Design of a 3D Microwave
Imaging System
Drew Jaworski
Advisor: Dr. Yong Zhou
Fall 2011 – Senior Design I
Why Microwave Imaging?

Electromagnetic Imaging Systems

Vision


X-Ray


Expensive
Microwave


Limited to surfaces
MRI (quantum mechanics)


Ionizing radiation
Infrared (“thermal”)


Nature doesn’t always know best!
Non-ionizing, penetrating, less expensive!
Applications


Medical imaging (cancerous tumors, etc)
Industrial scanning (forging defects, etc)
Project Specifications

Design of a 3-dimensional microwave
imaging system




Vector network analyzer signal analysis
Automated data acquirement and
processing
Biomedical focus, but adaptable for other
imaging applications
Multiplexed antenna array
Project Constraints

Size


Budget


$300 from department + personal funds
FCC Regulations


Entire system less than 1[m]*1[m]*1[m]
Medical device band: 3.1[GHz]–10.6 [GHz]
Many others related to trying to manage
the above constraints
Electromagnetic Overview

Plane-wave approximation



Imaging subject located in far-field of
antenna array, perpendicular to
propagation of waves
Simplifies analysis at expense of
system size
Scattering through media

A result of multiple layers of
diffraction and refraction, in the
case of the complex human body.
Images courtesy of:
http://en.wikipedia.org/wiki/File:Linear.Polarization.Linearly.Polarized.Light_plane.wave.svg
http://commons.wikimedia.org/wiki/File:Huygens_brechung.png
Vector Network Analyzer

Measures a Two Port Network

Returns S-Parameters (Scattering Parameters)



S11 – Return Loss
S21 – Insertion Loss
Parallel antennas connected to VNA ports



Calibrate response
Place object between antennas
Result is how the object affected the
electromagnetic radiation between the two
antennas

Results can be manipulated with software algorithms
to give dielectric properties of the object!
Inverse Scattering Solution

Repeat with multiple antennas (4x4 array, in this case)


Rotate object between antenna arrays
Result is a set of matrixes of scattering parameters for a 32-port
network (for a range of frequencies!)


Can be manipulated to produce a discretized graphical representation of
the dielectric properties in different regions between antenna arrays
Inverse Scattering Problem – Microwave Tomography



We know the forward transmitted radiation (aka Incident Fields)
We have information about the received fields (aka Scattered Fields)
Now we want to know what made them change!


Very complex calculations that are demanding of computing resources
Fortunately, much research has been published that has mathematically
and/or computationally simplified the solution process (relatively)
Automated Data Analysis

Labview


Automate collection of data
Several colleagues have worked out the details



Rotation mechanism – Juan Nava, Miguel Rivera
TTL communication (for multiplexer) – Julio Vasquez
Matlab


Process data
Numerous published algorithms can be
implemented and tested
Frequency Selection




Often limited by hardware technology (switch/antenna bandwidth)
Biomedical focus – human tissues
Estimates vary, best to come up with your own and justify
accordingly
Begin with what spectrum is available


FCC
“Medical Systems: These devices must be operated in the frequency
band 3.1-10.6 GHz. A medical imaging system may be used for a
variety of health applications to “see” inside the body of a person or
animal. Operation must be at the direction of, or under the supervision
of, a licensed health care practitioner.”


http://transition.fcc.gov/Bureaus/Engineering_Technology/Orders/2002/fcc02
048.pdf
Begin with properties of human body

Database of dielectric properties of numerous types of tissue available
from Italian National Research Council site:

http://niremf.ifac.cnr.it/tissprop/
Dielectric Properties Database

Skin (wet and Dry), Muscle, Fat, and Bone

Major constituents most body parts
Wavelength and Attenuation Constant versus Frequency
0.14
Wavelength
[cm] – SkinWet
4
Wavelength [cm] SkinDry
3.5
0.12
Wavelength
[cm] – Muscle
3
2.5
0.08
Highest λ
2
0.06
1.5
Highest alpha
0.04
1
Lowest λ
Lowest alpha
0.02
intersection
intersection
0
0.5
0
Frequency [GHz]
Attenuation Constant [1/cm]
Wavelength in Tissue [m]
0.1
Wavelength
[cm] – Fat
Wavelength
[cm] – BoneCortical
Attenuation
Coefficient [1/cm] –
SkinWet
Attenuation
Coefficient [1/cm] –
SkinDry
Attenuation
Coefficient [1/cm] –
Muscle
Attenuation
Coefficient [1/cm] –
Fat
Attenuation
Coefficient [1/cm] –
BoneCortical
Antenna Array Multiplexer

