CAVITY DESIGN CONSIDERATIONS FOR
SOLID STATE MICROWAVE OVENS
ELECTROMAGNETIC FIELD SIMULATION
GEOFFREY TUCKER FOR RAYMOND GUO
RF SYSTEMS & SOLUTIONS / RF APPLICATIONS, POWER PRODUCTS
SESSION FTF-HMB-N1994
17 MAY 2016
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SOLID STATE MICROWAVE OVEN
•
Solid-state transistors can generate high power microwaves that feed into
the cooking chamber via microstrip patch antennas
•
Transistor oven benefits over traditional magnetron ovens:
Microstrip patch antennas
- High Resolution Control
- Precise energy delivery in multi-feed system
Cavity
- Higher quality nutrition
Load
- Faster meal preparation
Glass shelf
- Versatile food adaptability
- Reduced mechanical BOM
Power
Amplifier
- Reduced Size & Weight
- Form factor design flexibility
β12
Calculate the highest efficiency
Power
Amplifier
Power
- Lower logistics cost
Smart control
- Durability
algorithm
- Consistent heating parameters over lifetime
RF
Source
Power
Amplifier
- Reduced supply voltage (4000KV to 50V)
- Reduced maintenance
•
RF performances (power and efficiency) are significantly improving and
cost will decline as adoption increases
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Amplifier
β13
AC/DC
Converter
Power
Power
Amplifier
β14
main
Phase shifter
Closed loop control system
Electronic alteration of
the electromagnetic
field pattern
AGENDA
•
Principle of Microwave heating
• Microwave Cavity
• Effect of Load Properties
• Effect of Feed Sources
• Antenna Properties
• Dual-Source Microwave Oven
• Quad-Source Microwave Oven
• Next Step
Note that the simulated results were obtained using HFSS software
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PRINCIPLE OF
MICROWAVE HEATING
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FOOD DIELECTRIC PROPERTIES AND ABSORBED POWER
•
Electromagnetic waves may be absorbed in many different ways
(Conduction loss, Ionic polarization, Dipole molecular vibrations and
rotations etc.)
•
In a microwave oven, the electrically dipolar water molecules absorb
most of the microwave energy resulting in a consequential rise in the
temperature
−
In low frequency electric fields the dipoles easily follow the changes in the field
and their orientation changes in phase with the field.
−
In higher frequencies the inertia of the molecules and their interactions with
neighbors make changing orientation more difficult and the dipoles lag behind
the field.
•
The dielectric loss factor implies the conversion of electrical energy into
heat
•
The dielectric loss tangent for the material and average microwave
power absorbed may be written as
ε2= ε1tan δ


1
P  [(   0 2 ) E  E *dV  0 2 H  H *dV ]
2
where V is the volume of the load, σ is the electrical conductivity, ε2 and
µ2 the imaginary parts of the permittivity and permeability respectively.
E and H are the absorbed electric and magnetic fields.
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ABSORBED POWER AND PENETRATION DEPTHS IN WATER
Water is a kind of anti-magnetic material with µ2=0, and
the way of dielectric polarization plays a dominant role
in power absorption

1
Pwater   0 2 E  E *dV
2
Penetration depths of microwaves in water:
Dp 

2 2 2
[(1  ( 2 / 1
1
)2 )2
 1]

1
2
•
Microwave ovens operating around 2.45GHz directly
heat only to a depth of a couple of centimeters
(Dp=1.4cm) in water.
•
Frequency(2.45GHz) used is a compromise between
efficiency (high frequency) and energy penetration (low
frequency).
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1.55cm
HOT AND COLD SPOTS
•
The waves propagate in the cavity and form traveling
waves but there is no EM field in some parts because
of superposition of many reflected waves inside a
cavity, which can cause the food to heat in some
places but to remain cool in others.
•
Uniformity of the electromagnetic field distribution
leads to uneven heating of food.
•
LEDs show the energy distribution in cavity.
•
Simulation result shows the energy distribution in the
water and cavity.
•
The changes of frequency, phase and amplitude of
multiple sources can control electromagnetic field
distribution(hot and cold spots)
•
Heat propagation inside the medium
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Hot spot
Cold spot
THERMAL PROPERTIES
The heating of the material depends not only on the
absorbed power, but also on the thermal properties
Heat
capacity
Diffusion
•
Heat
conduction
Phase
change
heat
Temperature
Heat
radiation
Dielectric properties
Convection
Microwave ovens are specified today on their ability
to heat one liter of water
EM field distribution
Relative Dielectric
Constant
ε1
Relative Dielectric Loss
Constant
Water
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(@25˚C)
7
ε2
12.48
The dielectric properties of load are dependent on
the temperature.
• Since the dielectric properties are temperature
dependent, this influences again the
electromagnetic field distribution
• For simplicity, we just consider its starting EM
field distribution and set the temperature to be
25°C.
• The key parameters of water(@25°C) can be
seen in the table below
Loss Tangent
tan δ
0.154
Penetration depths(mm)
13.8
Relative permeability
0.999991
Conductivity(S
iemens/m)
0.0002
MICROWAVE CAVITY
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ENERGY DISSIPATED IN CAVITY WALLS
•
The microwave energy into the cooking chamber is
absorbed in water and dissipated in the walls of cavity.
• Efficiency ηa can be estimated using relation:
a  1 

