A TRANSPONDER FOR HARMONIC RADAR TRACKING OF THE
BLACK VINE WEEVIL IN BEHAVIORAL RESEARCH
R. D. Brazee, E. S. Miller, M. E. Reding, M. G. Klein, B. Nudd, H. Zhu
ABSTRACT. The black vine weevil (BVW), Otiorhynchus sulcatus (Fabricius), is a major economic insect pest for growers of
ornamental nursery crops and small fruits. Development of management strategies by entomologists and growers has been
hampered by a lack of behavioral information on movement of BVW within agroecosystems. Although insects can be tracked
using tag-and-release methods, the BVW is active primarily at night, cannot fly, and can be difficult to relocate. Harmonic
radar technology has been used in entomological research and was investigated for applicability to the BVW problem. An
insect-mounted, miniature transponder was developed to facilitate location in conjunction with a commercially available
harmonic radar transceiver detector. The transponder, powered by a 0.917 GHz signal from the detector, returns a 1.834 GHz
signal when detected. The transponder consists of a Schottky barrier diode with an inductively loaded monopole antenna and
is lightweight at about 27% of a BVW body mass. In field trials, insects were successfully released, relocated, and recovered
after several days.
Keywords. Antenna, Insect, Instrumentation, Nursery, Pesticide.
T
he black vine weevil (BVW), Otiorhynchus sulcatus (Fabricius), is a particularly troublesome pest
for growers of ornamental and other nursery crops.
Shetlar (1995) provides an informative overview of
the problem, noting that the BVW ranges across much of
North America from Maine to the Carolinas and west to
Washington and Oregon. The BVW feeds on over 100 different kinds of trees, shrubs, vines, and flowers, with the preferred hosts in nurseries growing ornamentals typically being
Taxus, hemlock, and rhododendron. In addition to field and
landscape plantings, it commonly infests containerized perennials in greenhouse and polyhouse production settings.
Adults often feed along leaf margins, producing typically
crescent shaped notches, while the larvae prefer to feed on
young tender roots. Adult BVW are oblong oval in shape,
about 13 mm long, and while they cannot fly, they are very
active walkers.
With regard to control, knowledge about the behavior and
movement of this insect in and around nurseries is very
incomplete. This lack of knowledge causes growers to apply
insecticides against BVW after the resident population
Article was submitted for review in April 2004; approved for
publication by the Information & Electrical Technologies Division of
ASAE in February 2005. Presented at the 2004 ASAE Annual Meeting as
Paper No. 041033.
Mention of proprietary product or company is included for the reader’s
convenience and does not imply any endorsement or preferential treatment
by USDA-ARS.
The authors are Ross D. Brazee, ASAE Member Engineer, Senior
Research Scientist, Ethan S. Miller, Former Research Associate, Michael
E. Reding, Research Entomologist, Michael G. Klein, Research
Entomologist, Barry E. Nudd, Electronic Technician, and Heping Zhu,
ASAE Member Engineer, Agricultural Engineer; USDA-ARS Application Technology Research Unit, Wooster, Ohio. Corresponding author:
Heping Zhu, USDA-ARS Application Technology Research Unit, Ag. Eng.
Bldg., OARDC, 1680 Madison Ave., Wooster, OH 44691; phone:
330-263-3871; fax: 330-263-3670; e-mail: zhu.16@osu.edu.
should have been eliminated because they fear migration of
the insect into nurseries from outside sources. These
preventative treatments are costly in terms of time, labor,
materials, and environmental risks, and may be unnecessary.
Clearly, data are needed on BVW behavior to aid development of improved or alternative management strategies and
more effectively time and target those control measures.
Tracking of the insects is an essential part of behavioral
studies.