Julio Vasquez’s RF multiplexer design intended for this
project


Overlapping semesters meant his prototype was not yet
completed and could not be used immediately
Microstrip antenna array with integrated multiplexer
switch hierarchy



Avoids requirement of numerous expensive and tangled
SMA patch cables
Integrates network of SPDT switches into antenna array
4x4 microstrip antenna array



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1 SMA connector (patched to VNA)
15 SPDT RF switches (operating up to 8[GHz])
16 microstrip patch antennas
8 TTL-level (5V) control lines
Antenna Array with Multiplexer
RF Layout Guidelines

Line Widths

3.08[mm]


Curves



Ideally smooth curves
radius >= 3*lineWidth
Ground fills

Not completely necessary


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50Ω impedance
Relatively noise-free environment
Noise reducing padding around experiment setup
Not feasible for hand-produced prototype
Tapered impedance tranformers


Linear (“triangular”) is best for wideband operation (Pozar)
λ/4 ~ λ used in design (as long as could be reasonably fit)
Multiplexer versus Switch Network

Fully featured DC-12[GHz] multiplexer


$700 ~ $1700
Single SPDT RF switch IC


$1 ~ $3
M/A-COM technology solutions


MASW-007107
Pros



Large variety of models available
Distributed by Mouser
Cons

Small package size (GaAs DIE ~ 4[mm]*4[mm]
MASW-007107
Obtained from IC Datasheet: http://www.macomtech.com/DataSheets/MASW-007107.pdf
MASW-007107 (continued)
Switch Network Hierarchy
UWB Microstrip Antenna

Two-port network theory (one-port input
network, in this case)

S11 measures “return-loss” [dB]



Lower is better, -10[dB] indicates half of the input power is
lost in the network
Return Loss is power radiated from antenna (hopefully)
and other losses.
Bandwidth is measured where S11 crosses the
-10[dB] point

Design is UWB when (BW / Fcenter) >= 25%
UWB Microstrip Antenna

There are many published designs for UWB microstrip antennas

Most use complex ground geometries


(continued)
Usually explain it as something to keep the phase response level across the
useable band
After trying several designs, I began modifying the geometries in an
attempt to find something new
UWB Microstrip Antenna



(continued)
BW = 979[MHz]
Fcenter = 5.595[GHz]
-10[dB] BW => 17.52% (close, but not UWB)
UWB Microstrip Antenna

(continued)
Fractal and/or self-symmetry based designs

Intended to induce multiple resonance frequencies
XY Plot 1
patchAntennaV1
0.00
ANSOFT
Curve Info
dB(S(1,1))
Setup1 : Sw eep
$feedW='1.4mm' $patchHeight='15mm' $patchWidth='20mm'
-10.00
dB(S(1,1))
MY1: -10.0000
-5.00
-15.00
-20.00
-25.00
-30.00
3.50
4.00
4.50
5.00
5.50
Freq [GHz]
6.00
6.50
7.00
7.50
Inspired by: Miniaturized UWB Monopole Microstrip Antenna Design by the Combination of Guisepe Peano and Sierpinksi Carpet Fractals, IEEE AWPL, 2011
Budget (proposed)

Double-sided FR-4 boards (2x)


MASW007107 RF Switches (50x)


$37.50 + shipping <Mouser Electronics>
Commercially Manufactured PCBs


$12.66 + shipping <Parts Express>
$150 <Dorkbot PDX PCB group order>
All other supplies already in possession
Gantt Chart – SD1
9/19/15
Select Frequency Operation
Range
Decide Switching Design/Product
Learn HFSS
Decide Antenna Design
Layout Array PCB Design
Simulate Design Explicitely
Modify PCB Design Accordingly
Produce Protoype Board
9/169/30
10/110/15
10/1610/31
11/111/15
11/1611/30
12/112/15
12/1512/30
Gantt Chart – SD2 (proposed)
1/11/15
Layout Final Design
Send Layout Out for Manufacturing
Test New Boards
Establish Mathematics of System
Program Algorithms
Test System and Collect Data
Prepare for Research Symposium
Final Report and Presentation
1/151/31
2/12/15
2/162/29
3/13/15
3/163/31
4/14/30
5/15/31
Future Work


Finalize UWB antenna candidate design
RF Layout of antenna array


Produce a prototype (using materials on hand)
Export Gerber file and have it manufactured
commercially


Develop mathematics of Imaging System


$1 per square inch (min. 150 square inch order)
Microwave Imaging (2011), Matteo Pastorino
Begin making microwave images!