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State-of-the-art advances in the field of radar technology have made it possible to design noninvasive sensing devices of vital signs. This paper presents the effects of antenna characteristics on an ultra-wideband (UWB) radar-based respiratory rate measurement system for tele-health-monitoring applications. The performance of three types of antenna for a UWB radar respiratory rate measurement system was studied in this paper. In this context, the proposed system was comprehensively evaluated using two experimental sets. First, a mechanical setup was used to provide a series of controlled frequency and distance motions for measurement test. The error analysis for various parameters associated with pulmonary activities such as breathing frequency and chest displacement was performed. Results indicated that the proposed sensing system used effectively as a respiratory rate measurement is less affected and shows less error when an antenna with directive radiation pattern, low cross-polarization, and stable phase center was used. Due to its good radiation characteristics and small form-factor, this antenna was selected to study the respiratory rates of ten test subjects. The results were compared with a standard respiratory-rate measurement using a respiration belt transducer. The measurement results using the proposed and the standard systems showed excellent agreement. The mean error value of the respiratory rate measurements was 0.03 Hz with a statistical standard deviation of 0.03 Hz.

Vivaldi antenna is widely known as a broadband antenna. In this paper, we investigate a modified Vivaldi antenna with improved cross polarisation working in the ultra-wideband (UWB) frequency range (3.1-10.6 GHz) to be used as multiple probes for microwave tomography system. Our study includes investigation of radiation characteristics of the antenna, antenna design steps, fabrication sensitivity effects on the antenna performance and proposing and implementing a twenty-four antenna element system for fast data acquisition, including a novel method for frequency selection in microwave tomography applications. We also studied the fidelity parameter of the antennas inside the twenty-four element setup. The mutual coupling of adjacent elements, in spite of close proximity, is less than -17dB and fidelity variations for the antennas located in front of transmitter are less than 10%.