Series Diode As a Possible Design for Rectenna Applications

28 April 2003

Ricardo M. Silva

“Series diode as a possible design for rectenna applications”

“Mystery unraveled”

Throughout the past few months, we have examined various options of circuit layouts in order to optimize the conversion of RF (Radio Frequency) energy to DC (Direct Current). The RF to DC conversion has been studied in the past (Ref. W.C. Brown) and it has been implemented using a device called a Rectenna (Rectifying Antenna).

In the pursuit of an original and efficient design, we had experimented with the possibility of designing a rectenna which consisted of a 1/4 wave monopole antenna in series with a diode which was also in series with a load. This design was similar to a simple ½ wave series rectifier used in household AC (Alternating Current) to DC power supply.

Unfortunately, this design wasn’t able to provide any DC voltages at our operating frequencies of around 1GHz.

The following, provides an explanation as to why this design had failed.

During the past week, I was reading about signal mixers in order to get an additional insight on how rectennas work, and it finally struck me what the missing piece was from our series circuit design.

The figure below, Fig. 1, is a PSPICE representation of the series rectenna.

Fig.1 Series diode rectenna

1) V2 is the voltage induced on the ¼ monopole antenna

2) R1 is the antenna resistance

3) C2 is introduced here to provide the DC isolation from V2

4) D1 is the series diode

5) C1 is part of the load and it shunts the AC to ground

6) R2 is the series load

The circuit was simulated and the voltage was taken across the load. Figure 2 shows the output.

Figure 2, DC output of Series Diode Rectenna

The initial DC voltage, for t < 4 milli seconds is due to the charging of C2, the isolation capacitor, and it should be ignored for the steady-state output of the circuit.

It is clear from the simulation that the VDC output is zero. The reason why it is zero, is because the circuit lacks a DC return path (Ref. Laverghetta). Remember, C2 is introduced to simulate the DC isolation that an antenna has from the transmitting antenna, and because of the DC isolation, there isn’t a DC current loop.

To prove this, Fig. 1 was modified with the introduction of L1. L1 was chosen to be large so that the AC signal wouldn’t be shorted to ground. But since it is an inductor, its DC resistance is almost zero, providing the necessary DC return path. DC will flow through D1, R2 and L1.

The modified circuit is shown in Fig.3.

Fig. 3 Corrected circuit with DC return path L1

The circuit was re-simulated and all parameters were kept the same except for the introduction of L1. The output is displayed in Fig. 4.

Fig. 4 Corrected series diode rectenna output

The output is roughly 7VDC and it is what you would expect from a 10 VAC signal.

CONCLUSION

A series diode rectenna could be implemented, as long as provisions are made to supply a DC return path to the load.

REFERENCES

1) W.C. Brown, “The history of power transmission by radio waves,” IEEE MTTS; vol. MTT-32, Sept. 1984

2) T. S. Laverghetta, “Practical Microwaves,” Howard W. Sams & Co. Inc., 1984, page 110.