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• UCREFRP Catalog Number: 1113
• Author: Tyus, H. M.
• Title: Acquisition of Habitat Preference Data By Radiotelemetry, ‎(Proceedings of a Workshop on the Development and Evaluation of Habitat Suitability Criteria)‎.
• USFW Year: 1986.
• USFW - Doc Type: \
• Copyright Material: NO

antenna is approximately 3 m long at 50 MHz. These elements are mounted on a
boom that ranges in length from about 1.5 m for a 2-element Yagi antenna to
nearly 4 m for a 5-element Yagi antenna at 50 MHz. The antenna should be at
least 0.5 wavelength from the nearest large object in order to be highly
directional and effe tive in radio signals (gain). A 50 MHz antenna must be
mounted at least 3 m above the ground because ground reference may interfere
with performance. Yagi antennas are most effective when used from a fixed
location because of he constraints discussed above and the large size of the
antenna. The loop an enna does not have as much gain or directivity as a Yagi
antenna, but it is ad quate for close work once a fish has been located. Loop
antennas are recommended for fish work because they can be made small, and
they are more rugged and less sensitive to interference from objects in their
proximity than Yagis. Loop and Yagi antennas are bidirectional and .require
readings from two di ferent locations so that triangulation can be used to
locate the signal source. A simple bidirectional loop antenna for 50 MHz is
about ~ m in diamet r. Smaller diameters can be used, but some loss of
sensitivity is to bee petted.
After the antenna has received the radio signal, the signal is relayed
through coaxial cabl to the receiver where it is converted to an audio or
visual signal. Coaxi 1 cables have unavoidable efficiency losses due to their
construction. These losses can be minimized by the proper choice of cable,
keeping cable lengths short, and frequent inspection to make sure the cable is
not flattened or nit ed. Connectors also cause a loss of efficiency. Some
investigators have u ed coaxial splitters to separate the signal from one
coaxial cable to two cables. This method allows one antenna to be shared by
both a search and a inpointing type receiver. Since this can result in a
loss of received sign 1 strength, it should be avoided. Better techniques are
to use two trackers or to split the earphones to receive both signals
simultaneously (a di ferent signal for each ear). Other investigators have
used signal strength oosters between the antenna and receiver. However, care
must be taken so that the noise level is not boosted so much that it interferes
with signal receptio Listening to increasing amounts of static does not
improve tracking!
Losses associate with the transfer of signals between the antenna and
the receiver are us ally small, but they can significantly affect field
results. For example water, dirt, or corrosion on the conductors can diminish
the signal strength. These effects must be recognized and avoided by careful
upkeep of equipment, especially in high conductivity waters where signal
propagation is suboptimal.
The radio signal is converted in the receiver to an audio, visual, or
other type of display useful to monitoring personnel. Receiver design involves
many considerations and currently approaches theoretical limits for factors
such as sensitivity (primarily limited by thermoelectric noise generated
within the receiver) and selectivity (ability to differentiate the desired
signal from other signals).
There are three principal types of receivers: (1) the "search" (or scan)
receiver, which will simultaneously pick up signals from any transmitter in
use; (2) the "tracking" (or pinpointing) receiver, which is used to locate and
identify individual transmitters; and (3) the "programmable" receiver, which
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