Microwave Power Transfer

5.1 Microwave power Beaming

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Features of Microwave WPT• High power oscillators (kW~GW)

Magnetron, Gyrotron, etc. Inexpensive & easy maintenance.

• High energy conversion efficiency

DC-RF conversion efficiency is 50％~90％ minimizing heat load.

• Phase controllability

Beam forming and steering by a phased array. No need of mechanical Beam forming and steering by a phased array. No need of mechanical

control.

• High transmittance though the atmosphere

Transmittable though clouds, fogs, and dielectric medium.

• Large beam divergence

Resulting in short transmission distance, large receiver area.

• Impact on human body

At some frequencies, resonantly absorbed by H2O molecules, resulting in

heating inside the body. Regal safety threshold for stationary exposures at

GHz ranges is 10 W/m2. (1% of solar radiation)3

2. Microwave Oscillators2. Microwave Oscillators

Magnetron

Copper anode block

Resonating cavity

Operational principle 8-cavity magnetron

- Inexpensive, high power and very efficient ~ 80% of DC-AC conversion efficiency.- High output power of 100 kW@915 MHz has been obtained. - However, frequency is not precisely controllable and waveform/frequency is not stable.

A magnetic field parallel to a hot filament

5

Field Effect Transistor (FET)

Source Gate Drain

Self-excited oscillation circuit with

Compact, light weight, efficient ~90%.Frequency is stable and precisely controllable.

Structure of a GaAs FET amplifier

eff g2f v Lπ=frequency

veff : mean electron velocityLg : gate length

Semiconductor GaAs Base

n-GaAs active layer

6

Self-excited oscillation circuit with positive feedback

Comb-shape GaAs FET chip (NEC Corp.) 440×970 μm. 2 W output with Lg=0.5 μm, Wg=3mm@18 GHz.

Electron depletion region

GaN (Gallium Nitride) FETworks at 10 times higher voltages and higher electron mobility than GaAs transistors

Pow

er /

WPo

wer

/ W

Pow

er /

WPo

wer

/ W

Toshiba 6 GHz 174 W GaN Power FET

SiC, GaN high frequency devices• Broadcasting• Ground base for satellite communication• Cellular wireless infrastructure• High power radars• Public mobile radio• Military & aerospace

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Frequency /GHzFrequency /GHzFrequency /GHzFrequency /GHz

Pow

er /

WPo

wer

/ W

Pow

er /

WPo

wer

/ W

Gyrotron JAEA Gyrotron SpecificationsFrequency 170 GHzMax Output Power 1.5 MWOutput Duration 0.1 ms – 1000 sBeam profile GaussianBeam waist 20 mmElectrical efficiency 60%

１MW Gyrotron (Japan Atomic Energy Agency）8

3. Demonstration3. Demonstration

Power transfer to a model helicopter

light-weight & efficient rectenna receiver

Microwave-powered helicopter,1964

Raytheon Co. (USA) & Dr. William Brawn. 2.45 GHz.

Manufactured a rectenna array of 90% conversion efficiency .

It flew 10 hours at 18 m altitude.

10

High Power Transfer at Goldstone, US

Microwave power transmission demonstration at Goldstone, California, 1975

NASA JPL & Raytheon: 30kW, 2.45 GHz for 1mile distance @84% rectenna efficiency.

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Rectenna panels

12

WPT to an airplaneActive phased array transmitter (1 kW)

MILAX (Microwave Lifted Airplane eXperiment), 1992

Kyoto Univ., Nissan Motors, etc. A plane flew for 40 sec, 400 m distance at 15 m

altitude. Tested active phased array, 2.4 GHz GaAs-FET amplifier, 4 bit-digital phase

shifter, micro-strip antenna.13

Rocket experiment in space

METS (Microwave Energy Transmission in Space) rocket experiment, 1993

Kyoto Univ., Kobe Univ. etc., in plasma environment in the ionosphere at 150～200

km altitude, power was sent from Mother ship to daughter ship using the same

transmission-reception system as MILAX ones.

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4. Transmitter(Phased Array Antenna)(Phased Array Antenna)

Phased array antenna

θstr θstr

λ(δ/2π)

d

Concept of phased array antenna

strsin2d

λ δθ

π=

Adaptive array antennas for cell phone base. (Smart antenna) Directionality is designed to form a cell of communication area.

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Retro-directive function

α αα α

Sending back phase conjugation waves

λ(δ/2π)

d

sin2d

λ δα

π=

( )in 0 sinE E tω δ= + ( )out 0 sinE E tω δ= −

λ(δ/2π)

d

Pilot signal detection

Hardware retro-directiveSoftware retro-directive

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Active control MPT system (Univ. of Tokyo)

2.45GHz

Target

5.8GHz

Receiver

System

2.45GHz

Pilot

Signal

5.8GHz

Energy

Beam

Transmitter

System

Phase

difference

Tracking

System

Digital phase

control

PC

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Auto-tracking MPT system using an active phased array antenna.

