7979

• UCREFRP Catalog Number: 7979
• Author: Tyus, H. M.
• Title: Fish Radiotelemetry
• USFW Year: 1982.
• USFW - Doc Type: Theory And Application For High Conductivity Rivers.
• Copyright Material: NO

work, the omnidirectional straight (or whip) antenna can be used to receive
signals from all directions. For monodirectional tracking, either a loop or
Yagi antenna may be used. The loop antenna resembles a hoop on a stick, while
the Yagi type consists of several short cross pieces mounted on a vertical
pole. The short sections (elements) are mounted perpendicular to the vertical
plane and attached at their midpoints (Fig. 2).
The 0.25 wavelength antenna at 50 MHz fora simple whip antenna is about
1.5 m. The whip antenna usually rests on some object on the ground plane. A
ground reference plane placed perpendicular to the whip antenna aids in effi-
ciency. The whip antenna is more sensitive than a loop, but less sensitive
than a Yagi. Its advantages are simplicity of design, low cost, and ease of
mounting. This antenna has the least air resistance and, for this reason, may
be preferred for aircraft use. The convenience of a whip antenna may make it
more desirable than a Yagi, and many investigators believe the loss in effi-
ciency may not be significant (Knight, pers. comm.). In addition, the Yagi
antenna may pick up significant noise in some locations and mask the transmit-
ter signal, especially if antennas are stacked (i.e., if more than one antenna
is mounted on a single pole) (Range, pers. comm.).
Loop and Yagi antennas are used to locate a signal source by rotating the
antenna until the maximum.(or minimum) signal strength is detected. For the
Yagi antenna, the gain (efficiency) increases with an increasing number of
elements in the antenna although the rate of increase becomes less with each
element that is added. Each element of the 0.5 wavelength antenna is approx-
imately 3 m long at 50 MHz and about 1 m long at 150 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 effective 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 the constraints discussed above and the large size
of the antenna. However, Yagi antennas have been used successfully from a 4
to 5 m boom on small aluminum boats (Range, pers. comm.).
The loop antenna does not have as much gain or directivity as a Yagi
antenna, but it can be made much smaller and more rugged and is less sensitive
to interference from objects in its proximity. Loop and Yagi antennas are
bidirectional and require readings from two different locations so that
triangulation can be used to locate the signal source. A simple bidirectional
loop antenna for 50 MHz is about 1 m in diameter. Smal]er diameters can be
used but some loss of sensitivity is to be expected.
After the antenna has received the radio signal, the signal is relayed
through coaxial cable to the receiver where it is converted to an audio and/or
visual signal. Coaxial cables have unavoidable efficiency losses due to their
construction. These losses can be minimized by the proper choice of cable and
by keeping cable lengths short. Connectors also cause a loss of efficiency.
Some investigators have used coaxial splatters 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 pinpointing type receiver, Since this can result in a
loss of received signal strength, it should be avoided. Other investigators
have used signal strength boosters between the antenna and receiver. However,
care must be taken so that the noise level is not boosted enough to interfere
with signal reception (Range, pers. comm.).
Losses associated with the transfer of signals between the antenna and
the receiver are usually 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 waters of high conductivity 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 theoretics] 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 two principal types of receivers: 1) the search (or scan)
receiver which will pick up a variety of signals (ideally from any transmitter
in use); and 2) the tracking (or pinpointing) receiver which is used to locate
and identify individual transmitters. Any major field study should involve
use of receivers with search and track capabilities, especially if several
transmitters are used. A search mode] can prevent loss of a fish if tem-
perature changes or other factors cause a transmitter to emit a different
frequency.- The scan receiver, however, cannot be tuned as precisely as a
pinpoint receiver and may not have as great a range in terms of strength of
signal received or distance.
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