## 2 Measuring Electromagnetic Shielding and Related Parameters
A general overview of possible measurement setups was given by [PERSON] et al., who differentiate them into open-field (free-space) methods, shielded-box methods, shielded-room methods and the coaxial transmission-line methods, to which the ASTM D4935-18 belongs [31]. Their evaluation of the different methods is given here in brief, showing that different methods may have their advantages and disadvantages for different samples and frequency ranges under investigation:  
* Open-field (free-space) method--distance of 30 m between device and receiving antenna; wide variations due to differences in product assembly. * Shielded-box method--metal box with sample port in one wall, receiving antenna inside, transmitting antenna outside; difficult electrical contact between test specimens and shielded box, limitation of the frequency range to about 500 MHz, poor correlation between tests in different laboratories. * Coaxial transmission line--e.g., ASTM D4935; standard method for planar specimens, time consuming (several minutes to hours per spectrum, depending on the measurement method), typically in the range from 10 kHz to 1 GHz. * Shielded-room method--similar to shielded box method, anechoic chamber of typical ground area 2.5 m\({}^{2}\), large test specimens needed between transmitting and receiving antenna [31]. As mentioned before, the conductive and magnetic properties of the samples play an important role for the shielding effectiveness of a material, although even insulating materials with embedded conductive particles will have a certain shielding effect. While the conductivity of a rigid material can unambiguously be measured, this is much more complicated for textile materials. For low conductivities, a teraohm-meter can be used, as described in DIN 54345-1, working with circular electrodes and exerting a defined pressure on the fabric with defined electrode distances. For higher conductive fabrics, different suggestions can be found in the literature. [PERSON] and [PERSON] used contacting bars at a defined distance upon which they exerted a defined load to measure the in-plane resistance of a woven fabric [32]. In addition, solder lines were applied on the textile fabric along the positions of the contact bars to further increase the contact between measurement instrument and conductive coating on the fabric. The problem of high contact resistances was also mentioned by [PERSON], who found that, even in circular electrode systems such as a teraohm-meter, and in three-electrode systems used for volume or surface resistance measurements, high contact resistances can significantly influence the measured surface or volume resistance [33]. Even in four-probe measurements, normally used for measurements of small resistances or, more generally, resistances that are small in comparison to the contact resistance between sample and measurement, deviations from the real value occur if the contact resistance is too high, causing problems during bioimpedance measurements with textile electrodes and similar dry electrodes contacting the skin [34]. This is even more problematic regarding conductivity measurements on nanofiber mats on which no large pressure can be exerted without the danger of damage. While several papers reporting on the electric conductivity of electrospun nanofiber mats mention the four-probe method [35; 36; 37], only few of them explain the contacting process. Some papers describe measurements based on commercial four-point probes, e.g., the Jandel RM3 Test unit [38; 39], which try to exert a standardized pressure onto textile and similarly soft samples. Some authors mention the use of a conductive silver paste to create a reliable contact [40; 41; 42], which is in our experience quite often carried out, but unfortunately not always mentioned in the resulting papers, although it is a good method to enlarge the contact area without squeezing the fibers. Magnetic properties are usually measured contact free and nondestructively, so that the aforementioned contact problems do not occur here.