f 0 Q0

f0 Q 
278.8mm
0
Unloaded Q0’
where f0 and Q0 are resonant frequency and quality
factor of the cavity with material within it, while f0’ and
Q0’ are defined for the cavity without any material
inserted within it. Both are simulated in the type of
eigen mode.
Q0 of the cavity with 0.38L water (Limited by computer
memory, not 1L water) within it is 338, while the Q0' is
1.2x104. The microwave energy that turn into heat of
the load has an efficiency about 83.2%;
• The material of cavity walls is stainless steel
(σ=1.1X106s/m) with thickness of 0.4mm.
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470.8mm
470.8mm
Loaded Q0
ELECTRIC FIELD MODE PATTERN IN CUBOID CAVITY
Mode #2
2.46031GHz
2.46031GHz
Mode #3
Mode #4
Mode #5
2.46032GHz
2.46032GHz
2.47700GHz
x-y plane
Frequencies
eigen value
Mode #1
470mm
x-z plane
470mm
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278mm
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ELECTRIC FIELD MODE PATTERN IN MODIFIED CUBOID
Mode #1
Frequencies
eigen value
2.4407GHz
prismoid
Mode #2
Mode #3
Mode #4
Mode #5
2.4418GHz
2.4496GHz
2.4510GHz
2.4527GHz
x-y plane
20.0
20.0
309.5
399.5
376.4
286.4
x-z plane
359.5
330.7
25.0
136.4
159.5
359.5
10.0
348.0
•
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Mode #3 which focuses Most energy in the center is the
most optimal EM field pattern for microwave oven heating
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y-z plane
290.7
Unit: mm
319.5
199.5
176.4
319.5
308.0
ELECTRIC FIELD PATTERN INFLUENCED BY SMALL LOAD
Mode #2
Mode #3
Mode #4
Mode #5
2.4421GHz
Q=2054
2.4448GHz
Q=115
2.4474GHz
Q=242
2.4536GHz
Q=2496
2.4540GHz
Q=1065
x-z plane
x-y plane
Frequencies
eigen value
Mode #1
Just within 0.03L water
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CONCLUSION
The prismoids around the cavity can focus more EM energy in the center.
Lowering density of magnetic field intensity distribution on the walls of cavity can reduce
microwave energy losses in the walls
Smaller quality factor Q means higher efficiency and better impedance matching. The load has
different energy absorption efficiency in different field patterns (modes).
In the simulation results illustrated, an empty cavity displays a strong mode pattern at 2.45GHz.
But placing a small load (0.03L) in the cavity collapses the mode entirely.
That’s what to be discussed next!
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EFFECT OF LOAD
PROPERTIES
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PROPERTIES OF DIFFERENT KINDS OF “FOODS”
One
antenna
with 250W
power
1L water @2.45GHz
1L sea water (4S/m) @2.45GHz
1L water (100kg/m3) @2.45GHz
•
The larger the size of the material is, the better the impedance matching will be.
•
In sea water, the dielectric polarization plays a dominant role compared to Ionic polarization or
conduction loss
•
15
4.3L water @2.45GHz
The effective dielectric constant varies with density of water, this influences again the EM field
distribution
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PROPERTIES OF DIFFERENT KINDS OF “FOODS”
One
antenna
with 250W
power
1L water(ε1=40, ε2=12.48) @2.45GHz 1L water(ε1=81, ε2=0) @2.45GHz 1L water (divide into 4 parts) @2.45GHz 1L water (cubic sample) @2.45GHz
16
•
The lower the permittivity of the material, the higher the values of the field.
•
The impact of the imaginary part of the complex permittivity(ε2), directly related to the penetration
depths
•
Number, shape and position of the load have impact on electric field distributions
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MICROWAVE OVEN SYSTEM
Different “foods” give different input reflection coefficients and are consequently likely
to require heating in different frequencies. This implies that amplifiers should be able
to deliver peak power and efficiency at any frequency in the band, as the cavity
power absorption optima can occur at any frequency.
Simulating the responses of a cavity loaded with different quantities and types of
“foods” provides a characteristic fingerprint, which will be helpful for microwave ovens
adapt to various loads (shape, dimensions, permittivity, conductivity and so on).
Microwave ovens should use the smart control system to spread the power in the
most optimal way to heat a particular food, which maybe suggested to use different
feed sources for the heating process.
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EFFECT OF FEED
SOURCES
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THE EFFECT OF FREQUENCY OF THE FEED SOURCE
Changes of 10 MHz in that frequency(2.45GHz), corresponding to an imprecision of 0.4%
2.44GHz (0-3.79dBV/m)
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2.45GHz (0-3.89dBV/m)
•
The changes of frequency are very important in the
electric field distribution inside the oven cavity.
•
When the frequency increases, the number of
oscillations of the electric field increases, mainly due
to the decrease of the wavelength.
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2.46GHz (0-3.94dBV/m)
THE EFFECT OF PHASE OF THE TWO FEED SOURCES
P1=250W, P2=250W
Port 1
Port 2
β1=0°, β2=0°(0.8-3.22dBV/m)
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β1=90°, β2=0°(1.4-3.27dBV/m)
β1=180°, β2=0°(0.9-3.25dBV/m)
•
The changes of phase can influence the EM field distribution and strength
•
When the phase changes, hot spots and cold spots of the load varies,
mainly due to the mutual interference of the microwaves.
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THE EFFECT OF AMPLITUDE OF THE TWO FEED SOURCES
β1=180°, β2=0°
Port 1
Port 2
P1=50W, P2=250W(1.17-3.23dBV/m)
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P1=150W, P2=150W(0.8-3.14dBV/m)
P1=250W, P2=50W(0-3.05dBV/m)
•
The changes of Pin can influence the EM field distribution and strength,
also due to the overlay of the microwaves
•
The placement (orientation, location) of the antennas also need to
be taken into account
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Different
directions of
polarization
ANTENNA
PROPERTIES
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MICROSTRIP PATCH ANTENNA
Patch Antenna
Coaxial Feeder
I1
Ground
The feeding source point determines the direction
of polarization of the antenna
“
23
” in the figure shows the direction of
polarization
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PHASE CONTROL OF MICROSTRIP PATCH ANTENNA SOURCES
Antenna 1
Antenna 2
160mm
Ground
Shifting phase in multi-feed
system is a effective way to
direct energy to load
β1=0°, β2=0°
β1=90°, β2=0°
β1=180°, β2=0°
P1=100W, P2=100W
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β1=270°, β2=0°
AMPLITUDE CONTROL OF MICROSTRIP PATCH ANTENNA
SOURCES
β1=180°, β2=0°
P1=0W, P2=100W
P1=50W, P2=100W
P1=100W, P2=100W
P1=150W, P2=100W
Changing amplitude in multi-feed system is a another way to
influence the distribution of energy
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P1=200W, P2=100W
MUTUAL COUPLING IN TWO ANTENNAS
I2
I1
Antenna 1
Vs1
Antenna 2
Vs2
Excitation voltage Vs1 and Vs2 of
the two antenna elements :
Vs1  V1e j1
Vs 2  V2e j 2
Zg2
Zg1
Vs1 / Vs2  12e j12
Z g1  Z g 2  Z 0
Excitation
voltage
source
I1
Vs1
Zg1
Source
internal
Impedance
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Coupled
voltage
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I2
V12
Vs2
Z11
Zg2
Antenna
Self-impedance
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The incident power ratio:
1
Re(
2
V21
Z22
V12 Z11
 122
)(
1
)
2
1



