3.3.1 Designed antenna S-parameters

Figure 16 shows the reflection coefficient response of the antenna obtained from CST simulation compared with the calculated response of the equivalent circuit model by using Agilent ADS software in addition to the measured reflection coefficient. Good agreement was between the results of simulated, measured and ADS model. The antenna resonates at two bands 1.95 GHz (f1) and 2.45 GHz (f2). The circular patch is designed to radiate at 2.45 GHz by the direct feed with the transmission line placed behind substrate 2. Whereas, 1.95 GHz resonance frequency is designed to radiate due to the capacitive coupling between the circular patch on the top of substrate 1 and the circular slot located on the ground plane, where in 1.95 GHz case the disc antenna is considered as a feeder for the circular slot. The performance of the proposed antenna was simulated and optimized by commercial EM software CST Microwave Studio. A prototype of the proposed antenna was fabricated and tested. The reflection coefficient of the antenna was measured by R&S ZVA 67 Network Analyzer. It is noted that the simulated and measured results of the input impedances of the antenna are in good agreement. Only, a small shift in the measured S-parameters was observed due to the connector soldering, fabrication tolerance, the adhesive between the two layers of the antenna and the layers alignment in fabrication process.

Figure 18. There is a good agreement between the simulated and measured results

In this design, a scheme used in [49] is employed to achieve a dual-band impedance transformation at the two frequency bands (f1 and f2). This scheme is used to match between a complex and frequency-dependent rectifier input impedance (ZRec) and a real impedance of the antenna (ZAnt) by using four different sections (Section 1–4) as shown in Figure 19. The matching technique can be summarized in

Step 1: Achieve the conjugate matching between the load values at both resonant frequencies, that is, moving the two impedance values of the load (rectifier) on the Smith chart to be located on the same real circle with the imaginary parts are

Each section is characterized by two values Z and θ, where Z is the section characteristic impedance and θ is the section electrical length. The function of the first section (Section 1) is to make the real value of the rectifier input impedance at

equal on both sides of the Smith chart as shown in Figure 20.

Rectenna Systems for RF Energy Harvesting and Wireless Power Transfer

Step 3: Real to real impedance transformation.

Step 2: Cancel the imaginary part of the impedances at f1 and f2.

of the radiation characteristics.

DOI: http://dx.doi.org/10.5772/intechopen.89674

3.4 Rectifier-antenna matching

the following steps:

Figure 19.

Figure 20.

167

Matching steps indicated on smith chart.

Dual-band matching circuit.
