1

Abstract—This paper discusses an implementation of a far-field

wireless power transmission system using microwaves in the ISM

band. In this project, antennas transmit and receive power at a

frequency of 2.45 GHz over distances up to more than 60 cm. The

design, fabrication, and testing of a microstrip patch array

antenna is illustrated in this paper. Furthermore, three different

rectifier designs are discussed and compared based on their

performance: a peak rectifier, a full-wave rectifier, and a voltage

doubler rectifier. Finally, this paper discusses the overall results of

the system including how input power and distance between

antennas affect the output voltage of the system. The entire system

is able to illuminate a light-emitting diode (LED) wirelessly at

maximum distance up to 40 cm between the antennas.

Keywords—RF; microwaves; ISM; antenna; microstrip patch;

rectifier; voltage doubler rectifier; full-wave rectifier; peak

rectifier.

I. INTRODUCTION

ireless power transmission has been a topic of research

interest for several decades. Many technology leaders are

investing substantially into this technology to be able to add

wireless charging capability to their devices. In general,

wireless power transmission technology falls into two

categories: near-field and far-field. In near-field technologies,

power is transferred wirelessly over short distances using

inductive coupling with coils. This category is widely used to

add wireless charging capabilities with charging mats to smart

phones and other handheld devices. However, techniques used

in this category can only transmit power over a very short

distance. On the other hand, in far-field technologies, power is

transferred via electromagnetic radiation such as microwaves

and laser beams. In contrast to near-field, far-field technologies

provide power transmission over much longer distances but

have considerable propagation losses.

This paper describes a far-field wireless power transmission

system. In this system, antennas transmit power using

microwaves in the ISM band. As shown in Fig. 1, a signal

generator will generate power at 2.45 GHz and will be

connected to a transmitter antenna which will transmit the

power using microwaves to the receiver antenna. The power

received at the receiver antenna will be connected to a rectifier

which will convert the received microwave signal into DC

power. Then, the converted DC power will be connected to a

load such as a light-emitting diode (LED). In Section II of this

paper, the transmitter and receiver antennas will be discussed,

while Section III will focus on the rectifier circuits tested in this

project. The overall system performance will be explored in

Section IV, followed by conclusion in Section V.

Fig. 1. Microwave wireless power transmission block diagram

To ensure safety from high-levels of electromagnetic

radiation, the system designed in this project is FCC (Federal

Communications Commission) compliant. Part 15 of

subchapter A in chapter 1 under title 47 of FCC

telecommunication rules and regulations shows the maximum

power that can be safely transmitted between the two antennas

[7]. For the gain of the antenna used in this project, 28 dBm is

the maximum power that can be safely transmitted which will

not be exceeded in this project. All the antennas and rectifier

circuits are home-built; thus, equipment authorization is not

required by FCC.

II. ANTENNA

To achieve our objective, which is transmitting power using

microwaves, a device was required that can transmit, receive

and handle a certain amount of power. In other words, an

antenna is required to achieve this objective. There are many

types of antennas such as dipoles, parabolic, and microstrip

patch antennas. Each one of them has its own features,

advantages and disadvantages. In this project, a microstrip

patch antenna was chosen because it has many advantages that

will help this project to be efficient. In particular, microstrip

patch antenna has a light weight and small size compared to

other antennas. Also, directivity is one of the most important

features of microstrip patch antenna. Directivity helps to

transmit maximum power possible into one direction.

Moreover, if considered fabricating the antenna with large

quantities, microstrip patch antenna has one of the lowest

fabricating costs [2].

Microwave Wireless Power Transmission

System

Omar Alsaleh, Yousef Alkharraz, Khaled Aldousari, Talal Mustafawi, and Abdullah Aljadi

Prof. Bradley Jackson

California State University, Northridge

November 16th, 2018

W

2

A. Design

To design a microstrip patch antenna, different parameters

must be considered to design the antenna properly, as shown in

Fig. 2. The dielectric constant 𝜀𝑟 of the material used to

fabricate the antenna and the thickness of that material are not

free variables, so it has to be chosen before doing any

calculation. A lower dielectric constant will lead to a wider

impedance bandwidth. All of the parameters must be optimized

for the type and the height of the material.