Entomological radar technology offers some interesting
possibilities for adapting or developing research tools for
investigating insect behavior, populations, and migrations
(Beerwinkle et al., 1994; Wolf et al., 1993, 1995). It is
immediately clear that radar applications in nursery insect
research present different challenges than, e.g., long-range
migration studies, since tracking must be done in more
restricted areas with plant populations of wide variety. Since
the BVW prefers to live in or near these sorts of habitats,
harmonic radar appears to have potential as a tracking
research tool. Unlike a conventional radar system, which
depends on the target to reflect sufficient radio energy for
detection, harmonic radar systems use tuned reflectors
mounted on targets of interest. The reflectors are powered by
an incoming radar signal from a tracking transceiver, or
detector. For entomological studies, tiny transponders must
be attached to insects that are either mass-reared or captured
and released for study. The harmonic radar detector sends a
signal at a known fundamental frequency and receives return
signals from the target transponder at twice the transmitted
frequency (the 2nd harmonic, as described later). The
reflector circuit on the target typically consists of an antenna
and a Schottky barrier diode, which effectively multiplies the
received signal to produce the 2nd harmonic signal detected
by the receiver. This mode of operation prevents excitationsignal backscatter from interfering with detection of the
returned tracking signal from the transponder.
Transactions of the ASAE
Vol. 48(2): 831−838
2005 American Society of Agricultural Engineers ISSN 0001−2351
831
Mascanzoni and Wallin (1986) used harmonic radar
technology for tracing released, individually marked specimens of various nocturnal species of ground-dwelling
carabid beetles in a cereal field. They were able to
continuously trace individual insects for periods of up to
8 days. Recapture rates for radar-tagged insects indicated no
significant effect on behavior compared with other tracing
methods. Of the total number of insects recaptured, 86% of
the tagged and 83% of the control insects were recovered
within a week. Their diode-equipped tags used antennas of
3 to 5 cm lengths, with weight for some tags of the order of
30 to 80 mg. Tracking ranges indicated from their data for
these tags were about 4 to 6 m. Overnight travel distances for
the various insects observed averaged 7.9 m with a range of
3.2 to 16.2 m.
Other applications of harmonic radar technology include
studies of bee flight and foraging patterns. Capaldi et al.
(2000) used harmonic radar to demonstrate ontogeny in
orientation flights of honeybees. The bees take repeated
flights at increasing distances to develop homing capability,
as they become successful foragers at about three weeks of
age. Osborne et al. (1999) studied dynamics and spatial scale
of bumblebee foraging flights. Transponders consisted of
Schottky diodes with vertical dipole antennas and loading
coils constructed from with 135 mm copper-plated springsteel wire, and weighed about 12 mg.
Roland et al. (1996) used harmonic radar to study effects
of habitat on movement of several species of flying insect.
Tags were fitted to the small Apollo and alpine butterflies, to
larvae and moths of the forest tent caterpillar, and to the
parasitic tachinid and sarcophagid fly, both of which attack
the tent caterpillar. The tags that were used consisted of a
Schottky diode and center-driven dipole antenna formed
from extremely fine aluminum bonding wire. The tags
increased weight of the insects by about 0.1% to 0.9% and
typically weighed about 0.4 mg. Using a 1.7 W continuous
transmitted radar signal, they report detection ranges up to
50 m.
Boiteau and Colpitts (2001) investigated the effect of
electronic tag weight on flight performance of the Colorado
potato beetle. They found that transponders should weigh no
more than 23% to 33% of the beetle’s body weight to have
minimal impact on the number and quality of upward flights.
They also reported from chamber trials that beetles were
incapable of upward flight beyond 11.8 N m−2 average wing
loads.
With the aid of harmonic radar, Svensson et al. (2001)
monitored long-range, pheromone-mediated flight behavior
of male turnip moths (Agrotis segetum) under natural and
mating disruption conditions. Under various treatments,
moths could be followed over a variety of terrains for up to
77 min, corresponding to a total track length of 7350 m and
average ground speeds ranging from 0.7 to 5.4 m s−1. Tag
weight was about 8 mg, amounting to less than 10% of the
insect’s body weight.
