SIEVENPIPER et al. : HIGH-IMPEDANCE ELECTROMAGNETIC SURFACES 2067 surface. It is only on the textured surface, with its unusual surface impedance, that significant TE transmission signal Transmits levels can be obtained. Horn The tM data has variations of 10-15 dB. but remains relatively flat over a broad spectrum. The variations are produced by multipath interference, or speckle, which occurs in coherent measurements whenever multiple signal paths are Horn Microwave Absorber present Multipath interference can be distinguished from other effects because it is characterized by narrow-band fading, Fig 13. Reflection phase measurement performed using a pair of hom whose details depend on the exact antenna position. The antennas. The anechoic chamber used in these experiments measured 30 cm transmission drops off at low frequencies because the small X 30 cm x 60 cm, and the hole for the sample measured 10 cm X 10 probes are inefficient at exciting long wavelengths A typical TM surface-wave measurement on a textured surface is shown in Fig. 12(b). The size of the sheet and the Out-of-Phase measurement technique were the same as those used for the flat metal surface. The structure consisted of a triangular array 2 2 A of hexagonal patches as shown in Fig. l, with a period of 2 2.54 mm and a gap between the patches of 0. 15 mm. The thickness of the board was 1.55 mm and the dielectric constant In-Phase was 2.2 The transmission is strong at low frequencies, and exhibits the same multipath interference seen on the metal surface. At I 1 GHz, the transmission drops by about 30 dB, indicating the Out-of-Phase edge of the TM surface-wave band. Beyond this frequency the transmission level remains low and fat, eventually sloping oward at much higher frequencies because of weak coupling Frequency [GHz to TE surface waves. The te band edge is not apparent in Fig 14. Measured reflection phase of a two-layer high-impedance surface this measurement, but the region corresponding to the surface- wave bandgap is indicated on the graph by an arrow A tE transmission measurement of the same textured sur- A reference measurement is taken of a surface with known face is shown in Fig. 12(c). It was taken using a pair of renection properties, such as a flat sheet of metal, and all subsequent measurements are divided by this reference. A transmission is weak at low frequencies, and strong at high factor of m is added to the phase data to account for the requencies, the reverse profile of the TM data. A sharp Jump a reflection phase of T of 30 dB occurs at 17 GHz, indicating the te band edge Beyond this frequency, the transmission is flat, with only small The reflection phase of the high impedance surface is shown in Fig. 14. At low frequencies, it reflects with a T phase to Tm surface waves. so there is an additional transmission shift just like a metal surface. As the frequency is increased, peak at 11 GHZ, at the tm band edge, where the density of the phase slopes downward, and eventually crosses through TM states is high. Both TM and TE probes tend to couple zero at the resonance frequency of the structure. At higher slightly to the opposite surface-wave polarizations. However, frequencies, the phase continues to slope downward, and the cross coupling is greater with the TE probe because it lacks and-T/2, indicated on the graph by arrows, plane waves are he symmetry of the vertical monopole A surface-wave bandgap is measured between the TM band reflected in-phase, rather than out-of-phase. This range also edge at 11 GHz and the TE band edge at 17 GHz. Within this corresponds to the measured surface-wave bandgap, indicated Currents cannot propagate across the surface, and any induced edges falling approximately at the points where the phase currents radiate from the surface crosses through丌/2and-丌/2, respectively C. Low-Frequency Structures B. Reflection Phase With two-layer construction, the capacitors are formed by The reflection phase of the high-impedance surface can be the fringing capacitance between two metal plates lying edge- measured using two microwave horn antennas, as shown in to-edge, usually separated by a few hundred microns. If the Fig. 13. The measurement is done in an anechoic chamber substrate has a dielectric constant between 2-10. and the lined with microwave absorbing foam. The two horns are period is a few millimeters, the capacitance is generally on placed next to each other, aimed at the surface. Two windows the order of a few tens of femtofarads. With a thickness of are cut in the chamber, one for the antennas, and one for the a few millimeters, the inductance is a few nanohenry, so the surface under test resonance frequency is on typically the order of about 10 gHzSIEVENPIPER et al.: HIGH-IMPEDANCE ELECTROMAGNETIC SURFACES 2067 surface. It is only on the textured surface, with its unusual