Effects of Output Filter to RF–DC Conversion Efficiency in a Rectenna
Naokii Shinohara1 and Ryosuke Narasaki1
1
Research Institute for Sustainable Humanosphere, Kyoto University,
Gokasho, Uji, Kyoto, 611-0011, Japan, shino@rish.kyoto-u.ac.jp
Abstract
A single-shunt rectifying circuit is often used as a receiver for wireless power transfer. This type of circuit can
theoretically rectify microwave input at 100% efficiency with only one diode due to the enhancing effect of the output
filter. In this paper, we indicate the role of the output filter composed of a diode and a capacitor connected in parallel to
a distributed line through ADS simulation. In addition, we indicate the optimum parameters of the single-shunt
rectifying circuit to increase the radio frequency–direct current conversion efficiency.
1. Introduction
For a wireless power transfer (WPT) via radio waves, particularly via microwaves or energy harvesting, a
rectenna or rectifying antenna is always used [1]. In a rectifying circuit, the rectenna receives the radio waves in an
antenna and converts the radio frequency (RF) to direct current (DC). Numerous rectennas have been developed
worldwide [2]. In a rectifying circuit, a single-shunt rectifying circuit is often used, which is composed of a diode and a
capacitor connected parallel to a λg/4 distributed line. λg is the effective wavelength of an input radio wave (microwave
or millimeter wave; Fig. 1). The λg/4 distributed line and the parallel capacitance work in tandem as an output filter.
The single-shunt rectifying circuit can theoretically rectify microwave input at 100% efficiency with only one diode
because of the enhancing effect of the output filter [3]. A real diode and a real circuit introduce loss factors. In particular,
the V-I characteristics of the diode determine the RF–DC conversion efficiency [4]. The characteristics of the RF–DC
conversion efficiency hold for a connected load instead of the input power.
VD_peak =VL/2
Microwave (I1)
VD
Diode off
ID
I2
ID_peak
=2I1_peak
Diode off
on off
IL
DC
on
Rectifier (Microwave ? DC)
(Shingle Shunt Rectifier)
I1
Receiver
on
I2=|I1|
Microwave
Antenna
on off
LPF
Filter for
Higher
Harmonics
ID
IL
(if necessary) To Load R
L
I2
DC/DC
or DC/AC
?g/4 line
Diode
C
Z L=
(DC)
R
L
1+ j ? CRL
⎧Z 2
→∞
Z L + jZ0 tan(β ⋅ λg 4) ⎪ 0 ⎯C⎯
⎯→ ∞ Open for odd harmonics
= ⎨ZL
Z = Z0
Z 0 + jZ L tan(β ⋅ λg 4) ⎪
C →∞
⎯→0 Short for even harmonics
⎩ Z L ⎯⎯
Fig. 1 Block diagram of a rectenna with a single-shunt rectifying circuit and its theoretical waveform.
978-1-4673-5225-3/14/$31.00 ©2014 IEEE
Higher harmonics controlled by the output filter with the λg/4 distributed line and the parallel capacitor
theoretically enable 100% efficiency of the single-shunt rectifying circuit [4]. However, a recent theory states that a
rectifying circuit composed of a diode and a capacitor connected parallel without the λg/4 distributed line can rectify
microwave input at 81.1% efficiency [5]. According to the theory, the capacitor makes a bias voltage of the diode. The
rectifying circuit composed of a diode and a capacitor connected parallel is not a simple half-wave rectifier. The λg/4
distributed line between the diode and the capacitor controls the higher harmonics. The relationship between the
rectifying circuit with and without the λg/4 distributed line is the same as that between class B and class F amplifiers.
On the basis of the theory of the class F amplifier, the capacitor is insufficient for controlling the harmonics. We at
Kyoto University proposed and developed a class F rectifying circuit with resonance circuits (stabs) instead of
capacitance (Fig. 2) [6]. In this paper, we verify the role of the output filter of the single-shunt rectifying circuit, and
propose a method for increasing the RF–DC conversion efficiency with an improvement of the output filter.
(a)
(b)
Fig. 2 Concept of class-F load rectenna. (a) Normal single-shunt; (b) proposed class-F load type [5].
