Harmonic Radar Tag Design for
Tracking the Nezara Viridula
(Southern Green Stink Bug)
Ben Cannon
2007 SURE Participant
Adviser:
Dr. Anthony Martin
Outline
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Motivation for the Project
Current Trends in Insect Tracking
Introduction to Harmonic Radar
Basic Harmonic Radar Tag Design (Pros & Cons)
Improvements to the Basic Design
Simulation Results and Comparisons
Conclusions
2
Motivation for the Project
• The southern green stink bug is
responsible for damaging a variety
of crops (cotton, tomatoes, beans,
etc...)
• They prefer to feed on developing
fruits, piercing them and causing
them to become deformed and
damaged.
Southern Green Stink Bug
Photo credit: Extension Entomology,
Texas A&M University
• It is believed that tracking the
stink bugs’ movements through the
crop fields can lead to a better
understanding of their behaviors
and ways to combat them.
3
Unmanned Tracking
SR20 Unmanned Aerial Vehicle
Photo credit: Clemson UAV Laboratory
• The Clemson University
Unmanned Aerial Vehicle (UAV)
Laboratory has taken an interest in
the issue.
• Their Goal: to track the movement
of stinkbugs through crop fields
with UAV on a GPS guided path
• Mounting the tracking equipment
to the UAV eliminates the need for
a large base station in the center of
the field
• Reduces the range to a predictable
value (approximated by the height
of the UAV)
4
How to Track Insects: Harmonic Radar
• Most common/successful method currently used today
• The most basic system consists of a transmitting antenna,
a passive “tag” placed on the object to be tracked, and a
receiving antenna
• The passive tag requires an antenna and a nonlinear
element (diode) at its terminals
• The transmit and receive antennas should be mounted on
the UAV and the passive tags should be mounted onto the
stink bugs
5
Harmonic Radar Concept
• For this analysis, a fundamental frequency of 8.2GHz (“fo”) was
chosen to illuminate the tag.
• An electromagnetic field incident upon the tag’s antenna will induce
a current along its length which will drive the diode at the antenna’s
terminals
• Due to the non-linearity of the diode, it should produce voltage at
harmonic frequencies of the current that is driving it—the largest
being at the second harmonic (16.4GHz)
• This second harmonic current is then re-radiated through the tag
antenna and picked up by the receive antenna
• One can differentiate between backscatter from linear elements
(ground, foliage, etc.) at “fo” and the backscatter from the tag at
“2fo,” thus, locating the insect.
transmit antenna
fo = 8.2GHz
tag antenna
diode
receive antenna
2fo = 16.4GHz
6
Basic Tag Design
Honeybee Equipped with
Basic Tag
Photo credit: Rothamsted Research Group
• The basic antenna design is a halfwavelength (of “fo”) dipole trimmed
down to a resonant length
• At the dipole terminals is a low-barrier
beam lead Schottky diode (small in
size and high switching speed)
• A tuning inductor across the diode
“cancels” out the diode’s capacitive
reactance at the fundamental frequency
• The basic design is simple and elegant,
however, improvements can be made
to the tag to increase its efficiency and
performance.
7
Method of Moments (MoM)
Simulation of Basic Tag Antenna
Segmented Antenna from
MoM Simulator (EZNEC)
• The antenna portion of the tag was
simulated in MoM based software.
• The MoM software breaks the antenna
up into several pieces (small compared
to a wavelength) and calculates
antenna parameters by summing up the
contributions from currents in each of
the pieces. Moreover, it’s a numerical
solution to an integral equation.
• From the simulation we can look at
important antenna parameters such as
input impedance and far-field radiation
patterns
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Simulation Results at 8.2GHz
E-Plane Radiation Pattern
Antenna
orientation
Gain:
2.1 dBi
Input Impedance:
72.25 + j 0.14 Ω
-3dB Beamwidth:
78.4 deg.
9
Simulation Results at 16.4GHz
E-Plane Radiation Pattern
Gain:
3.67 dBi
Input Impedance:
414 – j 253 Ω*
-3dB Beamwidth:
49.2 deg.*
* not desirable
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Harmonic Balance Simulation of Tag
• It is important to study how power is transferred from the
tag’s antenna to the diode (and back to the antenna)
• Agilent Advanced Design System (ADS) contains a
harmonic balance simulator which is designed to simulate
non-linear circuits.
• Find a Thévenin equivalent circuit representation of the
antenna and run a harmonic balance simulation on the
antenna and diode/tuning inductor load.
11
Thévenin Equivalent for Tag Antenna
Trial Version
V1
SIN
R2
L3
C2
217
62.6n
1.44f
R1
L2
C1
73
11.3n
33.2f
+
Ζtotal
–
• Antenna’s input impedance “Ζtotal” can be represented as two
series R-L-C networks in parallel.
• Tune each series so that “Ζtotal” equals the antenna impedance
at both 8.2GHz and 16.4GHz.
• Sinusoidal voltage source magnitude is determined by how
much power is incident on the antenna.
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Diode and Tuning Inductor
HSCH5340ben
L1
• Diode model was created to represent the Agilent
HSCH-5340 low-barrier beam lead Schottky diode
• Value of 1.5nH was assigned to the tuning inductor
• These elements complete a circuit with the Thévenin
equivalent antenna and a harmonic balance
simulation can be run on the complete circuit.
13
Screenshot of ADS Harmonic
Balance Setup
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Evidence of Power Being Delivered to
Antenna at Harmonic Frequencies
Spectrum of Magnitude of Vout and I_Probe for Vin = 0.5V
The harmonic balance shows the frequency components of
the voltage and current between the antenna and the load.
15
RMS Power Dissipated in Load at
8.2GHz vs. Drive Level
Dissipated Power in Watts
4.50E-02
4.00E-02
3.50E-02
Actual Power Dissipated
(RMS)
3.00E-02
Power Available to
Conjugate Matched Load
2.50E-02
2.00E-02
1.50E-02
1.00E-02
5.00E-03
0.00E+00
-5.00E-03 0
1
2
3
4
5
Vin in Volts
*A measure of how well the load is match up to the antenna for receiving power
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Second Harmonic Power Delivered to
Antenna by Diode vs. Drive Level
Second Harmonic Power Delivered (Watts)
2.00E-05
0.00E+00
0
1
2
3
4
5
6
-2.00E-05
-4.00E-05
-6.00E-05
Second Harmonic RMS Power
Delivered to Antenna
-8.00E-05
-1.00E-04
-1.20E-04
-1.40E-04
-1.60E-04
Vin (Volts)
• There is a certain level (Vin ≈ 0.75 V) where the diode should be
driven.
• Driving the diode any harder does not result in much more power
being delivered back to the antenna.
• We should choose a transmitter height/distance and EIRP so that we
excited the harmonic tag at a level where Vin ≈ 0.75 V.
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* Negative values just indicate that power is being supplied rather than dissipated.
Design Improvement:
The Trap Dipole
• As seen earlier, it is not optimal to re-radiate second
harmonic current through a full-wave dipole (narrow
beam, high input impedance).
• Add parallel L-C networks to the antenna length that
resonate at 16.4 GHz
• This will result in a high impedance (theoretically ∞ )
at 16.4GHz, “trapping” the 16.4GHz current to the
length “a”
• 8.2GHz current will see the traps as inductive loads.
• Choose length “a” to be that of a half-wavelength
resonant dipole at 16.4GHz
• Choose length “b” to be that of an inductively loaded
resonant dipole at 8.2GHz
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• A two-band resonant dipole!
MoM Simulation of Trap Dipole
• Re-run a MoM simulation on the new design to see how
its antenna characteristics compare to the basic design.
8.2GHz Current Sees an Inductively
Loaded Dipole
Evidence of 16.4GHz Current Being
Trapped Between Loads
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Simulation Results at 8.2GHz
