Description:FIELD OF INVENTION

[01] The present invention generally relates to a circuit which derives energy from RF or microwave energy sources. To be specific, the invention describes a circularly polarized patch antenna that scavenge equal amount of energy in all directions. The key element of the invention is a rectenna optimized for receiving incoming microwave signals of low power density and to convert it into DC voltage. The two rectennas designed in this work operates at 1800 MHz and 2450MHz.

BACKGROUND OF THE INVENTION
[02] Stand-alone devices such as wireless sensor nodes located in difficult access environments have to work without the human intervention for many years. In these situations, many troubles arise such as battery replacement or recharging, in addition to the size and weight.

[03] The problem increases tremendously when the number of devices is large and distributed in a wide area or inaccessible places. Therefore, a feasible method is to use wireless power transfer technologies to supply energy to remote sensors.

[04] The technology for harvesting and recycling wireless power is essentially based on radio frequency identification, or RFID. A transmitter sends a burst of radio-frequency energy that both carries information to a chip and can be converted to dc electricity to power it.

[05] In microwave power transmission, the antennas have well-defined polarization, and high rectification efficiency is enabled by single frequency high microwave power densities incident on an array of antennas and rectifying circuits.

SUMMARY OF THE INVENTION

[06] Objective of the present invention is to develop rectenna, which derives energy from RF or microwave energy sources.

[07] The rectennas developed in this work was able to exhibit dc conversion efficiencies up to 65% at microwave power densities of 0.22 mW/cm2.

BRIEF DESCRIPTION OF FIGURES

[08] Other features and advantages of the present invention will become apparent from the detailed description of the invention, which follows when considered in light of the accompanying drawings in which:
[09] Fig. 1 Structure of circularly polarized rectenna
[10] Fig. 2 Photograph of rectenna
[11] Fig. 3 Schematic of a complete rectenna measurement setup
[12] Fig. 4 Structure and dimensions of rectenna
[13] Fig. 5 Branching in rectifier section
[14] Fig. 6 Measurement setup using two led’s in series placed across the output terminals of the rectenna
[15] Fig. 7 Photograph of circularly polarized rectenna for 1.8 GHz
DETAILED DESCRIPTION OF FIGURES
[16] The design of the circularly polarized patch is done using Ansoft ADK (Antenna Design Kit) and simulated in Ansys HFSS is shown in Fig 1. The rectenna is etched on a low loss substrate with (r = 3.74845, thickness = 1.58 mm, tan =.003496). It uses the dual diode rectifier topology. it contains two Schottky diodes mounted with a differential topology and 50 characteristic impedance microstrip feed line. The dimensions of the patch are calculated as per the equations for microstip square patch antennas.

[17] The photograph of the rectenna (rectenna fabricated using low loss substrate) is shown in Fig 2. Once an LED was connected across the load terminals to find whether it was glowing or not. The power was fed to the transmitter antenna from the network analyzer with a variation of power from 0dB to 10 dB and it was found that the LED started glowing, when the input power level reached 4 dB. At 8dB input power level the LED was glown with good brightness. The LED used for this was the common red LED.

[18] The schematic representation of the complete rectenna measurement set up is shown in figure 3. It consists of an RF power generator, an amplifier, a standard horn antenna as transmitting antenna, the rectenna, load resistance and a volt meter for measuring the output voltage across the load. Rectenna is placed at a distance of 10 cm from the linearly polarized horn antenna and is illuminated at its broadside. The signal generator is attached to the amplifier on the transmitter side. The voltmeter measures the output dc voltage obtained.

[19] The structure and dimensions of the rectenna is shown in Fig 4Using the voltage difference between V1 and V2 across the resistor load, the DC voltage is determined without reference to the RF ground plane. The distance between each diode and the point P0 is so adjusted that at 3.6 GHz it become equal to a quarter wavelength resulting in a small impedance at the input of the circuit (P0) and both diodes exhibit high impedances at this frequency. The point P0 behaves like a short circuit at this frequency. The dimensions L1, L2, L3 and L4 is obtained from the Ansoft Antenna design tool.

[20] Branching in the rectifier section is shown in the Fig 5. The dimensions L5 and L6 are calculated as per Wilkinson's Power divider rule and L7 and L8 are the lengths of quarter wave length stubs. For splitting the power equally into two branches, the lengths and widths of branches are calculated as per Wilkinson's Power divider rule, so that L5 = .75(g/4); L6 =.25 (g/4). The dimensions L7 and L8 are taken as L7 = .75(g/4) and L8 =.25 (g/4 ) such that the total stub length is (g/4). Note that g= (/ eff) is the effective wave length inside the substrate. W1 is the width corresponding to Zo (50) and W2 the width corresponding to .

[21] Two LEDs, in series, placed across the output terminals of the rectenna is shown in the Fig 6. When the rectenna (rectenna fabricated using low loss substrate) was illuminated with microwave power from a mobile phone, it delivered a maximum voltage of 4V at closer distance. A 1 F capacitor was placed across the output terminals for smoothening the output. Then the peak voltage was too much increased. Two LEDs, connected in series, were then placed across the output terminals of the rectenna. Both of the LEDs were glown with good brightness and then the output voltage across the LEDs in series were measured as 3V.
[22] The photograph of the circularly polarized rectenna for 1.8 GHz is shown in Fig 7. The patch Dimensions of the antenna is estimated by Ansoft ADK and was simulated using the full wave electromagnetic simulator HFSS (High Frequency Structure Simulator).
, C , C , C , C , Claims:I/We claim:

1. This system produces a voltage of 3 volts by RF energy scavenging
2. Circular polarization is achieved
3. Simplicity in the design of impedance matching.
4. A new kind of substrate is introduced with very low loss tangent and loss value.
5. The two rectennas designed in this work operates at 1800 MHz and 2450 MHz.
6. Designed prototype is fabricated and tested to harvest enough amount of RF energy for the portable applications