Radio Tags
Radio Tags
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Radio Tags

Radio-tags are what Biotrack supplies to researchers for fitting to animals so that they can study them. A radio tag consists of a transmitter circuit, that may include a sensor, a cell (battery) that provides the power, and it is then protected with potting and furnished to make attachment to the animals as easy and kind as possible. The attachment techniques are described in detail under the Radio Tags link, whilst here you can find out about transmitters, sensors and how to start tags.

Transmitters

Radio transmitters are small sealed units that contain the electronic circuitry needed to produce radio signals. For more details of their specifications please see the pdf files.

The TW-3 has a 'two-stage' transmitter circuit board with separate oscillator and amplifier/antenna matching circuits. It has an independent pulse-forming circuit (CMOS astable multivibrator) on a second board and is built mainly from surface mount components.

The TW-4 has a 'two-stage' transmitter circuit board with separate oscillator and amplifier/antenna matching circuits. It has an independent pulse-forming circuit (astable multivibrator) and is built from some of the smallest surface mount components available.

The 'Pip' range of transmitters are 'two-stage' transmitter circuit boards with separate oscillators and amplifier/antenna matching circuits. It has an independent pulse-forming circuit (astable multivibrator) and is built from some of the smallest surface mount components available, including a surface mount crystal. The inclusion of this small crystal is the main advance in the Pip, and enables 0.3g to be shaved off the weight of our small tags.

Sensors

Posture sensing
Posture-sensing circuits can be added to all but the very smallest radio tags to provide information about which orientation an animal is in. For example, most birds perch in an upright position, but fly horizontally, so this sensor can determine whether they are perching or flying.

The principle component in the circuit is a mercury tilt-switch which switches between two distinctly different pulse rates (fast and slow). The pulse rates can be set to any reasonable values, but typically the slow pulse would be about 50 pulses per minute, and the fast pulse 70 - 80 pulses per minute.

Activity sensing
Activity-sensing circuits are used when there are not such obvious changes in posture, e.g. many mammals alter their position from side-to-side as much as up and down, or even upside-down, so the sensing does not have such a clear meaning. However, many researchers find that it is interesting to know if an animal is active, without necessarily seeing it. The same tilt-switch is used as in the posture sensing, but in a horizontal position, so that any movement tends to cause a change in pulse rate.

Temperature sensing
Temperature-sensing circuits can be used for more than just temperature measurements. Indeed, the accurate measurement of temperature with the necessarily simple circuitry in animal radio transmitters is fraught with difficulties. The basic principle of temperature telemetry is that pulse interval is determined by sensor temperature, and usually the sensors are thermistors. Biotrack temperature-sensing tags are used mainly in studies of activity or behaviour (or for mortality-sensing as described in another section of this document), rather than for absolute temperature measurements.

Mortality sensing
Radio tags can be used to discover much about what an animal does when it is alive, but equally, a radio tag can help to discover when an animal is dead, and thereby how and why it died. A standard radio tag will permit the death of an animal to be discovered, simply by virtue of the tag having remained in one place for longer than expected, and by the absence of any of the fluctuations in signal strength that occur as an animal moves. However, it may be some days before the death is discovered, and in studies where causes of mortality are the prime research interest, such a delay is not acceptable (because the body may have decomposed or been damaged to the point where it is more difficult to determine the cause of death). In these mortality studies, there are special circuits that can be added to the transmitter to provide specific signal patterns when an animal dies. These circuits are either 'activity-mediated' or 'temperature-mediated'.

Starting tags

Start-up wires
Most Biotrack tags are supplied with 'start-up' wires. These are not as convenient as reed switches, but over very large sample sizes, have proved more reliable. Whereas a small proportion of reed switches have been known to develop faults, a well-soldered, properly covered soldered wire connection is utterly dependable. 

Magnet Switches
On most tags the reed switch is a 'normally closed' type. These switches are fitted in line with the power supply, and no power is consumed when the tag is switched off. When the magnet is removed the contacts touch and the transmitter starts to pulse. You can turn the transmitter off again by replacing the magnet. We recommend that you listen to the tag while you do this to check that the magnet is in the right position.


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