Generally, no great amount of detail was given on
transponder properties, such as antenna impedance, efficiency, or far-field characteristics near the ground. Despite the
desirability of having such information, those numbers are
not always easy to obtain, considering the variety of needs for
each tracking application and the difficulties associated with
high-frequency measurements. Thus, the primary goals of
this study were: (1) to determine the potential of harmonic
832
radar technology for tracking of the BVW, given its
ground-dwelling and cover-seeking tendencies, and (2) to
identify and develop the transponder-antenna combination
that would be required.
THE HARMONIC RADAR SYSTEM
THE RADAR TRANSCEIVER
The radar transceiver chosen for this research was a
RECCO Type R5 detector (RECCO AB, Lidingö, Sweden)
that is designed for locating avalanche victims wearing
special transponders. The system transmits a 0.917 GHz
fundamental excitation signal from an integral Yagi-Uda
antenna at about 1.7 W, receiving a 2nd harmonic response
at 1.834 GHz from any transponder within range and located
in the same direction as the main forward lobe of the antenna
transmission pattern. The operator receives an audible signal
of transponder response via a detector headset. Detector
transmitted field strength was measured with its Yagi-Uda
antenna held horizontally level 1.32 m (4.04l, where l is the
fundamental signal wavelength) above a concrete floor,
using an RF field strength meter (AlphaLab, Inc., Salt Lake
City, Utah) in high-pass mode. Total midline horizontal and
vertical power density corrected for background was
14.7 mW cm−2 at 8.18 cm (l/4) from the antenna.
REFLECTOR-TAG REQUIREMENTS
The reflector tag, or transponder, package must have the
least possible interference with the movement and behavior
of the insect, while enabling detection at least within its
normal range travel distances. It was anticipated that a
detection range for a BVW of 3 to 4 m would be acceptable,
depending on the insect’s location above the ground. The
combined receiving and transmitting antenna of the transponder must ideally be simple, lightweight, flexible, and
small as possible. A particular constraint is that the
transponder should have a minimum of protrusions that could
impair insect activity.
Figure 1 is a simplified schematic diagram of a reflector
tag circuit, of which the Schottky barrier diode is an essential
element. The DC path around the Schottky diode, which may
include an inductor, conductive strip, or transmission line,
provides a pathway for dissipating accumulated charge,
allowing the diode to operate at zero-voltage bias (Riley and
Smith, 2002). The antenna should be as near as possible to an
optimum match with the diode to realize efficient power
transfer.
Antenna
Schottky Diode
DC Path
Figure 1. Reflector tag schematic diagram.
TRANSACTIONS OF THE ASAE
THE SCHOTTKY DIODE
The Schottky barrier diode is useful for frequency
multiplication applications owing to its low junction capacitance and nonlinear characteristic at low signal levels.
Models for diode-antenna interaction at high frequencies are
complex, since a nonlinear differential equation is encountered (Kanda, 1980). However, the problem is tractable, and
Kanda (1980) made some comparisons among analytical and
numerical solutions for short-dipole, antenna-driven systems
with nonlinear loading, giving transfer function characteristics over a range from 1 kHz to 10 GHz. For our transponder
development, it was adequate to assume that the diode
impedance had resistive and capacitive (approx. 1.0 pF)
components and make use empirical analyses and simulations.
A voltage-current characteristic model for the Schottky
diode (Kanda, 1980) is:
( αv
i (t ) = I s e
o (t )
)
−1
(1)
where Is is the saturation current (assumed to be 2 × 10−9 A),
a = q/nkT ≅ 38 V−1 [q is the electronic charge (1.6 × 10−19
C), n is a dimensionless diode ideality factor (approx. 1.05),
and k is Boltzmann’s constant (1.38 × 10−23 J/K)], T is the
absolute temperature (approx. 290 K), and vo (t) is the applied
voltage. It is presumed that the diode is part of a complete circuit including the antenna. The effect of diode nonlinearity
in generating signal harmonics can be seen by application of
a simulated incoming radar signal voltage:
vo (t )= Vo sin (2πft )
(2)
in equation 1, with f = 0.917 GHz and the amplitude (Vo )
chosen arbitrarily to normalize results. Fourier transformation of the resulting i(t) yields the frequency domain plot
shown in figure 2.