Active phased array transmitter

Transmitter specifications

Parameters valuesmicrowave frequency 5.8 GHzwavelength, λ 51.7mm

5-element active phased array antenna

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wavelength, λ 51.7mmtotal transmission power 3.5 Warray pitch, d 110 mm (d/λ=2)diameter of the array, D 330 mmSteering angle ,θ 9°Divergence angle ,θd 9°

Beam forming

Measure and computed beam profiles

ComputedMeasured

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Beam steering(x, y, z) = (160, 0, 1040)

(x, y, z) = (113, 113, 1040)

Computed beam steering

calculated measured

(x, y, z) = (80, 139, 1040)

(x, y, z) = (0, 160, 1040)

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5. Receiver(Rectifying Antenna: Rectenna)(Rectifying Antenna: Rectenna)

Theory of Rectenna Design

Antenna

Dipole and microstrip patch antennas are used because of

- simplicity of structure

- miniaturization in the size and weight

Schottky diode (SBD, Metal-semiconductor junction)

- low VT of 0.15-0.45 V 順方向電圧 (cf. 0.6-1.7 V in PN-junction diode)

- very fast switching action of 100 ps (10 GHz)

RECTENNA is the coined word for “RECTifier” and “antENNA”.

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- very fast switching action of 100 ps (10 GHz)

dipole rectenna

Theory of Rectifier Circuit Design

R s

R LV g

+

-VL

+

-

I 1 I 2

I R

+-

I 1Input

Filter

Output

Filter

Output Smothing Capacitance

λ/4 Microstrip Line

ZW, β

R S

D C o

C 1RF-IN

Harmonic Filter

R L

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input filter

- prevent any of harmonic wave from flowing back to the antenna.

- allow current flow at the fundamental frequency.

output filter

- reflect odd harmonics f, 3f, 5f ・・・.- allow even harmonics 0, 2f, 4f ・・・ to flow without any voltage drop

Block diagramIdeal equivalent circuit

Output Smothing Capacitance

and Lowpass FilterShottkey-Barrier Diode

Output Smothing Capacitance

and Lowpass Filter

λ/4 Microstrip Line

ZW, β

R S

D C o

C 1RF-IN

Shottkey-Barrier Diode

Harmonic Filter

R L

Rectification with a single diodenodeanti-node

VDC

VT

VBR

breakdown voltage VBR

thermal voltage VT

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Rectification with a single diode

Typical diode V-I curve

VT

VBR

2T LV R

2Ld

V R

VT

VBR

Half-wave Rectification with a single series diode

Energy loss factors and conversion efficiency

Co

nv

ers

ion

Eff

icie

ncy

　η

100%

Diode Max.

Efficiency

VR Effect

V f Effect

Higher order

Harmonics Effect

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- η decreases with a decrease in Pin because fractional loss due to diode forward voltage

drop VT increases. (Vfeffect)

- When diode voltage Vd > VBR, η also decreases with Pin because reverse current is

induced. (VR

effect) It takes maximum when VDC ~VBR/2 or Vd ~VBR.

- At impedance matching condition with optimal external load RL, there is no reflected

wave.

Input Power P in

L

R

R

V

4

2

Co

nv

ers

ion

Eff

icie

ncy

2

BR

L4

V

R

Dipole Rectenna array on membranelight, thin, flexible & polarity free

Multi-directional dipole antenna (NASA Langley Research Center)

H-shape dipole on membrane(UT and JAXA/ISAS)

0.5λ

0.55λ

0.12λ

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Polarity matching in reception antennaLinear polarization

Yaw angleΨ

No attitude (Yaw angle) changeNo attitude (Yaw angle) changeNo attitude (Yaw angle) changeNo attitude (Yaw angle) changeLinearly polarized waves (Generally longer transmission distance)

Horn antennaParabola antenna Patch antenna

With attitude (Yaw angle) changeWith attitude (Yaw angle) changeWith attitude (Yaw angle) changeWith attitude (Yaw angle) changeCircular polarized waves (independent of antenna attitude, wave polarity)

Circular polarized patch Circular polarized patch rectenna

Monopole/Dipole rectennaPatch rectenna

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Micro-Strip Antenna (MSA) y

x

z

#4

#1

#3

#2

F

E

H

a

b

Polarization

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・used for wireless communication receivers on aircraft, spacecraft, and automobile and for GPS receivers. ・directionality vertical to the antenna plane・Light weight, high power

Linearly polarized MSA

Interior electromagnetic field of

Rectangular MSA

a is normally half wavelength. It works as an

array of dipole antennas. The ground plate

reflect re-radiation. More volume(a*b*t),

more band width.

