Minimum Number Of Frequencies Research Articles

Reconstructing Dispersive Scatterers With Minimal Frequency Data

Reconstructing the permittivity of dispersive scatterers from the measurements of scattered electromagnetic fields is a challenging problem due to the nonlinearity of the associated optimization problem. Traditionally, this has been addressed by collecting scattered field data at multiple frequencies and using lower frequency reconstructions as a priori information for higher frequency reconstructions. By modeling the object dispersion as a Debye medium, we propose an inversion technique that recovers the object permittivity with a minimum number of frequencies. We compare the performance of this method with our recently developed deep learning based technique (Sanghvi. et al., IEEE Trans. Comp. Imag., 2019) and show that given a properly trained neural network, single frequency reconstructions can be very competitive with multifrequency techniques that do not use neural networks. We quantify this performance via extensive numerical examples and comment on the hardware implications of both approaches.

POISED-5, a portable on-board electrochemical impedance spectroscopy biomarker analysis device

Point-of-care medical devices offer the potential for rapid biomarker detection and reporting of medical conditions, thereby bypassing the requirements for offline clinical laboratory facilities in many cases. Label-free electrochemical techniques are suitable for use in handheld diagnostic devices due the inherent electronic detection modality and low requirement for processing reagents. While electrochemical impedance sensing is widely used in tissue analysis such as body composition measurement, its use in point-of-care patient testing is yet to be widely adopted. Here we have considered a number of issues currently limiting the translation of electrochemical impedance sensing into clinical biosensor devices. Specifically, we have addressed the current requirement for these sensors to be connected to an external processor by applying a minimum number of frequencies required for optimized biomarker detection, and subsequently delivering analytics within the measurement device. The POISED-5 device was evaluated using a sensor for the ovarian cancer biomarker cancer antigen 125 (CA125), demonstrating performance comparable to standard laboratory equipment, with direct interpretation of response signal amplitude substituting traditional impedance component calculation and model fitting.

The effect of multipath distortion on the choice of operating frequencies for high-frequency communication circuits

Harmful multipath distortion on high-frequency facsimile services and telegraphic services operating at high speeds occurs when the received signal is composed of two or more components arriving by different modes over the same great-circle path with comparable intensities, but having travel times which differ by an amount equal to an appreciable fraction of the duration of a signal element. The dependence of multipath distortion on the relationship of the operating frequency to the MUF is discussed and a new term, the multipath reduction factor (MRF), is introduced which permits calculation in terms of the MUF of the lowest frequency which can be used to provide a specified measure of protection against multipath distortion. The MRF has a marked path-length dependence and is calculated as a function of path length for representative values of the other parameters involved by making use of an ionospheric model. It is then shown how the MRF can be used in connection with world-wide MUF prediction material to determine the minimum number of frequencies which must be assigned to a high-frequency communication service of continuous availability operating at high speed. Some comparisons with observations are discussed, and finally conclusions are drawn concerning manner of operation and choice of operating frequencies to reduce or to eliminate harmful multipath distortion.

Vertical radiation and tropical broadcasting

National stations in the Tropics should use short-wave vertical incidence aerials consisting of 16-element binomial arrays; with 5-kW transmitters these would be capable of serving 90,000 square miles of territory. Co-channel stations should be separated by 1,500 miles. The stations require 2 or 3 frequencies in the 2·5, 3·5, 5 and 9 Mc/s bands depending on location; so far as possible the 3·5 and 5 Mc/s bands only should be used. Use of the minimum number of frequencies and frequency changes assists listeners. Stations intended to serve small, densely populated areas can operate in the medium-wave band.