*Related Work*

In literature, a number of structural health monitoring (SHM) techniques exist to evaluate the integrity of pipes aiding in improving safety and lifespan of the pipeline infrastructure [5,6]. The different indicators that are monitored for elucidating the pipeline health are cyclic pressure, surface temperature, humidity, and vibration [5,6]. Among these indicators, pressure is the most important and found to be the primary cause of damage to the pipelines [7].

A number of techniques have been proposed to monitor the pipeline pressure; such as piezo based resistive sensors, micro-electromechanical systems (MEMS)-based capacitive sensors, and fiber Bragg grating (FBG)-based negative pressure sensors [7–10]. The fabrication process of all the sensors is very complex, which either requires a hefty clean room process or very expensive tools for precision. For example, the MEMS-based capacitive sensors require expensive substrate material, chemical etching, wire bonding, and separate housing [10]. Additive manufacturing is a viable alternative of this process, which has been previously proven for electronic system and component design [11]. Three-dimensional printing technique was previously used for designing multiple components in a pressure sensor [12–14]. However, a fully 3D-printed pressure sensor is still a feat to achieve. Most of the published research is focused on various other applications and gas pipeline is not the primary focus, which leads to over complexity, limited communication range, and scalability issues.

One of the major limitations of these sensor units is that a direct wired connection is required for data acquisition. The direct probing or wired connection limits the application of the sensors and poses a problem for inaccessible environments, like under ground buried pipelines or large infrastructure with a requirement of thousands of sensing nodes.

A number of wireless sensing approaches have been proposed as an alternate solution to overcome the disadvantages of the wired sensing methods [15,16]. The wireless methods monitor the pressure in the pipeline by deploying a large number of active sensor nodes. The integrated nodes on the surface of the pipes transmit the data wirelessly to the interrogator. The disadvantage of active sensors is the need for an on-board battery, charging and/or replacement requirements increase the implementation and maintenance cost of the system [15–17]. Battery-free, low-cost, real-time sensing is ideal to provide a robust and economical system with longer life span and lower maintenance [18,19].

Inductor-capacitor (LC) resonator-based passive sensor tags have been proposed in literature for monitoring the pressure based on capacitive loading, which in turn shifts the resonance frequency [20]. LC resonant sensors operate in near-field configuration and have a limited interrogation distance [21]. The conventional far-field passive pressure sensors operate at a single frequency, which are prone to clutter due to multi-path interference and have a lower signal-to-noise (SNR) due to self-jamming [18]. To overcome these limitations, the harmonic communication approach is desired, that eliminates self jamming, reduces multi-path interference, lowers down phase noise, and increases the SNR. Harmonic-based sensors have been reported in literature for sensing temperature, and pH [22,23]. In this work, a passive harmonic RF tag with integrated pressure sensor is presented for the first time for the real-time monitoring of the pipeline infrastructure.

In this paper, a harmonic RF system is proposed for gas pipeline infrastructure that consists of an RF tag coupled with a pressure sensor and an RF interrogator. The harmonic RF sensor tag has a receiver (2 GHz) and transmitter (4 GHz) antenna, a harmonic doubler, and an integrated pressure sensor with the phase shifter. The integrated pressure sensor is designed with a cavity based parallel plate reactive impedance element/sensor that changes its reactance with the change in applied pressure from 0–20 psi. The designed sensor is fabricated using the additive manufacturing technique, which uses thin liquid crystalline polymer (LCP) substrate as its diaphragm. The phase shifter translates the change in reactance to the change in received 2 GHz signal's phase, with the power efficient modulation technique, which is backscattered at harmonic frequency of 4 GHz. A schematic of the proposed sensing system over the pipeline infrastructure is shown in Figure 1.

**Figure 1.** Proposed passive wireless sensing system for the pipeline infrastructure.