Fig. 2. Microstrip patch antenna [3]

Considering the parameters shown in Fig.2, the width was

calculated using:

W =c

2𝑓√(εr+1)

2

where c is the velocity of electromagnetic waves in free space,

which equals to 3x1011 mm/s, and f is the operating frequency

in GHz. Then, the effective dielectric constant εreff was

calculated using:

εreff =εr + 1

2+εr − 1

2[1 + 12

h

W]−1

2

where 𝑊

ℎ>1. Finally, the length was calculated using:

L =c

2𝑓√εreff− 2∆L

where ∆𝐿 can be found using [3]:

∆L

h= 0.412

(εreff+0.3)(W

h+0.264)

(εreff−0.258)(W

h+0.8)

.

The microstrip patch antenna was designed to operate at 2.45

GHz. the parameters were calculated for that frequency using

the formulas mentioned earlier. The calculated parameters, as

shown in Table I, are approximated, and they must be optimized

with the simulator to have better results and the best reflection

coefficient possible. A reflection coefficient is the amount of

power reflected from the antenna, and it needs to be matched to

transmit or receive maximum power, where less than -10 dB is

considered well matched. The length of the patch will affect the

chosen operating frequency [1]. The geometry of the antenna is

shown in Fig. 3.

Fig. 3. Single microstrip patch antenna geometry

The single microstrip patch antenna has a gain around 7 dBi

and a reflection coefficient less than -10 dB between 2.42 GHz

and 2.47 GHz, as shown in Fig. 4. To be able to focus more

power to increase the distance, a higher gain is required. To

achieve that, a 2x1 microstrip patch antenna array was designed

for better gain and bandwidth. However, the size of the antenna

is doubled.

Fig. 4. Reflection coefficient versus frequency for a single microstrip patch

antenna

TABLE I

SINGLE MICROSTRIP PATCH ANTENNA PARAMETERS

Parameter Value

Frequency 2.45 GHz

Dielectric constant εr 2.2

Thickness 1.5875 mm

Width of substrate 75 mm

Length of substrate 64.9 mm

Width of patch 65.5 mm

Length of patch 39.1 mm

Width of feed line 4.88 mm

Notch width 0.28 mm

3

The 2x1 microstrip patch antenna was designed using a

standard microwave T-junction power divider technique, as

shown in Fig. 5, where the feed line splits into two other feed

lines. A 50 Ω feed line splits into two 100 Ω line, and each 100

Ω feed line connects to a 70.7 Ω feed line. This method helps

to match the impedance accordingly. The 2x1 microstrip patch

antenna parameters is shown in Table II.

Fig. 5. 2x1 microstrip patch antenna array geometry

B. Fabrication & testing

The ease of fabricating microstrip patch antenna is one of its

own advantages. After the array was designed and the results

were satisfying, the antennas were fabricated to be tested. A

photograph of the fabricated microstrip patch array antenna is

shown in Fig. 6. The array was fabricated with an RT/Duroid

5880 with a thickness of 1.57 mm using the milling machine at

CSUN. The microstrip patch arrays were tested in the CSUN

anechoic chamber. It was observed that the simulation results

were slightly different compared to the measured results after

fabrication. The measured results showed a reflection

coefficient of -16 dB as shown in Fig. 7. The normalized

radiation pattern is shown in Fig. 8. The measured gain of the

antenna is 10 dBi. The simulated and measured results were

different due to manufacturing tolerance and simplifications

used for the simulated model.

Fig. 6. A photograph of the microstrip patch array antenna

Fig. 7. Reflection coefficient versus frequency for a 2x1 microstrip patch

array antenna

Fig. 8. Normalized radiation pattern of the 2x1 microstrip patch array antenna

III. RECTIFIER CIRCUITS

This section illustrates the comparison of peak rectifier, full

wave rectifier, and voltage doubler in terms of performance.

The diode has enormous impact on the rectifier’s performance

since it is the main source of power loss. To be suitable for this

microwave power transfer project, the diode must have low

turn-on voltage to operate efficiently. As a result, the diode

Avago HSMS-2820 was chosen for each rectifier in this project.

The smoothing capacitor was SMD Multilayer ceramic

capacitor. This model was used due to its High-Q and low

effective series resistance (ESR). Advanced Design System

(ADS) by Agilent Technologies Inc. was used to simulate and

design the circuits. The components were simulated accurately

considering parasitic parameters from manufacturer's data

sheets. The components were simulated taking into

TABLE II MICROSTRIP PATCH ANTENNA ARRAY PARAMETERS

Parameter Value

Frequency 2.45 GHz

Dielectric constant εr 2.2

Thickness 1.5875 mm

Width of substrate 167 mm

Length of substrate 75 mm

Width of patch 76 mm

Length of patch 39.1 mm

Width of feed line 4.88 mm

Notch width 0.28 mm

4

consideration the parasitic parameters from manufacturer's data

sheets. The fabricated circuits were designed with Roger

RT/Duroid 5880 substrate (𝜀𝑟= 2.2 and thickness of 0.79 mm).