Z g1  Z11
1
P1

P2 
V2 2 Z 22
1
1
Re(
)(
)
2
2
2
1


Z g 2  Z 22
2
2
MUTUAL COUPLING IN TWO ANTENNAS
•
The currents(due to the excitation sources and the coupled
voltages) I1 and I2 of the two antenna elements can be
expressed as follows:
I1 
•
V1e j1 ( Z g 2  Z 22 )  V2 e j2 Z12
I2 
( Z g1  Z11)( Z g 2  Z 22 )  Z12Z 21
V2 e j2 ( Z g1  Z11)  V1e j1 Z 21
( Z g1  Z11)( Z g 2  Z 22 )  Z12Z 21
Isolation I12between two antennas is the ratio of received
power PR1by antenna 1 to transmitted power PT2 by antenna 2
Z11  Z 0
(1  S11 )(1  S22 )  S12 S21
(1  S11 )(1  S22 )  S12 S21
Z12  Z 0
2S12
(1  S11 )(1  S22 )  S12 S21
Z 21  Z 0
2S21
(1  S11 )(1  S22 )  S12 S21
Z 22  Z 0
(1  S11 )(1  S22 )  S12 S21
(1  S11 )(1  S22 )  S12 S21
1
2
Re( I1'  ( I1' )*  Z11)
Z
Re( Z11)
PR1
12
I12 (dB)  10lg( P )  10log( 2
)  10log(
)
2
2
2
*  j12
*
j12
T2
1
*
[ Z 0  Z11  12 Z 21  12 ( Z 0  Z11) Z 21 e
 12 ( Z 0  Z11 ) Z 21e )] Re( Z 22 )
Re( I 2  ( I 2 )  Z 22 )
2
2
S12 Re[(1  S11)(1  S22 )  S12S21]
I12 (dB)  10log(
)
2
2
{1  S22  122 S21  212 Re[(1  S22 )S21*e  j12 ]} Re[(1  S11)(1  S22 )  S12S21]
Where
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I1'
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 V2 e j2 Z12

( Z g1  Z11)( Z g 2  Z 22 )  Z12Z 21
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Z g1  Z g 2  Z 0
MUTUAL COUPLING IN FOUR ANTENNAS
•
The currents(due to the excitation sources and the coupled voltages) I1 ,I2,I3 and I4 of the four antenna
elements can be expressed as follows (If Z g1  Z g 2  Z g 3  Z g 4  Z0 ):
Z12
 I1   Z 0  Z11
  
