Measured radiation patterns are shown in Figure 4. The received power for the retrodirective array is relatively flat from -25 to 25 degrees (a 50 degree sector) while the received power starts to drop beyond 3 degrees for the normal non-retrodirective antenna array. It has a narrower scanning beam (50 degree sector) than that reported by Rangelov et al.8 (60 degree sector) because the patch loaded with shorting pins is larger than the conventional patch. Therefore, the distance between the array elements cannot be λ/2 because they would overlap. This causes side lobes in the radiation pattern and a narrower beam.

Figure 4

Figure 4 Monostatic radiation patterns for the retrodirective and normal arrays at 5.8 GHz.

To verify wideband retrodirectivity a monostatic measurement is made at 5.9 and 6 GHz (see Figure 5). Similar to the results in Figure 4, received power at both frequencies is relatively flat over a 50 degree sector.

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Figure 5a

 

Figure 5b

Figure 5 Monostatic radiation patterns for the retrodirective array at 5.9 (a) and 6 (b) GHz.

Conclusion

A wideband circularly polarized sequential rotation antenna is used as a transmitting array in retrodirective system designed for wireless charging of portable devices. A circular microstrip patch antenna with four shorting pins, to suppress surface waves, is used as the array element. In addition to surface wave suppression, sequential rotation applied to the array element improves impedance matching and 3 dB axial ratio bandwidth. Beam scanning capability is demonstrated through monostatic radiation pattern measurements showing a uniform power level available to charging devices regardless of location within a 50 degree angular sector and over a wide frequency ranging from 5.8 to 6 GHz. This provides more efficient power delivery to devices without prior knowledge of their location than antennas with a fixed beam.

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