2. Output Filter Composed of Distributed Line and Capacitor
RF-DC Conversion Efficiency
Loss at Diode (%)
According to the rectifying theory [5], one diode can theoretically rectify a microwave at an efficiency of 81.1%
irrespective of the length of the distributed line. Fig. 3 indicates the simulation results of RF–DC conversion efficiency
and loss at a diode of the rectifying circuit with various lengths of the distributed line between a diode and a capacitor
connected in parallel with the ADS simulator. The frequency (f) was 2.45 GHz continuous wave, and the input power
(Pin) was 1 mW. We used the diode parameters of HSM-2860 (Avago). The capacitance (C) was 820 pF, line
impedance (Z) was 200 Ω, and the connected load (RL) was 2 kΩ. The DC cut capacitor of 820 pF was installed after an
input port. Input impedance was matched in each length of the distributed line. The efficiency was approximately 75%
irrespective of the length of the distributed line. This result supports the rectifying theory [5].
Efficiency
Loss at Diode
Length of Distributed Line at Output Filter (deg)
Fig. 3 Simulated dependence of RF–DC conversion efficiency and loss at a diode on the length of the distributed line
between the diode and capacitor. f = 2.45 GHz, Pin = 1 mW, C = 820 pF, Z = 200 Ω, RL = 2 kΩ.
Time dependence of the voltage of a connected load and a diode are shown in Figs. 4(a) and 4(b), respectively.
At the first short period, the voltage of the load and the diode become negative, approximately −0.3V, which is the turnon voltage of the HSM-2860 diode. At the bottom of the negative voltage, −0.3V, the voltage become larger, which is a
bias of the diode with the capacitor. These simulation results also support the theory [5].
(a)
(b)
Fig. 4 Time dependence of voltage of the (a) connected load and (b) diode. f = 2.45 GHz, Pin = 1 mW, C = 820 pF, Z =
200 Ω, RL = 2 kΩ.
RF-DC Conversion Efficiency (%)
820 pF of the capacitance is theoretically sufficient to use harmonics for increasing the RF–DC conversion
efficiency. Next, we simulated the rectifying circuit with small capacitance of 1 pF, 3 pF, and 5 pF. The simulation
results are shown in Fig. 5. With the exception of capacitance, the parameters were the same as those shown in Fig. 3.
In this simulation, the RF–DC conversion efficiency depended on the length of the distributed line. The single-shunt
rectifying circuit with 90° of the distributed line (λg/4) rectified the microwave input at maximum efficiency.
Harmonics are used to theoretically increase the RF–DC conversion efficiency with a λg/4 distributed line. Therefore,
this simulation result supports the theory [3][5].
Length of Distributed Line at Output Filter (deg)
Fig. 5 Simulated dependence of RF–DC conversion efficiency and loss at a diode on the length of the distributed line
between the diode and capacitor. f = 2.45 GHz; Pin = 1 mW; C = 1 pF, 3 pF, and 5 pF; Z = 200 Ω; RL = 2 kΩ.
3. Conclusion
A single-shunt rectifying circuit can theoretically rectify a microwave with full wave efficiency. A key circuit is
an output filter composed of a diode and a capacitor connected parallel to a distributed line. The capacitor makes a bias
voltage at the diode, and the theoretical RF–DC conversion efficiency is 81.1%. Harmonics are theoretically used to
increase the RF–DC conversion efficiency through the distributed line, in particular the λg/4 distributed line. Using
ADS simulation, we investigated and verified the rectifying theory. Harmonics were key in increasing the efficiency. To
use the harmonics effectively, large capacitance is required. The optimization of the output filter is needed to increase
the efficiency of a single-shunt rectifying circuit and a rectenna.
4. References
1. N. Shinohara, ““Wireless Power Transfer via Radiowaves (Wave Series)”, ISTE Ltd. and John Wiley & Sons, Inc.,
Great Britain and United States, , Jan. 2014.
2. N. Shinohara, “Rectennas for Microwave Power Transmission”, IEICE Electronics Express, Vol.10, No.21, 2013,
pp.1-13.
3. R. J. Gutmann and J. M. Borrego, “Power Combining in an Array of Microwave Power Rectifiers”, IEEE Trans.
MTT, Vol.27, No.12, 1979, pp.958- 968
4. T. W. Yoo and K. Chang, “Theoretical and Experimental Development of 10 and 35 GHz Rectennas”, IEEE Trans.
MTT, Vol.40, No.6, 1992, pp.1259- 1266
5. T. Ohira “Power Efficiency and Optimum Load Formulas on RF Rectifiers Featuring Flow-Angle Equations”, IEICE
Electron. Express, Vol.10, No.11, 2013, pp.1-9
6. Hatano, K., N. Shinohara, T. Seki, and M. Kawashima, “Development of MMIC Rectenna at 24GHz”, Proc. of 2013
IEEE Radio & Wireless Symposium (RWS), 2013, pp.199-201.