Miniaturization (0.2 g) and evaluation of attachment techniques of telemetry transmitters
1 Swiss Ornithological Institute, CH-6204 Sempach, Switzerland
2 Zürich University of Applied Sciences, CH-8400 Winterthur,
Switzerland
3 Micro-Consult Inc., CH-2025 Chez-le-Bart, Switzerland
* Author for correspondence (e-mail: beat.naef{at}vogelwarte.ch)
Accepted 5 September 2005
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Summary |
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Key words: radio-telemetry, circuit design, transmitter attachment, field technique
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Introduction |
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Amongst the subjects that are too small to carry radio-tags are species of
global economic importance or of conservation concern, and excellent example
organisms for behavioural ecology, evolutionary biology or
physiology/endocrinology. For many purposes, alternative techniques are
lacking. For example, harmonic radar transponders
(Riley and Smith, 2002;
Cant et al., 2005
) can be built
extremely small but have serious disadvantages. First, all tags operate on one
frequency, which prevents the remote identification of individuals. Second,
the microwave signal is strongly suppressed by vegetation, which greatly
restricts the application of the technique. Thus, the usable ranges are short
(<50 m with a hand-held transmitter/receiver, <1 km in open area with
vehicle-mounted equipment). Passive integrated transponders (PIT tags) are
also very small and allow for individual identification. However, their
detection ranges are far too short (<1 m) to allow the range use of animals
to be recorded. Furthermore, neither of the two techniques allows for the
transmission of behavioural or physiological data. Therefore, further
miniaturization of VHF radio-transmitters fills a crucial gap, since it offers
both individual identification and data telemetry over considerable distances
with portable and relatively inexpensive equipment.
To study the range use and survival of juvenile barn swallows Hirundo rustica L. after fledging, we improved both range and life of miniaturized transmitters. We present construction details and technical data for a new ultra-miniature VHF transmitter. Specifically, we address two main issues of miniaturization. (1) We propose a design for the transmitter's circuitry that maximizes the output while keeping the number of components small and the average current drain low. In particular, the pulse-forming circuit and antenna-matching circuit offered potential for development. (2) We analyse the radiated power of the newly developed transmitter. With tags that are very small relative to the radiated wavelength, factors such as the size (mass) of the battery, and the technique of attaching the transmitter become increasingly important. So far, these effects have not been quantified.
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Materials and methods |
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We used the P-Spice® software (Cadence Design Systems Inc., San Jose, USA) to evaluate a variety of one- and two-stage circuits with respect to the ratio of radiated power to the number of electronic components. Since adjustable components for oscillators (capacitors, inductors) are large compared to standard components, a main aim of the evaluation was that the circuits should oscillate reliably without any tuning elements. Based on the modelling results, different spatial layouts were realized and tested in the laboratory and in the field.
Transmitter construction
All circuits were realized on a 100 µm double-side printed circuit
board. To allow modifications of pulse length and interval by exchanging
components, the timer capacitor and resistor were arranged at the periphery of
the board. All capacitors and resistors were of 0402 size (1.0 mmx0.5
mm, height 0.2-0.5 mm). We used laser-cut inductors of 0402 dimensions, except
for the large inductor L2, which was available as a wire-wound type of 0603
size (1.5 mmx1.0 mmx1.0 mm). We used two types of crystals
differing in dimensions and mass, and also in the resulting radiated power.
Details of all electronic components and manufacturers are given in
Table 1. For the antenna we
used either 13 cm of a 0.15 mm multistrand steel cord originally used for
fishing, or 7 cm of 0.03 mm steel wire (unwound from the multistrand
cord).
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Results |
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A marked reduction of the average current drawn was obtained with a
modified pulse-forming circuit. Astable multivibrators require 6-8 parts and
thus would add considerable volume and mass even if twin transistors and
smallest capacitors and resistors were used. Here we propose a new solution to
the problem of controlling beeper transmitters with a multivibrator consisting
of only three components. We improved the RC pulser by inserting a PNP
transistor between the `timer' capacitor C1 and the base of the RF transistor.
This transistor cuts the pulse width to about half the duration obtained with
the common RC pulser while keeping the pulse interval constant. Using the
smallest components (T1: SOT490, C1: 2x0402, R1: 0402), this
multivibrator has a volume of 3 µl and a mass of 10 mg.
Transmitter mass
For field use the circuitry was coated with two layers of acrylic varnish
(Tropicalising varnish, Electrolube®, Wentworth Inc., Berkshire, UK). The
average mass of the finished transmitters was 0.084 g (range 0.081-0.087 g,
N=5) with the small crystal and 0.135 g (range 0.125-0.140 g,
N=17) with the larger crystal. Both versions are considerably smaller
than any commercially available transmitter. A comparison of dimensions and
mass with the smallest commercial types is given in
Table 2, and a photograph of
the assembled circuitry is given in Fig.
1.
