# Measurement of Dielectric Properties of Thin Materials for Radomes Using Waveguide Cavities
###### Abstract  
We present waveguide cavity measurements used to evaluate several thin materials for use in radomes. In addition to the data on the materials, we show how these measurements can be performed with common laboratory equipment and simple calculations. We sought an approach that allowed candidate materials to be readily evaluated to deal with formerly selected materials becoming unavailable or cost-prohibitive. We used lengths of standard waveguide (WR650 and WR137 here) with readily manufactured irises and a vector network analyzer (Keysight N5225B here). To select the iris size and determine the limits of the simplifications in the equations used, we employed a full-wave 3D electromagnetic simulator (CST Microwave Studio). The equations required to calculate the dielectric properties of samples and their contribution to the equivalent system noise temperature from unloaded and loaded resonant frequencies and Q factors are shown. While these formulations can be found elsewhere, we did not find these assembled as conveniently in other studies in the literature. We also show that orienting the sample down the length of the cavity allows for higher-order modes to be fully utilized. We did not find this straightforward adaptation of the common cross-guide orientation in other works. Overall, the results allowed us to recommend three fabrics for use at the frequencies tested (1.7 and 5.6 GHz). The complete process is outlined to assist others in performing these measurements themselves. radomes; dielectrics; thin materials; permittivity; loss tangent; waveguides; resonant cavity; modes; wave; electromagnetics +
Footnote †: journal: _et alc  
It is important to note that the permittivity can be dependent upon temperature and frequency. As such, differing frequencies propagating through the material can experience different values of permittivity. High values of permittivity increase the losses from reflected waves [1,2]. Similarly, the loss tangent affects the amount of loss and noise in a system. As such, it is important that materials with low-loss tangents are used for radomes [3]. Therefore, dielectrics with low permittivity and loss tangents at various frequencies are the most appropriate for radome structures. The precise measurement of these materials to identify their permittivity can be challenging. Permittivity measurements can be performed on many types and forms of materials--bulk, thin, solid, liquid, ceramic, plastic, and even organics [2,4]. These different materials have various applications, including capacitors, transformers, switches, cabling, reactors, and cooling systems [5]. Each of these materials can have multiple measurement processes available, each with its own advantages and disadvantages, dependent upon the type of material and application. All methods mentioned in this paper utilize the modes of propagating waves through the material. This paper focuses on transverse electric (TE) and transverse magnetic (TM) modes, as shown in Figure 1[6,7]. Mode degeneracy is when multiple modes have the same propagation coefficient, causing overlap when viewing the modal signals. This can make it difficult to single out the chosen mode peak [8]. Choosing an appropriate method for measurement can be difficult and depends upon various factors. The authors in [2] provide a simple set of questions to help determine which measurement method is the best. These questions include the material category (bulk or thin), whether the sample under test requires non-destructive means of measurement, the type of frequency band (broad or narrow required, the measurement accuracy required (high or low), and what the dielectric losses are expected to be (high or low). These questions help to narrow down the requirements of the measurement and determine the most accurate method for not only the frequency range but also the thickness of the material [2]. [PERSON] and [PERSON] provide a comprehensive overview of the most popular methods for this measurement process in [4].