surface impedance, that significant TE transmission signal levels can be obtained.) The TM data has variations of 10–15 dB, but remains relatively flat over a broad spectrum. The variations are produced by multipath interference, or speckle, which occurs in coherent measurements whenever multiple signal paths are present. Multipath interference can be distinguished from other effects because it is characterized by narrow-band fading, whose details depend on the exact antenna position. The transmission drops off at low frequencies because the small probes are inefficient at exciting long wavelengths. A typical TM surface-wave measurement on a textured surface is shown in Fig. 12(b). The size of the sheet and the measurement technique were the same as those used for the flat metal surface. The structure consisted of a triangular array of hexagonal patches as shown in Fig. 1, with a period of 2.54 mm and a gap between the patches of 0.15 mm. The thickness of the board was 1.55 mm, and the dielectric constant was 2.2. The transmission is strong at low frequencies, and exhibits the same multipath interference seen on the metal surface. At 11 GHz, the transmission drops by about 30 dB, indicating the edge of the TM surface-wave band. Beyond this frequency, the transmission level remains low and flat, eventually sloping upward at much higher frequencies because of weak coupling to TE surface waves. The TE band edge is not apparent in this measurement, but the region corresponding to the surface￾wave bandgap is indicated on the graph by an arrow. A TE transmission measurement of the same textured sur￾face is shown in Fig. 12(c). It was taken using a pair of small coaxial probes, oriented parallel to the surface. The transmission is weak at low frequencies, and strong at high frequencies, the reverse profile of the TM data. A sharp jump of 30 dB occurs at 17 GHz, indicating the TE band edge. Beyond this frequency, the transmission is flat, with only small fluctuations due to speckle. The TE probes also couple slightly to TM surface waves, so there is an additional transmission peak at 11 GHZ, at the TM band edge, where the density of TM states is high. Both TM and TE probes tend to couple slightly to the opposite surface-wave polarizations. However, the cross coupling is greater with the TE probe because it lacks the symmetry of the vertical monopole. A surface-wave bandgap is measured between the TM band edge at 11 GHz and the TE band edge at 17 GHz. Within this range, neither polarization produces significant transmission. Currents cannot propagate across the surface, and any induced currents radiate from the surface. B. Reflection Phase The reflection phase of the high-impedance surface can be measured using two microwave horn antennas, as shown in Fig. 13. The measurement is done in an anechoic chamber lined with microwave absorbing foam. The two horns are placed next to each other, aimed at the surface. Two windows are cut in the chamber, one for the antennas, and one for the surface under test. Fig. 13. Reflection phase measurement performed using a pair of horn antennas. The anechoic chamber used in these experiments measured 30 cm 30 cm 60 cm, and the hole for the sample measured 10 cm 10 cm. Fig. 14. Measured reflection phase of a two-layer high-impedance surface. A reference measurement is taken of a surface with known reflection properties, such as a flat sheet of metal, and all subsequent measurements are divided by this reference. A factor of is added to the phase data to account for the reference scan of the metal sheet, which is known to have a reflection phase of . The reflection phase of the high impedance surface is shown in Fig. 14. At low frequencies, it reflects with a phase shift just like a metal surface. As the frequency is increased, the phase slopes downward, and eventually crosses through zero at the resonance frequency of the structure. At higher frequencies, the phase continues to slope downward, and eventually approaches . Within the region between and , indicated on the graph by arrows, plane waves are reflected in-phase, rather than out-of-phase. This range also corresponds to the measured surface-wave bandgap, indicated on the graph by a shaded region, with the TM and TE band edges falling approximately at the points where the phase crosses through and , respectively. C. Low-Frequency Structures With two-layer construction, the capacitors are formed by the fringing capacitance between two metal plates lying edge￾to-edge, usually separated by a few hundred microns. If the substrate has a dielectric constant between 2–10, and the period is a few millimeters, the capacitance is generally on the order of a few tens of femtofarads. With a thickness of a few millimeters, the inductance is a few nanohenrys, so the resonance frequency is on typically the order of about 10 GHz.