Z 0  Z 22
 I 2   Z 21
  Z
Z 32
31
 I3  
I   Z
Z 42
41
 4 
•




Z 0  Z 33
Z 34 
Z 43
Z 0  Z 44 
Z13
Z 23
Z14
Z 24
1
j
 V1e 1

j 2
V2e
 V e j 3
 3
 V e j 4
 4








Isolation I1,(2,3,4) is the ratio of received power PR1by antenna 1 to transmitted power PT2 , PT3 , PT4 by
antenna 2, antenna 3 and antenna 4
1
Re( I1  ( I1 )*
 Z11)
V1 0
2
PR1
I1,(2,3,4) (dB)  10lg( P  P  P )  10log(
)
T2 T3 T4
1
1
1
*
*
*
Re( I 2  ( I 2 )  Z 22 )  Re( I 3  ( I 3 )  Z 33)  Re( I 4  ( I 4 )  Z 44 )
2
2
2
The mutual coupling between four antennas can be calculated by Matlab!
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DUAL-SOURCE
MICROWAVE OVEN
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①
MUTUAL COUPLING IN DUAL-SOURCE MWO
•
Three ways to reduce the mutual coupling in antennas
1. Cross polarization with antennas arranged perpendicularly
2.Set up barriers to block coupling
3. Proper placement of antennas
port1
h
S
h=0, α12=1,β12=180°
70
85
100
115
130
145
160
mm
-10
-15
-20
I12
I21
S11
S22
-25
-30
dB
horizontal distance between antennas, S
30
-20
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-15
-10
-5
0
5
10
15
20
mm
-5
Isolation & RL
Isolation & RL
-5
S=133.3, α12=1,β12=180°
-10
-15
-20
-25
dB
I12
I21
S11
S22
vertical distance between antennas, h
port2
MUTUAL COUPLING IN DUAL-SOURCE MICROWAVE OVEN
②
port1
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port2
③
④
port1
port1
port2
Each dual-Source microwave oven model features the corresponding
serial number, like the number “②”.
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port2
MUTUAL COUPLING IN DUAL-SOURCE MICROWAVE OVEN
port2
port2
port2
port1
port1
port1
⑤
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⑥
⑦
The resonance frequencies of the antennas are decided by
the field patterns that could be excited.
Most of the resonances excited by Antenna I and Antenna II
occur at different frequencies.
When both feeds operate at the same frequency, which may
be hard to tune for getting low power return loss due to
impedance mismatch
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SIMULATION RESULT IN DUAL-SOURCE MICROWAVE OVEN
Placement
①
②
β12=0°
③
Same side
P1=250W
P2=250W
@2.45GHz
④
⑤
Opposite
side
⑥
Adjacent side
⑦
Methods
I21(dB)
I12(dB)
Load
loss(W)
PReturn1 (W)
PReturn2 (W)
Energy absorption
efficiency
Cross
polarization
-15.3
-11.6
449.6
22.4
10.1
89.9%
Metallic wall
-10.2
-8.7
420.2
48.27
16.7
84.0%
Metallic wall
+
Cross
polarization
-15.2
-13.8
455.2
8.8
20.6
91.0%
Metallic wall
+
Cross
polarization
-14.8
-14.7
389.9
20.5
78.7
78.0%
Cross
polarization
-27.7
-35.2
391.1
46.9
44.0
78.2%
Cross
polarization
-13.3
-15.8
290.5
82.7
115.3
58.1%
Cross
polarization
-24.6
-21.2
292.8
109.6
79.1
58.6%
Omit loss in the walls of cavity
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SIMULATION RESULT IN DUAL-SOURCE MICROWAVE OVEN
Placement
①
②
β12=45°
③
Same side
P1=250W
P2=250W
@2.45GHz
④
⑤
Opposite
side
⑥
Adjacent side
⑦
Methods
