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Prototype and Measurements

Based on the promising results afforded by the simulations, a prototype antenna was constructed. Each row of the wire-grid metasurface was formed from AWG20 copper wire and soldered into two large copper plates. The copper plates were then placed inside a rectangular horn antenna, along with the two matching wedges in the horn throat. Figure 2.9 shows photographs of the prototype. The wire grid exhibited dimensional inaccuracies of several percent, but the overall horn performance is highly tolerant of slight geometrical changes, as evidenced by the measurements that follow.

Photographs showing (a) fabrication of and (b) completed wire- grid metahorn antenna prototype. Reprinted, with permission, from Ref. 14, Copyright 2013, IEEE

Figure 2.9 Photographs showing (a) fabrication of and (b) completed wire- grid metahorn antenna prototype. Reprinted, with permission, from Ref. 14, Copyright 2013, IEEE.

Measured and simulated metahorn input reflection coefficient

Figure 2.10 Measured and simulated metahorn input reflection coefficient. While the measured data differ from the simulated data as a result of standing wave/measurement artifacts, both show that the horn maintains a low reflection coefficient better than -15 dB across most of the band.

The metahorn’s reflection coefficient remained below -15 dB across its operating band, as shown by the measured and simulated data in Fig. 2.10. Pattern measurements, shown in Fig. 2.11, confirmed the excellent performance predicted by the simulations. The measured patterns show some slight asymmetries; these are a result of imperfect manufacturing (hand-soldering) and asymmetries in the antenna measurement range. The antenna was rotated 180° between two sets of pattern measurements to help isolate the asymmetries. Inconsistencies appearing on the solid and dashed curves at the same в value correspond to asymmetries in the range, while inconsistencies appearing on opposite values of в correspond to asymmetries in the metahorn itself.

Measured and simulated E-plane

Figure 2.11 Measured and simulated E-plane (a—d) and H-plane (e—h) radiation patterns for the wire-grid metahorn. The two measurements represent two orientations (rotated 180° relative to each other) of the metahorn to help separate errors in the testing range from errors in the horn itself.

Measurements of the cross-polarized radiation produced values that were somewhat higher than those predicted by simulations, but this is to be expected from an imperfectly manufactured horn. In spite of the imperfections, the peak relative cross-polarization remained around -25 to -30 dB across the operating band. Higher- quality manufacturing would improve these levels further.

The wire-grid metahorn exhibited successful operation with negligible intrinsic loss across more than an octave bandwidth. Compared to the trifurcated horn, which is the state-of-the-art feed horn for single linear polarization at C-band, the metahorn yields lower sidelobes and lower backlobes across the operating band. Moreover, this demonstration shows promise for lightweight metamaterial horns to replace circular corrugated feed horns for dual-polarization over the super-extended C-band, where such horns are heavy and expensive.

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