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Conventional transmitters consist of an antenna, a power source and a transmitter unit. Although this combination is fairly fundamental, the specific components chosen may vary between projects. In light of this, rather than attempt to recommend a particular type of transmitter, it is likely more useful to the researcher to describe the basic equipment options which are currently available for transmitters.
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The two most common transmitting antenna are whip antennas and loop antennas:
1. Whip antennas
Characteristics: Most frequently used. Omni-directional. Usually stainless steel with a Teflon coating. Must be light, strong, and generally withstand a tremendous amount of flexing.
Pros/Cons: Produce more uniform signal over a greater distance than do loop antennas. Potentially subject to breakage through metal fatigue or corrosion; however, whip antennas may be sandwiched between layers of a collar for protection when attached to rough species (e.g., bears).
2. Loop antennas
Use: With collars, tuned to radiate maximum signal at the exact neck circumference for which they are constructed (±5%).
Pros/Cons: Signal does not travel as far as whip antenna.
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1. Lithium and silver batteries
Use: Most commonly used for wildlife transmitters.
Pros/Cons: Potentially cumbersome.
Duration and nature of study must complement characteristics of battery life which is directly proportional to pulse period and inversely proportional to pulse width and signal strength (Lessard 1989).
2. Solar Cell
Use: Renewable energy source powered by sunlight.
Pros/Cons: Low weight, long-lived transmitter package.
Not recommended for:
i. Nocturnal species
ii. Species active under heavy cover.
iii. Species which occupy burrows or caves.
iv. Aquatic species which frequently dive (e.g., turtles)
The battery capacity, operational life and duty cycle requirements determine the radio frequency energy the transmitter circuitry can generate and deliver to the antenna (Beaty 1990). The theoretical relationship between battery capacity, current drain and operational life is expressed by the equation:
Discussion of transmitter range tends to focus on "Line of sight" (LOS) range. This is the maximum unobstructed distance between transmitter and receiver which produces an adequate signal. Range may be influenced by environmental conditions and geographic factors. High humidity, heavy fog, heavy rain, wet snow, and intervening vegetation will absorb energy from the signal. Radio waves reflecting from rock outcrops or water bodies will also reduce the signal's energy due to phase cancellation (Beaty 1990). Increasing the transmitter power output by four times results in a doubling in LOS range, and a subsequent fourfold decline in battery life.
In general, larger batteries equate to increased weight, increased life, and increased range. The relationship between battery life and signal range is an inverse one. A transmitter which emits a signal which can be received at long distances will not last as long as one which puts out a shorter-range signal. For wide-ranging species it may be preferable to use long-range, short-life tags, whereas for a study of species with small home ranges, long-lasting tags with shorter ranges may be more appropriate. The range and life of a transmitter is dependent on the size of the battery, which in its turn depends on the size of the study animal and the method of transmitter attachment. The best trade-off between weight, life and range of a transmitter will depend on the particulars of each study.
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Transmitters (tags) are available as complete units (including attachment options such as collars) or as components which are assembled and finished by the researcher. (Note: transmitters which are not assembled commercially may be subject to additional testing and certification requirements through Industry Canada.) Manufacturers generally package transmitter units in a metal can and/or cover them in an acrylic or epoxy resin coating to protect them from the elements (e.g., salt water) and from being damaged by the teeth, beak or claws of the animal.
Transmitters are available as one- or two-stage circuits. One-stage transmitters are useful for many applications due to their simple design and consequent low weight (as low as 0.5 g or less). Two-stage transmitters consist of a basic oscillator plus an amplifier, and must be powered by a minimum of 2.4 volts. Two-stage transmitters are larger, more complex and often more powerful than single-stage units. Functionally speaking, choosing between a one- or two-stage transmitter has several implications (Table 2).
Table 2. A comparison of one-stage and two-stage transmitters (adapted from Kenward 1987).
|
One-stage transmitter |
Two-stage transmitter |
|
|
|
|
|
|
|
|
Generally speaking, two-stage radio transmitters are best suited for wide-ranging animals, including birds, which are large enough to carry them. Animals which are too small to carry a two-stage transmitter, or have localized, relatively short movement patterns, can carry one-stage transmitters.
1. Positive Magnetic Shut-off Switches
Characteristics: A magnet is taped to the outside of the transmitter to prevent it from activating. The magnet is removed when the transmitter is deployed and transmission begins.
Pros/Cons: Simple activation. Not suitable for very small transmitters. More expensive than an unfused connection.
2. Unfused Battery Connection
Use: Unfused connection must be soldered closed to activate transmitter. Connection should then be covered with a protective epoxy.
Pros/Cons: Light weight - often used with very small transmitters (e.g., 0.5 g) to keep weight down. Requires practice to activate quickly. Connection leads may break if worried (i.e. if transmitters are exercised during storage). Increased time required for activation; soldering and epoxying can be difficult in the field. Cannot be deactivated if research animals are not successfully captured.
Ideally, transmitters should be stored on a wooden shelf with at least 2.5 cm distance between magnets on different collars to ensure that the magnets do not cancel one another out and activate the transmitters (Decker 1988). A stored transmitter should also be exercised 2 to 3 days/month in order to prevent the build-up of a `passivation layer' on the battery electrodes. A receiver should be used to check that all magnets are in place and all transmitters are turned off. Small transmitter tester units are also available from several suppliers.
A detailed log should be kept of each transmitter unit (including those in storage) giving receipt dates, storage times, testing and results, deployment date, number of relocations and any notes on unusual signal characteristics or animal behaviour (see Appendix for sample). If the transmitter fails, the log is invaluable when the failure is analysed by the manufacturer.