THE ANTENNA
Various types of antenna were considered for applicability
under the constraints imposed by the insect and the need for
the transponder to be as small as possible with acceptable
detection range. The antenna selected needed to be tuned to
a range of frequencies giving a bandwidth such that incoming
and outgoing power could be transferred as efficiently as
possible. Antenna configurations are usually expressed in
terms of number of wavelengths, with the relation between
free space wavelength (l0, m) and the speed of light
(c0, m s−1) as:
Vol. 48(2): 831−838
Normalized Response
Since the BVW cannot fly, its fused wing-cover surfaces
provide a favorable base for mounting a transponder. The
BVW frequents dark areas under leaves and mulch, where it
can find nourishment and protection. Thus, a low-profile
antenna is essential to minimize impairment of its movement, which is almost always in a forward direction.
Protrusion of transponder components beyond the boundaries of the insect’s body is tolerable provided there is
minimum interference with its maneuvering and climbing.
The BVW is about 8 to 11 mm long and has a mass of
approximately 73 mg. Only limited observations have been
made of its load carrying ability, and it was therefore assumed
that a transponder weighing more than 30% of BVW body
weight could impair its movement, as suggested by the work
of Boiteau and Colpitts (2001).
Frequency (GHz)
Figure 2. Fourier domain plot of Schottky barrier diode normalized current response to an applied sinusoidal voltage at a fundamental frequency
of 0.917 GHz (1st harmonic). Harmonics from DC (0th) through the
fourth harmonic are shown. The radar detector receives only the 2nd harmonic at 1.834 GHz.
λ0 =
c0
300
≈
f
f MHz
(3)
Kraus and Marhefka (2002) and Balanis (1997) are useful
references in antenna development.
PLANAR ANTENNAS
Microstrip or patch-antenna transponders were considered for this application owing to their potential for sizes
comparable with the insect and absence of projecting wires.
They are a type of planar antenna (fig. 3) used in high-frequency systems owing to their adaptability to small devices
and relative ease of fabrication to strict tolerances. However,
in the current application with frequencies of 0.917 and
1.834 GHz, there was concern that a patch antenna might still
be too large. The shorter dimension (L) of a rectangular,
half-wavelength patch antenna would be (notation of Kraus
and Marhefka, 2002):
L=
0.49λ 0
εr
(4)
where er is the relative permittivity of a dielectric substrate
sandwiched between the conducting patch and the ground
plane. Application of equations 3 and 4 to a prototype halfwavelength rectangular patch antenna on a printed circuit
board (PCB) with an epoxy-fiberglass substrate of relative
permittivity (er ) of 4 and a 1.834 GHz signal gave a value for
L of 40.1 mm. With a longer dimension (W = 2L) corresponding to 0.917 GHz and a PCB of 3.2 mm thickness, the antenna
mass would be 8.86 g, which would be prohibitive for this application. Use of a higher-permittivity substrate or superstrate and slotted patch and ground planes, to allow
meandering of induced currents, could reduce antenna dimensions substantially for fixed operating frequencies. However, the mass of the dielectric and conductors was expected
to be excessive, especially for ceramic substrates, with the
risk that the unit would become too top-heavy for a BVW to
carry.
Experimental transponders with patch antennas and
Schottky diodes similar to RECCO reflector tags were
fabricated, but reduced in size (scaled) in accord with
833
Figure 4. Half-wavelength dipole antenna.
Figure 3. Rectangular planar patch antenna.