I21(dB)
I12(dB)
Load
loss(W)
PReturn1 (W)
PReturn2 (W)
Energy absorption
efficiency
Cross
polarization
-15.7
-11.9
460.9
17.8
3.9
92.2%
Metallic wall
-9.9
-8.6
403.3
60.9
21.3
80.1%
Metallic wall
+
Cross
polarization
-14.7
-13.5
470.4
7.8
8.0
94.1%
Metallic wall
+
Cross
polarization
-14.5
-14.3
361.4
23.9
102.3
72.3%
Cross
polarization
-27.7
-35.3
395.6
45.8
40.7
79.1%
Cross
polarization
-13.7
-16.4
269.3
58.2
160.6
53.8%
Cross
polarization
-24.6
-21.3
302.0
93.5
85.7
60.4%
Omit loss in the walls of cavity
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SIMULATION RESULT IN DUAL-SOURCE MICROWAVE OVEN
Placement
①
②
β12=90°
③
Same side
P1=250W
P2=250W
@2.45GHz
④
⑤
Opposite
side
⑥
Adjacent side
⑦
Methods
I21(dB)
I12(dB)
Load
loss(W)
PReturn1 (W)
PReturn2 (W)
Energy absorption
efficiency
Cross
polarization
-16.4
-12.6
456.2
11.0
16.7
91.2%
Metallic wall
-9.1
-8.2
388.8
60.7
35.5
77.8%
Metallic wall
+
Cross
polarization
-13.6
-12.4
473.0
10.7
2.7
94.6%
Metallic wall
+
Cross
polarization
-13.7
-13.2
363.0
22.2
99.8
72.6%
Cross
polarization
-27.8
-35.4
397.8
47.9
37.0
79.6%
Cross
polarization
-14.6
-17.5
235.8
73.0
179.1
47.2%
Cross
polarization
-24.6
-21.6
304.3
73.9
105.2
60.9%
Omit loss in the walls of cavity
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SIMULATION RESULT IN DUAL-SOURCE MICROWAVE OVEN
Placement
①
②
β12=180°
③
Same side
P1=250W
P2=250W
@2.45GHz
④
⑤
Opposite
side
⑥
Adjacent side
⑦
Methods
I21(dB)
I12(dB)
Load
loss(W)
PReturn1 (W)
PReturn2 (W)
Energy absorption
efficiency
Cross
polarization
-17.3
-13.4
417.5
5.6
62.5
83.5%
Metallic wall
-7.8
-7.6
394.4
29.7
58.8
78.9%
Metallic wall
+
Cross
polarization
-11.0
-10.3
442.8
20.2
20.3
88.6%
Metallic wall
+
Cross
polarization
-12.2
-10.9
435.5
9.9
36.7
87.1%
Cross
polarization
-27.9
-35.6
392.3
55.6
36.1
78.5%
Cross
polarization
-15.6
-18.8
205.9
167.7
114.2
41.2%
Cross
polarization
-24.6
-22.1
287.8
65.1
136.3
57.6%
Omit loss in the walls of cavity
37
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RETURN POWER FROM EACH PORT
/W
①
②
③
④
⑤
⑥
⑦
PReturn1
150
100
50
200
/W
①
②
③
④
⑤
⑥
⑦
150
PReturn2
200
100
50
0
/°
0
45
90 135 180 225 270 315 360
Phase shifts, β12
0
/°
0
45
90 135 180 225 270 315 360
Phase shifts, β12
The energy is returned from each port due to impedance
mismatch and cross-talk.
38
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ENERGY ABSORBED BY LOAD
500
/W
Energy absorption
①
②
Load Loss
450
③
④
400
⑤
⑥
350
⑦
300
Improving heating efficiency
by minimizing impedance
mismatch and cross-talk
between feeding antennas
250
200
/°
150
0
45
90
135
180
225
Phase shifts, β12
39
The degree of absorbed
energy becomes more flat
when the value of the
reflection coefficient of two
feeds become closer
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270
315
360
QUAD-SOURCE
MICROWAVE OVEN
40
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port3
QUAD SOURCE MICROWAVE OVEN
•
•
•
port4
①
Load
Adding more microwave sources to a multimode cavity
