# Microwave NDT of Smart Composite Structures with Embedded Antennas
## 1 Introduction  
Antennas are extensively used in aerospace and space platforms for various applications encompassing radars, communication, positioning, etc. The concept of embedding RF antennas onto load-bearing aircraft skins was introduced a couple of decades ago for aerospace and defense applications and since then there has been keen interest concerning the integration of antennas in composite structures to improve structural efficiency and create smart structures. The term conformal load-bearing antenna structures (CLAS) is often used to describe such structures [1; 2; 3]. Over the years, several developments have been proposed in this regard such as surface antenna structures, composite smart structures and 3D integrated microstrip antenna structures to further enhance structural and electrical efficiency [4; 5; 6; 7; 8]. Recently, a frequency-modulated continuous wave (FMCW) radar front-end was embedded inside an aerospace composite [9]. The manufacturing process for highly integrated embedded antennas is very complex and can lead to irregularities. Furthermore, such structures are always prone to impact damage, loading, vibrations and other factors that could threaten the structural integrity of the component. This could inevitably affect the performance of the structure and may lead to the complete failure of the part. Therefore, ensuring the quality of the build and integrity of the antenna before putting it into service is essential. Also, given the nature of the composite antenna itself (a woven mesh) it could be damaged whilst in service, thereby requiring subsequent localizationand repair. This can be accomplished through non-destructive testing (NDT), which is well established and renowned. The study of antenna performance in the presence of impact damage, delamination and dynamic loading has been undertaken by various researchers in the past [10; 11; 12]. The metric to gauge the structural integrity of the antenna-embedded composite was largely based on the antenna measurement characteristics, such as radiation pattern, return loss, etc., all of which require a special infrastructure, equipment and the isolation of the structure during testing. The in situ inspection of antenna-embedded composite structures has not been explored in the literature, to the best of the authors' knowledge. Distinct advantages are associated with in situ monitoring, including the prediction of any impending failure in the structure and a relatively short inspection time frame. Within the realm of NDT, there are numerous techniques that can be employed to perform the structural integrity assessment of structures under test (SUT). However, with regard to the inspection of composite structures, several NDT modalities have limited applicability as highlighted multiple times in the past [13; 14; 15; 16; 17]. X-ray computed tomography (X-ray CT) is the gold NDT standard in this application for both quality check/control and inspection. X-ray CT is expensive and the machine can only accommodate patches (not an entire functional surface), which increases the need for an alternative method. As a result of their inherent advantages, microwave and millimeter waves have been successfully employed in the past to non-destructively test composites of various types including fiberglass, honeycomb, radome, etc. using waveguide apertures and planar resonant probes [17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27]. However, the microwave inspection of antennas embedded in composites is new and has not been investigated before. In this paper, we introduce microwave non-destructive testing as an alternative method for the inspection of CLAS for the first time. To illustrate the utility of microwaves for antenna-embedded structures, a microwave planar resonator probe operating in the ultrahigh frequency (UHF) range is utilized. Among various microwave probes, the planar resonator probe, being sensitive to current distributions, is deemed advantageous in this application [28; 29; 30]. This paper is organized in the following manner.