## 4 Algorithm Validation
### Validation by Measurement  
First of all, validation of the developed algorithm was carried out by means of measurement. The SE frequency dependencies were calculated analytically and measured for an enclosure with dimensions \(a=d=300\) mm and \(b=120\) mm, taken from IEEE STD 1597.2 standard. A total of three cases were considered. In case 1, the enclosure was empty, and, in its front wall, there was a squared aperture with dimensions \(w=l=80\) mm (Figure 17). In case 2, the aperture was the same, but the enclosure was filled with a 100 mm high conductive plate, located at a distance of 75 mm from the front wall (Figure 18a). In case 3, the enclosure was empty again, and, in its front wall, there was a slot with \(w=4\) mm and \(l=190\) mm, which was filled with a dielectric with \(\varepsilon_{r}\approx 3.3\). In all cases, the SE was determined relative to the E-field at the observation point located in the centre of the enclosure, i.e., at \(p=150\) mm. To measure the SE, we used a test setup that complies with the IEEE STD 299.1 standard. All measurements were carried out in a radio frequency anechoic chamber  
Figure 16: Algorithm of equivalent circuit transformation for a cylindrical enclosure with internal filling (Flowchart 7). Figure 17: Empty enclosure with a squared aperture of \(80\times 80\) mm\({}^{2}\). (Figure 19a). The SE values were determined from the transmission coefficient modules \(\mid\!S_{21}\!\mid\) between the two antennas, using the following equation:  
\[\mathrm{SE}=20\log_{10}\frac{\mid S_{21}^{0}\mid}{\mid S_{21}\mid}, \tag{37}\]  
where; index \"0\" denotes the values of \(\mid\!S_{21}\!\mid\) measured in the absence of the enclosure. The first antenna used in the test setup was an active Rusintell AI dipole with a maximum measurement error of 2 dB, which can only be used as a receiver. The dipole was placed outside of the enclosure and was vertically polarized. The dipole arms were oriented perpendicular to the wide wall of the enclosure, so that the maximum of the antenna pattern was directed towards the aperture. We did not place the receiving dipole antenna inside the enclosure, as it is quite large and could affect the field in the enclosure, i.e., shift the resonant frequencies, leading to incorrect SE results. The second antenna, which was used as a transmitter, was in the form of a small-sized quarter-wave monopole with a length of 25 mm and was placed in the center of the enclosure. Although IEEE STD 299.1 implies that the transmitting antenna must be outside the enclosure, the approach we have taken is consistent with the reciprocity principle of scattering and radiation problems and allows us to determine the SE quite accurately. The distance between two antennas was 1.5 m in all cases of \(\mid\!S_{21}\!\mid\) measurement, including those where there was no enclosure. A Rohde & Schwarz (Munich, Germany) ZNL20 vector network analyzer, with less than 1 dB error, was used to obtain \(\mid\!S_{21}\!\mid\) values. The test setup was used, starting from 500 MHz, due to the limited frequency range of the receiving dipole antenna. Figure 19: Test setups for measuring SE according to IEEE STD 299.1 (**a**) and using the indirect method (**b**). Figure 18: Enclosure populated with conductive plate (**a**), enclosure with dielectric filled slot (**b**). In the left photo, the enclosure has no wall. For the enclosure from case 1 (with a squared aperture), the SE was also obtained by indirect measurement using the methodology proposed in [44]. For this purpose, a stripline with a 50 \(\Omega\) impedance and a microwave resistor as a load was used (Figure 19b). The SE values were calculated, according to [44], based on the frequency dependence of the reflection coefficient modulus \(\left|S_{11}\right|\) of this stripline when it is placed over the aperture of the shielding enclosure. The values of \(\left|S_{11}\right|\) were determined with a Micran (Tomsk, Russia) R2M series scalar network analyzer, having less than 1.5 dB measurement error.