Metamaterial Transmission Line Resonators for Structural Moisture Sensing in Buildings Andreas Penirschke, Rolf Jakoby Institute for Microwave Engineering and Photonics Technische Universität Darmstadt, Darmstadt, Germany Email: penirschke@imp.tu-darmstadt.de Abstract— This paper presents novel metamaterial moisture sensors based on composite right/left-handed (CRLH) transmission line resonators, applicable for moisture measurements in brick and stone walls. Metamaterials are periodic structures consisting of series capacitors and shunt inductors with a periodicity well below the wavelength that can have properties similar to homogeneous materials. For sensing applications, metamaterial structures exhibit high sensitivity and the advantage of scalability in frequency and size as well as the ability of conformal implementation to the sensing task [1-2]. For proof of concept, a reflection type and a transmittance type sensor for implementation in small boreholes are realized and tested in a dedicated measurement setup. Keywords— metamaterial, resonant moisture detection I. INTRODUCTION Monitoring of the moisture content in buildings made of concrete, brickwork or other masonry materials is a challenging task. The curing process of concrete during construction is significantly affected its temperature and moisture level. Uncontrolled moisture transfer during construction can prevent the concrete from reaching its full strength. Monitoring of temperature and moisture level provides crucial information about hardening and setting process of concrete as well as the progress of deterioration mechanisms such as corrosion of steel reinforcement or carbonation and alkali-aggregate reaction [3]. After construction, high moisture contents over long periods can lead to serve deterioration of the room climate and may even cause damages of buildings when dissolved salts are present. Many historic buildings are threatened by high moisture contents in the walls. High quality measurements of moisture content can help to take appropriate decisions about interventions and remedial work. Often it is necessary to detect the moisture content in walls over a long period of time to check if executed repairs were successful [4]. A well known way to detect the moisture content in buildings is to use a nondestructive measurement system that measures at the surface, which is highly dependent on the room climate and other parameters such as the surface roughness and the pressure applied to the sensor electrodes [4]. A possibility for a nondestructive method is to detect the moisture content within a wall with a impulse radar system. The response of the moist wall depends on the moisture content, but the interpretation of the reflection pattern is very difficult. This paper describes novel microwave sensors using metamaterial transmission line resonators to detect the spatial moisture profile within the wall. A small borehole with a diameter of 8mm needs to be drilled into the wall to insert a coaxial probe. The microwave field radiates orthogonal to the direction of the hole. From the change of the reflected signal the effective complex dielectric constant and thus the moisture content can be detected, when the complex dielectric properties of the dry wall material is known. II. MOISTURE DETECTION USING MICROWAVES The moisture content can be calculated by a dielectric mixing formular that was proven to yield accurate results in [5]. As described in [4], the moist wall contains stone (s), water (w) and air (a). The effective dielectric constant can be determined by its volume fractions to ges a w r w s r s s r v v v v + + = , , , ε ε ε , (1) where v s , v w and v a are the volume fractions of stone, air and water respectively and ε r,s and ε r,w are the dielectric constants of the stone material and water. With the density ρ s of the wall material and the relation between density and mass m s =ρ s v s the moisture content can be calculated by % 100 ⋅ + = Ψ s w s w w v v v ρ ρ , (2) III. COMPOSITE RIGHT/LEFT-HANDED TRANSMISSION LINE SENSOR CONCEPT A well known way to detect the permittivity of a solid material is to use a resonant circuit consisting of a capacitor and an inductor. A change of the loaded capacitor changes its