E-Plane Radiation Pattern
Gain:
2.14 dBi
Input Impedance:
64.9 + j 0.6 Ω
-3dB Beamwidth:
78 degrees
20
Simulation Results at 16.4GHz
E-Plane Radiation Pattern
Gain:
2.39 dBi
Input Impedance:
80.9 + j 1.0 Ω
-3dB Beamwidth:
70 degrees
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Antenna Comparison
• The trap dipole design has more desirable characteristics
than the original design for re-radiating second harmonic
current
• As seen in the previous figure, the radiation pattern is
closer to being omni-directional
• The input impedance is almost purely real, with a much
smaller resistance. One can predict that this will improve
power transfer from the load to the antenna at the second
harmonic.
• Determine a new Thévenin equivalent circuit for the trap
dipole design
• Run a harmonic balance simulation on the new design to
determine if power transfer is truly more efficient/optimal
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Harmonic Balance Results and
Comparisons
RMS Power Delivered to Antenna
(Watts)
Second Harmonic Power Delivered to Antenna vs. Drive
Level
1.00E-04
0.00E+00
-1.00E-04 0
-2.00E-04
-3.00E-04
-4.00E-04
-5.00E-04
-6.00E-04
-7.00E-04
-8.00E-04
-9.00E-04
1
2
3
4
5
6
Without Traps
With Traps
Vin (Volts)
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Harmonic Balance Results and
Comparisons
Ratio of Power Delivered at Second Harmonic to Power Dissipated at
First Harmonic
0.6
Without Traps
0.5
With Traps
0.4
0.3
0.2
0.1
0
0.00E+00
1.00E-03
2.00E-03
3.00E-03
4.00E-03
5.00E-03
Power Available to Conjugate Matched Load
A good indication of efficiency.
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More Power but Less Gain… Is this
Truly an Improvement?
• Effective Isotropic Radiated Power (EIRP) the amount of power that would have to be emitted by an
isotropic antenna (evenly distributes power in all
directions) to produce the peak power density observed in
the direction of maximum antenna gain.
• EIRP = (Power)×(Gain)
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EIRP Example
• 0.428mW of power available at the antenna terminals (to a
conjugate matched load) of each design.
Powers:
Basic Tag:
re-radiates 63.2μW (2nd harmonic)
Trapped Tag: re-radiates 168 μW (2nd harmonic)
Gains:
Basic Tag:
3.78dBi  2.388
Trapped Tag: 2.39dBi  1.734
EIRP
Basic Tag:
0.151 mW
Trapped Tag: 0.291 mW
• Although the basic design has a higher broadside gain, the
trapped design has nearly twice the EIRP due to its
efficient power transfer between the antenna and load! 26
Predicting the Needed Equipment
(transmit  tag)
• Friis Propagation Equation:
Pr
  
 Gt Gr 

Pt
 4R 
•
•
•
•
2
Harmonic Balance predicts desired drive level (gives “Pr”)
MoM simulator gives tag antenna gain “Gr”
Estimate UAV flight altitude (R ≈ 10m)
Choose transmitter power and gain to match this EIRP:
Pt Gt  
Pr
  
Gr 

 4R 
2
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Predicting the Needed Equipment
(tag  receiver)
• Same thing in reverse…
• Harmonic Balance predicts second harmonic power
delivered to tag (gives “Pt”)
• MoM simulator gives tag antenna gain “Gt”
• Same UAV flight altitude (R ≈ 10m)
• Choose receiver power and gain to match these receive
values:
 Pr 
  

   Pt Gt 
 4R 
 Gr 
2
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Conclusions
• Ultimately an issue of power rather than elegant antenna
design
• MoM and Harmonic Balance simulators predict power
transfer and optimal drive level
• Simple improvements to basic design can yield a much
more efficient tag
• Free space propagation equation predicts necessary
tracking equipment
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Acknowledgements
• My adviser – Dr. Martin
• Dr. Noneaker & Dr. Xu
• Josh Lawrence
• Everyone involved in the SURE lecture/lunch series
• My fellow SURE participants
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