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Performance and power consumption
With the values for the multivibrator set as in
Table 1, the transmitter
radiated pulses of 10-20 ms at intervals of 800-1500 ms. The average current
drawn depended on the values of R1 and C1 and ranged from 15 to 35 µA
(Table 3). This is similar to
multivibrator or CMOS controlled transmitters, but at a much lower volume and
mass of the circuitry. Using the S852 transistor the average current drawn
could even be reduced to 8-9 µA (at a 0.01 duty cycle). Therefore, the
expected life of the 0.2 g tag (with a 337 size cell, 8 mA h) is 22 days for
the 15 µA version and up to 35 days in the 9 µA version.
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The pulse width was linearly correlated with the capacity of C1, independently of the resistance of R1. A 100 nF increase in C1 resulted in a 2 ms increase in pulse width (Table 3).
At a given value of C1, the pulse interval was linearly correlated with R1.
However, the slope of the regression varied in relation to C1. The detailed
regression coefficients and statistics are given in
Table 3. The circuit oscillated
reliably with values of R1 of 1.0-2.7 M, allowing duty cycles of down
to 0.008 to be set (i.e. one 16 ms pulse per 2 s). Lower duty cycles are
undesirable from the point of view of detectability of the signal in the
field.
Radiated power
The radiated power of the bare transmitters varied considerably in relation
to the type of crystal, antenna length and battery type. In the configuration
with the larger crystal, a 396 battery (transmitter mass 0.6 g) and a 13 cm
antenna, the average ERP was -15.8±2.0 dBm (mean ±
S.D., N=7), which is slightly better than the
-17 dBm predicted by the P-Spice simulation. The ERP of the new design was
equivalent to the average ERP of the control group of commercial types
(-15.9±3.7 dBm, N=16; measurements taken with transmitters
attached to the dummy body). The parts of the circuit (L3, C4, C5) that match
the antenna to the oscillator strongly improved the radiation at the nominal
frequency of 148.5 MHz. Fig. 2
shows the frequency spectrum of the new design compared to that of the circuit
lacking L3, C4 and C5 (i.e. the design of
Cochran, 1963). The output at
the nominal frequency was improved whereas radiation at frequencies above 180
MHz was strongly suppressed. Thus, the
-filter-like matching circuit
yielded a higher gain at the nominal frequency than any transistor amplifier
stage.
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Also measured on the dummy body, the average power of the smallest (0.2 g) configuration was -21.6±2.6 dBm (mean ± S.D., N=5). The reduction in ERP compared to the larger type was mainly due to the short antenna and to the smaller dimensions (see below).
We observed a strong non-linear increase of the radiated power with the mass of the battery attached to the transmitter (y=-1.34+19.33x0.538, r2=0.83, P<0.001; Fig. 3). By contrast, the current drain during pulses was constant over all dimensions of cells (average 1.81 mA at 1.55 V; min. 1.78 mA, max 1.84 mA). This indicates that the effect was not due to insufficient cell capacity but was caused by the increasing lack of ground plane area with decreasing cell size. According to this correlation, the transmitter's ERP was 4.8 µW with the smallest 337 cell (0.12 g) and 28.9 µW on a 2.3 g 390 battery, respectively. This indicates considerable physical constraints in maximizing the output while minimizing the total transmitter mass and volume.
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Discussion |
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Furthermore, the design significantly improves the ratio of ERP to the average current drawn and to tag mass and volume. In particular with radio-tags of 0.5-2.0 g, this yields a substantial gain in tag life per unit mass. For example, the new design in a 1.0 g configuration (i.e. with 392 cell, 18 µA) would run for 105 days, whereas commercial types have an expected life of about 60 days (e.g. ATS 2420: 55 days, www.atstrack.com). In free air (i.e. transmitter and antenna >2 wavelengths above ground) the radiated power of the 0.2 g tag gives an operational range of c. 2 km (at -135 dBm receiver input). Although this range was confirmed in field tests, we have not yet collected experience with tagged animals. Ground-to-ground ranges are hardly predictable because the signal strength depends greatly on the positions of transmitter and antenna, and may vary from 50-500 m.
Although the transmitter was primarily designed for tracking animals in the field, the circuit can be modified for many more research purposes. For example, very small implants could be constructed by using a short coiled or loop antenna. Furthermore, the circuit can be modified to transmit physiological or behavioural data by adding sensory devices.
Research implications
There is experience with radio-tagging small animals down to 10 g
(Naef-Daenzer et al., 2001
;
Bontadina et al., 2002
),
confirming the 5% rule for the acceptable transmitter load, at least for birds
and bats. The new transmitter will allow the application of radio-telemetry to
be expanded on taxa for which virtually no experience exists, for example
insects and other invertebrates, and small amphibians. Pilot studies with
smaller species have revealed that animals carrying very large and heavy loads
(stag beetle Lucanus cervus L, up to 30% of body mass;
Sprecher-Uebersax and Durrer,
2001
; Riecken and Raths,
1996
) did not behave like untagged animals.
It is difficult to draw general conclusions on maximum acceptable loads or attachment techniques, since radio-tagging probably affects different species differently. In addition to tag mass, the volume of the package is also an important issue if small animals are to be tagged. Therefore, researchers should carefully evaluate and eliminate transmitter effects. Minimizing the impact on study animals should be preferred over maximization of technical reliability.
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Acknowledgments |
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References |
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