Figure 5. Folded dipole antenna.
equation 2. Reduction was accomplished by using a mica
substrate with no attempt to slot an antenna of the size needed
for a BVW. The effective permittivity (er , eff ) was taken to
be the arithmetic mean of the permittivities of the substrate
and superstrate. Use of a single, thin layer of mica for the substrate (er = 6) and with air as the superstrate (er = 1) gave a
value for er, eff of 3.5, which enabled reduction of tag size to
about half that of a RECCO reflector. In trials, this transponder was lacking in both size and performance, with a mass of
94 mg and a range of only about 1 m.
Table 1 is a summary of ranging tests for several similar
reflector tags using Schottky diodes. Other than preliminary
response checks in the laboratory, most ranging trials were
done in an outdoor area. Tags were secured to a 40 × 85 mm
piece of 2 mm thick, rigid plastic, which served as a dielectric
support for the transponder, preventing its distortion due to
handling or ground contact. Range measurements were done
in a level turf area that was as free as possible from stray
electromagnetic fields. Tags were placed on the ground,
subjecting them to a relatively severe response test and with
ranging performed from the optimum direction according to
the transponder antenna pattern. For later tests, with tags
using monopole or dipole antennas, this direction would be
normal to the antenna axis. When ranging, the operator
located the point where a return signal could no longer be
detected. Since an operator needed to maneuver the detector
at approximately chest level, detection distances were
measured horizontally with a surveying wheel from the plane
of the forward end of the detector antenna to the transponder.
WIRE ANTENNAS
Several forms of wire antennas were considered as
potentially suitable for tracking the BVW. Among them were
the 1/2-wavelength dipole (fig. 4), 1/2-wavelength folded-dipole (fig. 5), 1/4-wavelength monopole over a ground plane
(fig. 6), and a magnetic loop (fig. 7). The bold, dark segments
in the drawings represent drivepoint locations. The expression relating wavelengths in a vacuum and in a transmitting
medium is:
λ = λ0 ×V f
(5)
Figure 6. Quarter-wavelength monopole antenna.
Figure 7. Small magnetic loop antenna.
0.95 for wire antennas. Equation 5 is fundamental for singleelement wire antennas and may be scaled to 1/2 or other
wavelengths.
Dipole and monopole antennas are resonant, such that
their feedpoint impedances:
Z A = ( Rs + Rloss ) + jX A
(6)
are completely resistive. A dipole antenna has the advantage
of being a single line, as opposed to a plane for a patch
antenna. From a size standpoint, at 1.834 GHz, the length of
a 1/2-wavelength dipole would be 77.7 mm, large compared
to the BVW. A folded dipole antenna, while having greater
bandwidth than a dipole, would also be 77.7 mm long. A
1/4-wavelength monopole at 38.3 mm is also large compared
to the insect. A magnetic loop, at 15.5 mm in diameter, approaches an acceptable size, but could limit an insect’s access
where Vf is the velocity factor, i.e., the ratio of the speed of
light in the new medium to that in a vacuum, typically about
Antenna
Large PCB
Small PCB
Single-layer mica
Mica sandwich
834
Table 1. Scaled transponders.
Substrate
Dimensions
Mass
(εr )
(mm)
(mg)
PCB (4)
PCB (4)
Mica (6)
Mica (6)
59 × 25
25 × 9
22 × 7
22 × 7
3500
1100
94
410
Range
(m)
7
1
1
0
Figure 8. Tapped inductor in a tuned circuit.
TRANSACTIONS OF THE ASAE
Overall
Dimensions
(mm)
Mass
(mg)
Dipole
78 × 15
30
Folded dipole
15.5 × 5
Antenna
Loaded monopole,
3t3-15 mm
19 × 4
Table 2. Wire antenna comparisons.
Simulated
Simulated
Range
Z0
Gain
(m)
(Ω)
(dBi)
20
4.6
44.5−j16.4
4.14
4
3.73+j540
−5.4
3.5
6.67−j115
−2.3
to narrow openings. From a purely radiation viewpoint, dipole antennas could be ideal for tracking insects, since they
are balanced, can operate without a ground plane, and are
generally easy to construct. A 1/10-wavelength dipole at
1.834 GHz would be 15.5 mm long, possibly an acceptable
size. However, it might be difficult to mount on an insect because its feedpoint is at the center and approximately half the
transponder length would protrude forward.
