Harmonic Radar Tag Design Harmonic Radar Tag Design for Tracking the Southern for Tracking the Southern Green Stink BugGreen Stink Bug

▪▪ Conclusions Conclusions ▪▪

▪ ▪ Comparison Comparison ▪▪

▪▪ Introduction Introduction ▪▪

▪ ▪ Harmonic Radar Harmonic Radar ▪▪

▪▪ Design Design Improvement Improvement ▪▪

▪▪ Basic Tag Design Basic Tag Design ▪▪

Southern Green Stink Bug

SS..UU..RR..EE.. 20072007Benjamin Benjamin CannonCannonCarnegie Mellon UniversityAdviser: Dr. Anthony

Martin

Photo credit: Extension Entomology, Texas A&M

University

• The southern green stink bug is responsible for damaging a variety of crops of economic importance. It is believed that tracking stink bugs’ movements through crop fields can lead to a better understanding of their behaviors and ways to combat them.

• The Clemson University Unmanned Aerial Vehicle (UAV) Laboratory has taken an interest in the project. Their Goal is to track the movement of stink bugs through crop fields by way of a UAV on a GPS guided path.

• Mounting tracking equipment to the UAV eliminates the need for base stations, thus reducing the range to a predictable value (approximated by the height of the UAV).

• A lightweight and efficient method for tracking the insects is needed. The solution: Harmonic Radar.

fo = 8.2GHz

2fo = 16.4GHz diodereceive

r

transmitter

tag

• The basic system consists of a transmitting antenna, a passive “tag” placed on the object to be tracked, and a receiving antenna.

• An electromagnetic field (at 8.2GHz) incident upon the tag’s antenna induces a current along its length which drives a diode at the antenna’s terminals.

• Due to the nonlinearity of the diode, it produces current 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 non-linear tag at “2fo,” thus, locating the insect.

• The basic tag antenna design is a half-wavelength (of “fo”) dipole trimmed down to a resonant length.

• At the dipole terminals is a low-barrier beam lead Schottky diode, chosen for its small size and high switching speeds.

• A tuning inductor in parallel with the diode cancels out its capacitive reactance at the fundamental frequency.

Honeybee with Basic TagPhoto credit: Rothamsted

Research Group

• It is not optimal to re-radiate second harmonic current through a full-wavelength dipole (narrow beam, high input impedance).

• Add parallel L-C networks to the antenna length that resonate at 16.4GHz. This will result in a high impedance (theoretically infinite) at 16.4GHz, “trapping” this second harmonic current to the length ‘a’ depicted in the figure below.

• 8.2GHz current sees the traps as inductive loads.

•Choose length ‘a’ to be that of a resonant half-wave dipole at 16.4GHz. Choose length ‘b’ to be that of an inductively loaded resonant dipole at 8.2GHz.

• What results is a two-band resonant dipole.

‘a’‘b’

Trap

Trap

Load

The Trap Dipole:

Second Harmonic Power Delivered to Antenna vs. Drive Level

-9.00E-04

-8.00E-04

-7.00E-04

-6.00E-04

-5.00E-04

-4.00E-04

-3.00E-04

-2.00E-04

-1.00E-04

0.00E+00

1.00E-04

0 1 2 3 4 5 6

Vin (Volts)

RM

S P

ow

er D

eliv

ered

to A

nte

nn

a (W

atts

)

Without Traps

With Traps

Ratio of Power Delivered at Second Harmonic to Power Dissipated at First Harmonic

0

0.1

0.2

0.3

0.4

0.5

0.6

0.00E+00 1.00E-03 2.00E-03 3.00E-03 4.00E-03 5.00E-03

Power Available to Conjugate Matched Load

Without Traps

With Traps

Second Harmonic E-

Plane Radiation Patterns

Without Traps

With Traps• The trap dipole design has more desirable

characteristics for re-radiating second harmonic current. The radiation pattern is closer to being omni-directional. Additionally, the input impedance is almost purely real, with a much smaller resistance value. Consequently, it is better matched to the diode for power transfer.

• As seen in the figures below, the trap dipole design efficiently delivers more second harmonic current to the antenna.

• An important parameter to consider is the Effective Isotropic Radiated Power (EIRP) of the tag = ( Power )( Gain )

• Although the basic design has a higher broadside gain, the trapped design has nearly twice the EIRP due to its efficient and well-matched power transfer between the antenna and the load.

• Finally, a simple improvement to the basic design yields much more efficient tag.