In this article the problem of field spreading when
the focal length (z0) becomes more than the aperture size (D)
in the fresnel zone wireless power transfer (WPT) is addressed.
One of the solution is to have the z0 ≈ D. However, the distance
of WPT can be meters or Kms and building the D so large is not
practically and economically feasible. This can be achieved with
the use of repeater system, i.e, the lens kept at the focus of the
previous lens. A comparison is made with the conventional WPT
in the Fresnel zone with one lens and repeater system with more
than one lens and the efficiency of power transfer is compared.
The best part of this repeater system is with a very small receiver
size (DRX = 3 cm), an efficiency of 19% (with 2 lens and always
the authors are talking about the RF-RF efficiency) is achieved
at 14.82 m. Also, with the increase in the repeaters, the efficiency
can go upto 97% theoretically. For practical implementation the
reflections from the ground and the losses from the fabricated
lens have to be taken. The future work will be to design Fresnel
lens with the unit cells tuned for the required amplitude and
phase. In future the repeater system using Fresnel lens can help
achieve higher efficiency with lower size of receiver antenna
aperture, thus making practical implementation of Fresnel WPT
more efficient replacing the overhead transmission cables for low
duty machines and sensors