H

tGround

Impedance Matching between MSA and output

ρ 0

a

x

y

ε r

F

G

S

R

R : Radiator element

S : dielectoric Substrate

G : Ground

F : Feeding point

Output impedance Z varies from 0

(center) to several hundred ohms

(open boundary) depending on the

feeding point location ρ0. Design ρ0

to have 50 Ω impedance.

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G

S

R

ρ 0

a

x

y

ε r

F

b

Tf

Fm

ρ 0

x

y

ε r

l s

l 0

w

b) Coplanar Feeding Method

a) Backside-Coaxial Feeding Method

c) ElectroMagnetic (EM) Coupling Feeding Method

to have 50 Ω impedance.

0150 tan

2Z

a

ρπ = − Ω

for 0.15< ρ0/a<0.85

a

ρ0

MSA Rectenna Design

Antenna Array (Right Face)

Dielectric

Medium

Combine

Patch antenna and rectifier circuit are

printed on a substrate which requires low

dielectric constant (εr = 1.2~5.0) and low

dielectric loss (tanδ=10-3~10-4), such as

the Teflon-fiberglass substrate.

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Rectifier Array (Rear Face)

Cupper (Ground)

Cupper (Antenna)Cupper (Rectifier)

MAS rectenna array

The size of antennas in its resonance

direction is λg/2.

r

gε

λλ 0=

Rectifier Circuit for MSA rectenna

Output Smothing Capacitance

λ/4 Microstrip Line

ZW, β

R S

D C o

C 1RF-IN

Harmonic Filter

R LFeeding Point

(DC out)

Stub

LPF

OutputTransmissionLine

Chip

Condenser

DiodeRF IN

Feeding Point

(DC out)

Stub

LPF

OutputTransmissionLine

Chip

Condenser

DiodeRF IN

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Diode

Chip Condenser

Stub

Feeding

pointRF-IN

Stub

Output Smothing Capacitance

and Lowpass FilterShottkey-Barrier Diode

An example of rectifier

The equivalent circuit is the same as

previous.

Stubs are used as capacitors for

input and output filters.

Flexible Patch Rectenna on a felt pad

light, thin, flexible, polarity free & efficient

Patch rectenna on felt (UT)

Rectification efficiency

Wearable patch antenna (NICT)

Patch rectenna on felt (UT)

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6. Future Applications6. Future Applications

Observation System for Gas Pipeline Network Using Unmanned Airship

Specifications of power and propulsion systems

Endurance

Thrust Force

Propulsion Power

Energy for 10h Cruise

10

584

8.17

224

h

N

kW

kW·h

Unmanned Airship (Unmanned Airship (Unmanned Airship (Unmanned Airship (φφφφ30m x 8m). 30m x 8m). 30m x 8m). 30m x 8m). Recharge with 10hr intervalsRecharge with 10hr intervalsRecharge with 10hr intervalsRecharge with 10hr intervals (power (power (power (power stations at every 100stations at every 100stations at every 100stations at every 100----200 km.)200 km.)200 km.)200 km.)

Energy for 10h Cruise

Microwave Power 30GHz

Energy Charging Time

Rectenna Efficiency

Rectenna Max. Power

Density

Rectenna Weight Density

Rectenna Surface Area

Rectenna Total Weight

Fuel Cell Energy Density

Fuel Cell Weight

224

200

1.1

0.5

1.5

1.5

60

90

500

450

kW·h

kW

h

-------

kW/m2

Kg/ m2

m2

kg

Wh/kg

kg

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Solar Power Satellite (SPS) system

1 GW Class SSPS Concept

Panel 2 km × 1.9 kmOutput power 1 GWOrbit GeosynchronousPowerPowertransmission 5.8 GHz

Total weight 20,000 tonsProblems ・EM interference with aircrafts,

birds.・cost of electricity・terrestrial energy balance

1 GW Class SSPS Concept in JAPAN

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Summary of Microwave WPT

MW class oscillators are available at inexpensive price. Frequency is stable and phase is controllable.

High power of MWT was demonstrated on the ground and in space.

A microwave beam can be formed and steered by a phased

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A microwave beam can be formed and steered by a phased array antenna, without mechanical gimbaling.

Non-mechanical Automatic tracking is realized through a retro-directive function.

Rectifying antenna (Rectenna) using a single SBD rectifyer and patch antenna is simple, light weight, inexpensive and flexible.