# Dual-Band Single-Layered Frequency Selective Surface Filter for LTE Band with Angular Stability
## 5 Fabrication and Measurement Results  
Figure 12a shows the fabricated C-FSS array, while Figure 12b,c displays the measurement setup of the fabricated structure with horn antennas and an anechoic chamber, respectively. To conduct measurements, Vector Network Analyzer (VNA) of Agilent Technologies model N5230A (Santa Clara, CA, USA), which operates in the range of 300 kHz-20 GHz, is used. Multiple techniques can be employed to determine the reflection and transmission properties of an FSS structure. Usually, horn antennas are arranged on opposite sides of the FSS structure to examine the transmission characteristic, whereas they are positioned on the same side to measure the reflection coefficient. The horn antennas can be adjusted to have either vertical or horizontal polarization, allowing for the measurement of both TE and TM transmission properties. The horn antenna is examined without FSS which results in a reference curve. This reference curve is employed to standardize the FSS frequency response, thus offsetting any losses that occur in free space.  
Although this setup enables the measurement, it may produce imprecise data because of edge diffractions. The occurrence of these diffractions is because, in most cases, the FSS dimensions are substantially lower than the beamwidth of the horns. So, choosing the array size of FSS is crucial. The size of the C-FSS array is 21.5 \(\times\) 21.5 mm\({}^{2}\), which is sufficient for testing scenarios.  
Figure 12: (**a**) Fabricated 4 \(\times\) 4 array structure of C-FSS. (**b**) C-FSS test setup for S\({}_{11}\) parameters. (c) C-FSS under test in anechoic chamber.  
In Figure 13a, the compared results of fabricated and simulated data are shown, and Figure 13b shows VNA results. The C-FSS exhibits distinct resonant behavior at around 2.1 GHz. The results differ slightly from one another. These differences can be related to variations in fabrication tolerances, errors in measurements, or external influences that impact the measurements.  
To obtain measurements at various oblique angles of incidence, the transmitting and receiving antennas are re-positioned along a circular trajectory, either in a clockwise or counterclockwise direction, to achieve the desired oblique angles in relation to the path of normal incidence. At a perpendicular angle, both antennas are aligned along the 0\({}^{\circ}\) line. For oblique incidence, the transmitting and receiving antennas are displaced in opposite directions, with one moving clockwise and the other counterclockwise, along a circular trajectory. This displacement allows the antennas to form the required angle with the normal path.  
This experimental result is shown in Figure 14a,b. The measured reflection and transmission coefficients are examined under oblique incidence for TE and TM polarization. The angular stability is examined up to 80\({}^{\circ}\).  
Figure 14: (**a**) Angular stability of reflection coefficient under TE polarized wave. (**b**) Angular stability of transmission coefficient under TE polarized wave.  
Figure 13: (**a**) Compared measured and simulated reflection and transmission coefficients under TE polarized wave. (**b**) VNA result of proposed C-FSS.