can result in increased heating power and field uniformity
if designed properly
Different locations of antennas were simulated to obtain
low coupling and uniform field distribution
Top face
Middle face
port1
Signals are sent through multiple antennas to combine
optimally at the location of a specific load.
port2
Bottom face
5.6-34.8dBV/m
Hot spot
Cold spot
Sweep by source phase from 0°to 360°(time)
41
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E filed distribution along the line
QUAD SOURCE MICROWAVE OVEN
②
4.2-34.0dBV/m
port3
port4
port2
port1
③
4.0-34.3dBV/m
port4
port3
port1
port2
42
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QUAD SOURCE MICROWAVE OVEN
④
5.4-35.6dBV/m
port4
port3
port1
port2
⑤
0.33-33.8dBV/m
port4
port3
port1
port2
43
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QUAD SOURCE MICROWAVE OVEN
⑥
5.0-34.7dBV/m
port3
port4
port2
port1
⑦
1.8-32.55dBV/m
port4
port3
port2
port1
44
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QUAD SOURCE MICROWAVE OVEN
⑧
3.6-34.6dBV/m
port2
port1
port3
port4
⑨
3.7-34.0dBV/m
port2
port4
port1
port3
45
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QUAD SOURCE MICROWAVE OVEN
4.6-34.7dBV/m
⑩
port2
port3
port1
port4
⑪
port1
46
port2
1.0-32.0dBV/m
port3
port4
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QUAD SOURCE MICROWAVE OVEN
⑫
2.3-36.2dBV/m
port2
port3
port4
port1
⑬
port4
3.5-35.5dBV/m
port3
port1
47
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port2
#NXPFTF
QUAD SOURCE MICROWAVE OVEN
port2
⑭
port4
port3
port1
4.9-33.6dBV/m
48
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QUAD SOURCE MICROWAVE OVEN
PReturn2
(W)
PReturn3
(W)
PReturn4
(W)
Load
loss(W)
Energy
absorption
efficiency
standard
deviation
σ
I1(2,3,4)(dB)
I2(1,3,4)(dB)
①
-12.6
-12.6
-12.1
-12.2
80.2
76.2
26.6
25.6
742.8
74.3%
12.54
②
-22.2
-27.8
-16.0
-24.2
16.4
27.8
16.9
41.9
820.5
82.1%
6.39
③
-15.3
-27.3
-24.1
-9.8
3.6
2.3
81.3
17.2
837.0
83.7%
6.97
β13=0°
④
-19.6
-26.45
-16.6
-29.8
18.6
36.0
48.2
109.4
745.2
74.5%
8.90
β14=0°
⑤
-18.1
-30.7
-23.9
-14.4
42.2
12.5
99.0
5.0
766.9
76.7%
7.15
⑥
-15.8
-21.1
-13.6
-23.6
1.2
23.8
93.5
42.8
776.8
77.7%
6.33
⑦
-35.9
-15.7
-42.9
-23.9
134.1
64.8
58.9
5.8
673.7
67.4%
8.42
P2=250W
⑧
-27.9
-21.4
-19.9
-24.6
49.4
33.0
10.8
58.1
779.3
77.9%
6.54
P3=250W
⑨
-17.2
-28.4
-16.6
-17.4
83.0
21.3
13.2
157.8
675.4
67.5%
5.97
⑩
-19.6
-21.4
-17.8
-20.3
32.4
55.9
49.9
54.2
750.5
75.1%
6.46
⑪
-17.9
-27.5
-19.8
-21.0
32.8
109.2
29.4
109.7
661.0
66.1%
7.04
@2.45GHz ⑫
-9.3
-11.9
-11.6
-15.4
65.9
28.3
59.1
31.8
762.6
76.3%
10.63
⑬
-18.3
-12.8
-17.5
-12.2
14.3
59.3
15.9
63.6
819.4
81.9%
13.19
#NXPFTF
-15.1
-12.4
-24.7
-28.4
30.4
42.4
15.4
51.4
787.3
78.7%
7.0
β12=0°
P1=250W
P4=250W
49
PUBLIC⑭
USE
I3(1,2,4)(dB) I4(1,2,3)(dB)
PReturn1
(W)
IN CASE ③, FOR EXAMPLE
950
/W
β12,β13=β14=0°
Energy absorption@2.45GHz
β13,β12=β14=0°
β14,β12=β13=0°
900
Load Loss
port4
port3
port1
34.75cos(12 
850
180
 0.8875)  815.1
800
750
port2