Transmitters may be refurbished (replacing the battery, canister, antenna and attachment and testing all components) or retrofitted by re-working a transmitter to new specifications, (e.g., changing a deer tag to a moose tag; Decker 1988b). Both of these procedures are best done by the company from which the transmitter was originally ordered.
Proper care and maintenance of transmitters is critical to justify the often expensive costs of field studies. There is little point in wasting opportunities to place radio tags on animals because the tag has failed to work for the desired time period.
ARGOS Platform Terminal Transmitters (PTTs) differ from conventional VHF transmitters in that they emit a much more complex and larger transmission which is repeated at longer intervals and received by an ARGOS satellite (Burger 1989b). PTTs can transmit diverse data such as temperature, activity count, dive count, length of last dive, time spent out of water, etc. Transmitters may be programmed to collect and compile data and then transmit it at specified times when the satellite's orbit takes it overhead. PTTs do not transmit the animal's location; this is calculated by the satellite or an Earth station from two or more transmissions. ARGOS satellite systems offer up to 20 locations per day (dependent on the transmitter's geographical position, Shaw 1991) with accuracies from 150 to 1000 m. Some researchers have reported that marked variations in accuracy and sampling frequency may occur within and among studies (Keating et al. 1991). Users may obtain data collected by the satellite either electronically (via modem or telex) or in the form of computer disks or tapes or hard copies. At present, PTT transmitters are available in weights as low as 25 g (from Toyocom, see Suppliers List).
A GPS (Global Positioning System) transmitter locates itself by receiving and triangulating signals from at least 3 of 26 possible satellites, then transmits its position (the animal's position) to the user. The accuracy of GPS location systems may vary with the density of the forest canopy (Rempel et al. 1995). GPS transmitters can also be programmed to compile location data for a specified length of time, then transmit all of the data at once when contacted by a special receiver operated by the user. In this way, several weeks of location data can be recovered during a single relocation flight. GPS transmitters can also be combined with the ARGOS system to download their data via satellite. At present, the size of GPS transmitters (1800g) limits their use to larger animals such as wolves and moose.
Wildlink radio transmitters store activity data in a computer within the radio collar (Kunkel et al. 1991). The user can communicate with the collar's computer by transmitting signals to a collar-mounted receiver, and can control the transmission of stored data to a standard wildlife telemetry receiver. These collars may also be equipped with remotely-controlled tranquilizer darts to allow recapture of animals (Mech et al. 1990). Limitations include failure of immobilization due to frozen drugs during excessively cold periods.
Radio-telemetry may also be used to provide information other than the animal's location. Virtually any information which can be expressed as a variable voltage can be transmitted (Osgood, no date). However, each additional component will add to the weight of the package and decrease its operational life.
Activity sensors vary the transmitter pulse rate (Pulse Interval Modulation or PIM) according to the animal's activity. There are two types of sensors: real-time and time-delay (Burger 1988).
Real-time sensors change the pulse rate instantaneously with the animal's activity. The orientation of a tip-switch built into the transmitter determines the specific pulse rates. In this way, the researcher is able to distinguish activities such as perched versus flying or head up versus head down. Alternatively, a `relative activity' type sensor will provide an increase in pulse rate as the animal's activity increases. The researcher must calibrate the pulse rate to specific activities or behaviours by correlating the pulse rate with visual observations of the animal's behaviour. Pulse interval timers provide accurate means of determining pulse rates. Transmitters with real-time sensors are also available with two distinct pulse rates which give an active/inactive signal.
In time-delay sensors, the tip-switch is incorporated with a counter. The transmitter pulse rate changes only if the switch is not triggered within a specified period of time. This type of sensor is most commonly used for mortality studies, but is also used to indicate hibernation or activity versus inactivity. Delay times for the counter may be set from seconds to several days. Researchers should keep in mind that the slower pulse rates are also more difficult to triangulate than faster rates, and that if a predator or scavenger moves the collared animal's body, the mortality switch may be delayed. Transmitters with variable pulse rates also use more power than those with steady rates.
VHF temperature sensors may be used to monitor either the animal's body temperature or the environmental temperature. Body temperature data may be useful in determining health or reproductive status, and ambient temperature may also be utilized for habitat selection or hibernation studies. Transmitters for body temperature may be placed subcutaneously, internally, within the inner ear, cloacally, or vaginally (Burger 1989). Transmitters for ambient or den temperature may be placed on a regular collar or harness. Size or weight limitations and the data precision required will also affect transmitter type and placement.
Temperature data are transmitted via PIM. The relationship between temperature and pulse rate must be carefully calibrated over the range of temperatures which the transmitter could encounter. Since this relationship may change as the transmitter ages, transmitters should be recalibrated at the end of the study. Temperature sensing transmitters may also be used to detect mortality. However, researchers must keep in mind that a carcass in direct sunlight may not initially register a temperature appreciably cooler than a live animal.
Pulse rates of light level indicator transmitters are controlled by a light sensor mounted within the transmitter. This allows researchers to calculate the amount of time spent under cover or in a burrow.
Pre-programmed Duty Cycles
Transmitters are available from several manufacturers which contain programmable microcontrollers. These allow the researcher to specify on/off cycles to increase battery life. When the transmitter is deactivated, it goes into a "stand-by" mode in which its power requirements can be reduced to 10% of its normal power usage. Using this technology, a transmitter can be programmed to turn itself off in the fall, during an animals period of hibernation, and then reactivate itself again come spring. Its life can also be prolonged by alternating active and inactive days. For a long term study, two ear-tag transmitters can be programmed so that one will turn itself on after the other is expected to fail.
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