Copyright © 1998 Province of British Columbia
Published by the Resources Inventory Committee
[Back to TOC] [Previous] [Next]
[Back to TOC] [Previous] [Next]
The function of a receiver is to receive the signal picked up by the antenna (to which it is connected by a coaxial cable), amplify it, and make it audible to the user. Receivers are available in a variety of sizes, weights and prices from a number of national and international suppliers. Study needs will determine whether data collection is best done manually by field personnel or whether an automated receiving station should be set up. Receivers are powered by replaceable and/or rechargeable batteries, and may also be equipped with a cigarette-lighter adapter for connecting to a vehicle's electrical system. Some models are equipped with scanners which may be programmed to switch between a number of different frequencies; this is ideal for studies with a number of animals which tend to wander. Strip-chart recorders or data loggers may also be incorporated into a receiving system, and are particularly useful for automated receiving stations.
Receivers may be damaged by static electricity from clothing or car seats and by radiated power from voice communication systems (Crow 1988). To prevent this damage:
It is also worthwhile to note that receivers are sensitive to moisture. This is an important consideration when try to locate animals in the rain.
It can be useful to adjust a receiver up or down in order to identify the best or most functional frequency for a given transmitter. It is not uncommon for a transmitter's best frequency to be slightly different from the one identified by the manufacturer. As well, a transmitter's frequencies may drift slightly.
[Back to TOC] [Previous] [Next]
Receiver antennas may be hand-held or mounted on a vehicle roof, aircraft or boat. A Yagi antenna is a directional `gain' type antenna which uses a number of parasitic directors in front of the `driven' element (the one connected to the coaxial cable) and a reflector behind the driven element in a defined mathematical relationship (Jones 1990). Directional antennas such as Yagis or `H' antennas concentrate the radiated energy to the front of the antenna. Minor lobes to the sides and rear are also produced.
Antenna beam width refers to the radial distance between the angles at which an antenna is held in which an audible signal is received (the `directionality' of the antenna). The greater the number of elements, the smaller the beam width. For example, a 3-element Yagi antenna has a beam width of 60o in the horizontal orientation, and a 2-element H antenna has a beamwidth of 100o in the horizontal orientation. Both antennas have wider beam widths in the vertical orientation (Burger 1991).
Regardless of the type of antenna, the elements must remain straight and parallel to one another to ensure maximum receiving efficiency. Antennas are tuned to a particular frequency, and antenna elements are only interchangeable if they are the same length as the original elements, and are interchanged between antennas of the same frequency range. Maintain antenna elements in perfect alignment; badly damaged elements are likely beyond repair.
The most commonly used hand-held antennas are the Yagi and the `H' antennas. Hand-held Yagis have 2 to 5 elements. Each additional director element increases the distance from which the antenna can pick up a signal. Loop antennas are small, hand-held antennas which are useful for close-in tracking of 1 km or less.
Large directional antennas with 5, 8 or 14 elements are usually used as vehicle mounted antennas or at fixed sites. Omni-directional (bipole) antennas may be mounted on a vehicle and used to determine the general vicinity of an animal. A precise location can then be determined with a directional antenna.
Both Yagi and H antennas have been used for relocating animals from the air. Antennas are mounted on fixed-wing aircraft with brackets designed to fit struts on commonly used types of aircraft. The operator uses a switch box to listen through the left or the right antenna or both to determine the direction of the incoming signal. The receiving system can be connected to the aircraft intercom system so everyone in the aircraft can hear the transmitter. Brackets to mount an antenna on a helicopter skid are also available (Telonics).
Brackets must be chosen carefully as strut sizes may vary within the same model of aircraft. The antennas should be centered on the struts with the tips of the antennas facing fore and aft, with the front of the antenna facing out toward the wing-tip and slightly downward (about 30o below the horizontal axis of the wing, Jones 1988). Some researchers recommend orienting the antennae at 90o to each other. Duct tape is often used to attach antenna coaxial cables along the outside of the wing and through windows or vents into the cabin. Antenna / coaxial cable attachments should also be secured or taped as they can become loosened by constant vibration and jeopardize the results of the flight. All equipment attached to aircraft is governed under aviation law and requirements may vary according to ownership, use and location. Researchers must ensure that their equipment and their means of attachment comply with the appropriate aviation standards.
Automatic direction finding systems incorporate a rotating antenna, a fluxgate compass and a receiver/datalogger that automatically determines and stores bearing and signal strength information on a particular animal for downloading.
Very large antennas with many elements may be installed at a fixed receiving station. Antennas at fixed sites are subject to lightning strikes and therefore should be well-grounded. Fixed receiving stations require some prior knowledge of the animal's range to ensure the best placement.
[Back to TOC] [Previous] [Next]
HTML Created: August 98
Copyright © 1998 Province of British Columbia
Published by the Resources Inventory Committee