Several constraints make it difficult to synthesize an
electrically small and efficient antenna. Feedpoint impedance becomes increasingly capacitive, making XA in equa−
tion 6 negative, necessitating inductive loading of the antenna. Two other limitations of small antennas are: (1) low radiation resistance (Rs approx. 0 W), and (2) low efficiency, a
problem compounded by low radiation resistance. Radiation
resistance seen by the load or source may be matched using
an autotransformer configuration, as shown in figure 8,
which represents the antenna as a tuned circuit with a tapped
inductor. The shortened antenna is the capacitor, and the
loading coil is the inductor.
The radiation resistance (Rs ) of the antenna can be lumped
into the Q of the inductor:
R
R
Q= s = s
(7)
X L 2πfL
which justifies the absence of a resistor in the circuit. Since
Rs is small for an electrically small antenna, the apparent
resistance (Rs ′) across the tap point can be related to the
actual radiation resistance as:
n +n 
Rs ’= Rs  1 2 
 n1 
2
(8)
where n1 and n2 are the numbers of turns in the coil, as depicted in figure 8. Thus, the antenna may be tapped to a value
that most nearly matches the Schottky diode. After the num−
ber of turns is adjusted to achieve resonance, the coil is tapped
to provide the required input impedance.
Figure 9. Folded dipole far-field radiation pattern.
Vol. 48(2): 831−838
Comment
1/10-wavelength loop at feedpoint.
15 mm whip; loaded by three turns on 6-32 machine screw;
tap at turn 3.
The transponder antenna requirement is similar to highfrequency (HF) mobile units in the intent to provide an
electrically small radiator with reasonable efficiency without
impeding insect mobility. A HF mobile antenna is often an
electrically short whip inductively loaded at the top, center
or bottom (Straw, 2002). Typically, a larger inductor is
required for center loading, whereas top loading must be
capacitive, with greater construction complexity. Thus a
simpler base-loaded antenna configuration was chosen for
investigation, allowing a smaller inductance. Practical
loading inductors typically range from 3 to 8 mm in length,
which for a whip less than 20 mm long, is a significant section
of the antenna. In computer simulations, loading coils were
generated from line segments rather than being represented
as complete, wire-wound inductors. In effect, this also
generated a small, normal-mode helical antenna topped off
with a whip (Kraus and Marhefka, 2002).
Copper wire was chosen for the antenna, since it can be
easily formed and is available in many sizes, although it is not
as durable as steel. However, steel is generally unsuitable for
high-frequency circuit or antenna applications due to its poor
conductivity compared with copper. Some early experimental prototypes were constructed with 26 AWG copper and,
later with 34 AWG copper wire, since fine copper wire is
lightweight and flexible.
Table 2 provides comparisons from among proposed wire
antennas for which computer simulations were done at
1.834 GHz. As indicated earlier, range distances were
measured horizontally from the forward end of the transmitting antenna, with transponders on the ground and the
detector held at chest level of the operator. The folded dipole
has a large inductive impedance component due to its loop
configuration. All of the antennas have a toroidal radiation
pattern, with the axis along the main radiating element. Figures 9 and 10 illustrate the far-field patterns of a folded dipole
Figure 10. Loaded monopole far-field radiation pattern with antenna
whip and coil superimposed.
835
and a loaded monopole, respectively. The images are not to
scale, representing only the general radiation patterns for either the electromagnetic field or power density.
