**Deepak Kumar \*, Saikat Mondal, Yiming Deng and Premjeet Chahal**

Department of Electrical and Computer Engineering, Michigan State University, East Lansing, MI 48824, USA; mondalsa@msu.edu (S.M.); dengyimi@egr.msu.edu (Y.D.); chahal@egr.msu.edu (P.C.)

**\*** Correspondence: kumarde2@msu.edu

Received: 12 November 2020; Accepted: 24 November 2020; Published: 27 November 2020

**Abstract:** In this paper, an efficient passive wireless harmonic communication system is proposed for the real-time monitoring of the pressurized pipelines. A pressure sensor is fabricated using the additive manufacturing technique and a harmonic radio frequency (RF) tag is designed to operate at the fundamental frequency (*f*𝑜) of 2 GHz that shifts the phase of the back reflected RF signal according to the applied pressure ranging from 0 to 20 psi. A power efficient phase modulation with virtually no losses is achieved using a hybrid coupler-based phase shifter that efficiently reflect back the incoming signal using an end coupled reactive impedance element/sensor. The phase delay introduced by the reactive element gets doubled with the second harmonic communication, which increases the sensitivity by a factor of two. The concept of harmonic backscattering is exploited to reduce the effects of multi-path interference and self jamming, as well as improving the signal-to-noise ratio (SNR).

**Keywords:** passive; pipeline; pressure; reactive impedance sensor; structural health monitoring; wireless

### **1. Introduction**

Pipelines are the safest and economically viable arterial networks for transporting natural gases and oils across the globe. The pipeline network expands at a faster rate due to the projected growth in population and increased rate of urbanization, creating a demand in supply of natural gas for domestic and industrial use [1]. The natural gases are gathered, transported, and distributed at various pressure levels via metals, composites, and plastic pipelines. Any material subjected to a high or a low cyclic pressure over a prolonged period of time induces a stress on the material. This stress over time compromises the safety and reliability of the pipeline and may lead to a catastrophic failure [2–4]. In order to prevent such failures, there is a growing need for the development of economical, real-time, scalable, structural integrity monitoring, and sensing system for the pipeline infrastructure implemented in refineries, chemical plants and manufacturing facilities.