A. Peak rectifier

In a peak rectifier, the diode is forward biased and current

flows through the load resistor during the positive half cycles.

During the negative half cycles, the diode is reverse biased.

Thus, the rectifier acts as an open circuit and current will not

flow through the load resistor. Therefore, the output will be

only the positive cycles [8]. The circuit diagram is shown in

Fig. 9, and a photograph of the fabricated rectifier is shown in

Fig. 10.

Fig. 9. Peak rectifier circuit diagram

Fig. 10. A photograph of peak rectifier

B. Full-wave rectifier

The classic full wave rectifier circuit is illustrated in Fig. 11.

The main purpose of the full wave rectifier is to convert the AC

signal to DC. As shown in the schematic, it is made of two parts.

The first part consists of 4 Schottky diodes that operate as

follows: during the positive half cycles, the diodes D2 and D3

become forward biased, the current flows through them into the

load resistor. During the negative half cycles, the diodes D1 and

D4 become forward biased and the current flows through them

into the resistor. The second part of the schematic is the

smoothing capacitor. To reduce the ripple voltage of the output,

a smoothing capacitor is connected in parallel with the load

resistor. The capacitor reduces the ripple by storing and

discharging energy in between the cycles which will result in

smoother DC output. A photograph of the fabricated rectifier is

shown in Fig. 12.

Fig. 11. Full-wave rectifier circuit diagram

Fig. 12. A photograph of full-wave rectifier

C. Voltage Doubler rectifier

The functionality of the voltage doubler rectifier is to double

the input voltage, so the output voltage would be twice the peak

input voltage and it can be represented as Vout= 2(VPeak-VDiode).

It consists of two capacitors and two Schottky diodes, as shown

in Fig. 13. The voltage doubler rectifier can be constructed of

two circuits which are the clamper and rectifier circuits [4]. The

clamper circuit appears during the negative cycles when the

diode D1 is forward biased. The rectifier circuit appears during

the positive cycles when the diode D2 is forward biased. C1 and

D1 rectify the signal during the negative cycles, whereas C2 and

D2 rectify the signal during the positive cycles. However, the

voltage that can be stored on C1 during the negative cycles is

transferred to C2 during the positive cycles. As a result, the

voltage on C2 can be twice VPeak minus 2VDiode [5]. Fig. 14

shows the fabricated voltage doubler rectifier.

Fig. 13. Voltage doubler rectifier circuit diagram

5

Fig. 14. A photograph of voltage doubler rectifier

D. Results of rectifier circuits

The three rectifiers were simulated and measured at 2.45

GHz at different load resistances and power inputs. Different

load resistances were used to simulate the output voltage at a

fixed power input of 7.2 dBm for the three rectifiers. As shown

in Fig. 15, the voltage doubler rectifier had the best results.

Furthermore, the rectifiers had similar results for input power

less than 4 dBm, as shown in Fig. 16. However, the voltage

doubler rectifier has better results for input power greater than

4 dBm. Thus, the voltage doubler was selected for the

microwave power transfer system.

Fig. 15. Simulated load resistance versus output voltage at 7.2 dBm for the

three rectifiers

Fig. 16. Measured input power versus output voltage for the three rectifiers

with a 1.1 kΩ load.

IV. OVERALL SYSTEM PERFORMANCE

A. System Simulator Application

To help simulating the system and predicting its

performance, a system simulator application was built using

MATLAB. The application takes several inputs and provide the

power received at the receiver antenna as an output. The inputs

to the application are input power, transmitting antenna gain,

receiving antenna gain, distance between the two antennas,

operating frequency, and polarization angle. Then, for the

second stage of the application, it calculates the DC output

voltage depending on the rectifier type. The system simulator

application graphical user interface is shown in Fig. 17 below.

The application uses the Friis transmission equation to calculate

the received power output which considers two antennas in free

space with no obstruction nearby [9]. The equation is as

follows:

PR =PTGTGRc

2

(4πR𝑓)2

Where: PT is the input power at the transmitter antenna.

GT is the transmitter antenna gain.

GR is the receiver antenna gain.

R is the distance between the two antennas.

𝑓 is the operating frequency.

c is speed of light constant.

Fig.17. System Simulator Application GUI

The application also indicates whether the user is in safe area

according to FCC regulations by calculating the power density.