ANTENNA SIMULATIONS
Computer simulations were performed using the NEC
(Numerical Electromagnetic Code; www.si-list.org/swindex2.html) EZ-NEC software and a free software alternative,
4nec2, to evaluate proposed antenna designs prior to any
actual testing. In setting up simulations with these systems,
coils were constructed stepwise by means of discrete wire
segments. EZ-NEC is less suitable than 4nec2 for simulations
requiring a discrete-wire model for an inductor, since 4nec2
supports more discrete-wire segments. A spreadsheet was
devised to aid in synthesizing discrete-wire coils of various
sizes and precisions. To standardize results, all data shown in
table 2 were based on simulations using 4nec2. Figures 9 and
10 were generated with 4nec2, which supports direct 3D
rendering.
ANTENNA SELECTION
Based on the simulations and preliminary physical testing,
the base-loaded monopole configuration was identified as
most suitable for BVW application from among the wire
antennas. This antenna was lightweight, durable, and physically small, and had adequate tracking range for the BVW. It
could also be mounted with the whip pointing rearward of the
insect, leaving its forward travel relatively unimpeded.
Design protocol for transponders with wire antennas was:
(1) to place as much wire on the insect as possible until
resonance was achieved, and (2) if resonance was not
attained, to load the antenna inductively until return signal
strength was maximized.
TRANSPONDER TESTING WITH INSECTS
INSTALLATION AND TRACKING TRIALS
Captive insects must be subdued before transponders can
be installed. One method is to place the insect in a
reduced-temperature environment until its activity is diminished. A simpler way was to release the BVW on a piece of
corrugated cardboard and push forceps into the cardboard
around its body to restrain it. Then its back was wetted with
water as a catalyst for a cyanoacrylate adhesive. After a small
drop of adhesive was placed on the diode, it was held in place
on the wetted area until secure. One disadvantage of this
method is that water tends to cause the adhesive to become
milky, possibly obscuring any identifying markings on the
transponder or insect. Figure 11 shows a transponder with its
loaded monopole antenna mounted on a BVW.
The radiation pattern of the transmitting antenna is shown
in figure 12, with the antenna’s central metal boom and
transverse elements superimposed. The pattern of the
Yagi-Uda antenna is predominantly in the “forward” direction, although, as shown in figure 12, minor side and rear
lobes occur in the field. Since the electric vector of the
electromagnetic field is parallel to the transverse elements of
the antenna, the maximum range was obtained when those
elements were parallel to the whip of the transponder antenna. The receiving antenna of the detector is also directional,
with a strong forward lobe. Detection requires “illumination”
by the transmitter and reception of the radiated 2nd harmonic
836
Figure 11. Transponder installed on a BVW. The trailing monopole antenna (next to its shadow) and the diode and loading coil (rear and side of the
insect) can be seen.
response. Thus, the actual detection pattern is a combination
of both antenna fields of the radar detector. It was essential
whenever possible to do a 360° sweep when a response signal
was first detected to ensure that the main lobes of the detector
antennas coincided with the actual direction of the target.
A monopole transponder has a toroidal radiation pattern,
with the straight antenna along the axis of the torus (fig. 10).
Since the antenna whip typically trailed the insect, the
weakest signals were received when the operator was directly
behind or in front of the insect. When searching for a tagged
insect, the operator set the detector sensitivity to its
maximum level and held the unit with the Yagi-Uda
transmitting antenna elements approximately parallel to the
horizon. The antenna was aimed slightly below horizontal,
and when a signal was detected, the operator moved toward
it while slowly sweeping the unit side to side. As the signal
became stronger, the operator reduced the detector sensitivity, allowing the antenna pattern directivity to become more
sharply defined. The radar did not have sufficient resolution
to precisely locate an insect, requiring visual inspection to locate insects that were be hidden under debris.
During the course of experimentation, several weevils
were released in a landscaped area for up to 4 days. In these
Figure 12. The far-field pattern of the detector’s Yagi-Uda transmitting
antenna, with the antenna’s driven, reflector, and director elements superimposed.
