predicted by standard formulas for horn-based FPAs. In particular, an FPA with scan loss below 1 dB while scanning up to ±17.5° (±44 beam-widths) is presented with directivity of 52 dBi complying with the needs for future sub millimeter imagers. The technique is validated via a physical optics code with excellent agreement.

A novel solution for the manufacturing of a reflectarray antenna with cosecant-squared radiation pattern is presented. The unit cell simply consists of a 3-D printed nylon grid inserted in between two thick metallic plates. To introduce the required phase delay, C and reverse C slots are laser cut into the plate illuminated by the feeder. This solution allows obtaining an efficient, robust, and compact antenna with low-cost manufacturing process, even for no mass production. Measurements confirm the feasibility of the proposed solution and show its performances.

In millimeter and submillimeter-wave radiometric imaging systems, a persistent goal is the increase in the speed of acquisition of the image while maintaining a high sensitivity. Typically, the highest sensitivity is achieved by cryogenically cooling the detectors, specifically in astronomical applications. However, for the purpose of low-cost imaging applications, it is desirable to operate at room temperature. Without cryogenically cooling, the electronic noise introduced by the detectors becomes dominant, making the detectors less sensitive. Resorting to detection architectures containing amplification circuitry might be impractical for implementation in large focal plane arrays (FPAs) fabricated in integrated technologies. This contribution derives the focal plane architecture that maximizes the imaging speed of radiometers operating at room temperature without using any amplification circuitry. It is shown that in such scenario a practical image acquisition speed can still be achieved when a very broad portion of the THz-band is exploited. Ultimately, the imaging speed is maximized when the FPA is undersampled, implying a tradeoff in the size of the optics. The analysis is substantiated by a case study with recently developed wideband leaky lens antenna feeds operating from 200 to 600 GHz.