The maximum power density allowed for general population

according to FCC regulations is 1 mW/cm2 at the operating

frequency of this project [6]. The power density is calculated

with the following equation:

S =PG

4πR2

Where: S is the power density.

P is the input power at the transmitting antenna.

G is the gain of the antennas.

R is the distance to center of radiation of antenna.

B. Overall system results

In this section, the voltage doubler rectifier was used along

with the two antennas to measure the overall system

performance. As shown in Fig. 18, the system consists of a

6

signal generator that is connected to the transmitting antenna.

Then the receiver antenna receives the transmitted microwave

signal and passes it to the rectifier which converts it into DC

power to light up the LED.

Fig. 18. A photograph of the microwave wireless power transmission system

The overall system results were measured in two different

setups. For the first set-up, the output voltage was measured for

a fixed signal generator power of 23 dBm and varying distance

between the two antennas with no load resistance. For this

measurement, the two antennas must directly face each other to

maximize the power delivered to the receiving antenna. Any

slight rotation of the antenna will result in power loss. As shown

in Fig. 19, the output voltage reaches above 5 V for short

distances and decreases with longer distances as expected. In

fact, a 1 V output was obtained up to a distance of

approximately 70 cm. For the second set-up, the distance

between the two antennas was fixed to 30 cm and the output

voltage was measured for varying input power. Fig. 20 shows

how the output voltage increases with higher input power. In

this setup, an LED was used to illustrate the results visually.

The maximum distance for which the system could illuminate

an LED was 40 cm at an input power of 23 dBm.

Fig. 19. Measured distance versus output voltage at a fixed signal generator power of 23 dBm

Fig. 20. Measured input power versus output voltage at fixed distance of 30

cm

V. CONCLUSION

A far-field power transmission system was built and tested

successfully as intended. The antennas were designed and

fabricated to achieve the purpose of transmitting power

wirelessly. Afterwards, the three rectifier circuits were built and

tested for better performance in terms of converting microwave

signal to DC power. The overall system succeeded in powering

up an LED wirelessly over a distance of up to 40 cm. However,

achieving high DC levels while staying in the safe

electromagnetic limits was challenging especially when

transmitting power in longer distances due to the FCC

limitations.

Far-field wireless power transmission may be widely relied

on to transmit power wirelessly in the future. Microwave power

transmission can be used in many different applications such as

charging smart phones and other handheld devices. Technology

leaders can enhance the antennas by decreasing its size and

increasing its efficiency. In addition, companies can improve

the efficiency of the rectifiers to get a better overall result. Such

an application may be developed and seen in a market product

in the following years.

REFERENCES

[1] D. M. Pozar, “A Review of Aperture Coupled Microstrip Antennas:

History, Operation, Development, and Applications,” University of

Massachusetts, Amherst, MA, rep., 1996. [2] V. Mohan Kumar, N. Sujith, and S. K. Behera, “Enhancementof

Bandwidthand Gainof a Rectangular Microstrip Patch Antenna,” A thesis

submitted in partial fulfillment of the requirements for the degree of Bachelor of Technology in Electronics and Communication Engineering,

pp. 1–53, 2010.

[3] M. P. Civerolo, “Aperture Coupled Microstrip Antenna Design and Analysis,” In Partial Fulfillment of the Requirements for the Degree

Master of Science in Electrical Engineering, pp. 1–118, Jun. 2010.

[4] A. Sedra and K. Smith, Microelectronic circuits, 6th ed. New York: Oxford University Press, 2009, p. 212.

[5] D. Harrist, "Wireless Battery Charging System Using Radio Frequency

Energy Harvesting", Submitted to the Graduate Faculty of The School of Engineering in partial fulfillment of the requirements for the degree of

Master of Science, p. 13, 2011.

7

[6] “RF Safety FAQ,” Federal Communications Commission, 11-Oct-2016. [Online]. Available: https://www.fcc.gov/engineering-

technology/electromagnetic-compatibility-division/radio-frequency-

safety/faq/rf-safety. [7] eCFR - Code of Federal Regulations. [Online]. Available:

https://www.ecfr.gov/cgi-bin/text-

idx?SID=17c17d56afef8ca1aa64e92282caf53b&mc=true&node=pt47.1.15&rgn=div5#se47.1.15_115.

[8] “Peak Detector,” All About Circuits. [Online]. Available:

https://www.allaboutcircuits.com/textbook/semiconductors/chpt-3/peak-detector/.

[9] P. Bevelacqua, “The Friis Equation,” Friis Equation - (aka Friis

Transmission Formula). [Online]. Available: http://www.antenna-theory.com/basics/friis.php.