TRANSACTIONS OF THE ASAE
trials, insects were always located, having moved anywhere
from a few centimeters to several meters, depending on location and duration of release. One insect carried the first prototype of the 3t3-15 mm loaded monopole antenna for over a
month without apparent ill effects. Transponders could be
readily removed from test insects with acetone. They remained relatively motionless for a period of time after removal, but soon resumed normal activity.
COPPER-PLATED STEEL WIRE ANTENNAS
Based on the foregoing work, development of a transponder with a base-loaded monopole antenna using copperplated steel wire was undertaken. The purpose was to take
advantage of the ruggedness of steel wire while maintaining
the needed conductivity by means of copper coating. This
method made use of the “skin effect” wherein conduction
occurs primarily at the outer surface of a wire at high
frequencies.
Steel guitar string wire of 152 mm diameter was chosen for
its light weight and spring-like characteristics. Induction
coils were cold-formed on a 6-32 machine screw at intervals
along a length of wire and stabilized by heating. The wire was
then precleaned as necessary and immersed in plating
solution in an electroplating unit. A 25 s exposure produced
a copper coating of 16 mm thickness.
Following final cleaning, antennas with 3-turn coils were
cut from the plated wire, with allowance for whips up to about
50 mm length. A Schottky diode was then soldered in place,
tapping the coil at its third turn, and the antenna whip was cut
to the desired length as for the previous monopole transponders. Transponder mass was typically 27 mg with a 48 mm
whip, about 32% of the mass of a BVW. A similar
transponder was fabricated with tinned-copper wire
(34 AWG) for experimental ranging comparison.
Preliminary ranging trials were done with the RECCO
detector as for other transponders, with results as summarized in table 3. The range was consistently greater for the
copper-plated steel (CPS) than for the tinned-copper (TC)
transponder. There may be several reasons for this: First, the
monopole whip of the CPS unit was slightly longer than that
of the TC unit. The tinning coat on the TC wire may have
been less conductive than the copper plating of the CPS unit
at the radar frequencies used. Finally, the TC unit can be
much more easily distorted or misshapen than the CPS unit,
whereby it could have been detuned to some extent. It was
concluded that CPS transponders, with their ruggedness,
weight, and response characteristics, would be suitable for
field tracking studies.
CONCLUSIONS
It was found that harmonic radar has potential as a
research tool for tracking the black vine weevil (BVW). Of
the transponder-antenna combinations studied, the Schottky
barrier diode with an inductively base-loaded monopole wire
antenna was identified as most suitable. Pending further
investigation, 15 mm appears to be the minimum useful
antenna whip length that would be required. The typical 3 to
4 m detection ranges using a harmonic radar transmitter/receiver set, such as a standard RECCO detector, were
considered to be acceptable for the BVW at the present stage
of entomological investigations. Overall, the transponder
Vol. 48(2): 831−838
Table 3. Comparisons of loaded-monopole transponders with
copper-plated steel (CPS) vs. tinned copper (TC) wire antennas.
Whip
Length Range
(m) Location of Transponder
(mm)
Wire
TC
CPS
CPS
TC
CPS
CPS
40
48
48
40
48
48
0.94
2.0
2.7
6.0
10.0
12.0
On soil surface, in shallow depression
On soil surface, in shallow depression
On soil surface, at the base of a tree
21.6 cm above soil surface, in open space
21.6 cm above soil surface, in open space
30.5 cm above soil surface, in open space
was lightweight and fairly simple to construct, and with a
mass of 20 mg was about 1/3 the mass of a BVW. In transponder trials, tagged BVW were successfully located and recovered up to 4 days after release in landscaped areas. Similar
transponders constructed with copper-plated steel wire were
found to have very acceptable range characteristics in addition to their resistance to deformation and detuning.
Design of insect-borne transponders requires careful
attention to mass, and to loading and dimensions of antennas.
Each insect species can be expected to present unique
challenges in terms of its habitat, size, and physiology, and
whether it flies, walks, or both.
ACKNOWLEDGEMENTS
The authors acknowledge helpful discussions with M. J.
Sciarini and R. C. Hansen.
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