70.67cos(13 

180
 1.561)  836.3
49.25cos(14 

180
 12.04)  794.5
700
/°
650
Power absorbed by the load could reach
907W, when β13=270°,β12=β14=0°
50
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0
45
90
135
180
225
Phase shifts, β
270
315
360
IN CASE ③, FOR EXAMPLE
Energy absorption@2.45GHz
β14=0°
51
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β14=90°
β14=180°
IN CASE ③, FOR EXAMPLE
Power return from each port @2.45GHz, β14=0°
Port3
Port1
52
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Port2
#NXPFTF
Port4
IN CASE ③, FOR EXAMPLE
dB
Port1
dB
Port2
-26.8-13.5dB
β14
β14
-30.3-15.3dB
β12
β13
β13
β12
dB
Port3
β14
β14
-7.2-21.8dB
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β12
β12
β13
53
dB
Port4
-10.3-25.6dB
#NXPFTF
Isolation @2.45GHz
β13
IN CASE ③, FOR EXAMPLE
2.45GHz , β12=β13=β14=0°
2.44GHz, β12=β13=β14=0°
2.45GHz , β12=0°,β13=90°,β14=45°
2.46GHz , β12=β13=β14=0°
54
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2.45GHz , β12=90°,β13=60°,β14=45°
CONCLUSION
Electric field distribution or pattern inside the microwave cavity depends on the
arrangement of antennas in cavity. Compared to all the models explored, case ③ may
have the highest energy absorbed efficiency, but it also depends on that type of load.
A smart selection of operating frequency and relative phase shifts enables minimization of
power loss due to impedance mismatch and cross-talk. The results show a clear way of
controlling the heating states.
MWO might be able to sweep phase and frequency (therefore field distribution pattern)
methodically over the widest range of states, which can be built into complex sequences
for heating different types of food effectively and uniformly.
In some cases it might be best simply to find the lowest return loss modes to ensure the
maximum power is retained within the cavity where it is available for heating the load.
55
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NEXT STEP
56
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NEXT STEP
Not all the microwave oven models are tuned to be the optima. Further improvements
need to be made.
The degree of energy absorption uniformity will be quantitatively described by
standard deviation σ, that can be calculated by energy absorption statistical features of
several equal volume divided from 1L water.
The circular polarization antennas which have rotating electric field of same
magnitude give us a probability of uniform heating and good matching, that makes it
worthy of study.
Series of experiments should be built according to the model simulation. Focus on
adjusting the size, orientation and location of the antennas. Lowering the reflection
coefficients at the resonant frequencies from each port is the first and vital step.
57
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RF HEATING
@ FTF
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NXP SOLUTIONS FOR SOLID STATE RF COOKING
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RF
Components
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Reference
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and highest ease of use
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RF COOKING @ FTF
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•
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•
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•
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