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

thus, the gain in efficiency of antenna operation is less at higher fre-
quencies. The precise relationship of these two contradictory effects is
unknown (Sinning 1979); however, the magnitude of effect probably varies with
environmental conditions.
The use of water rather than air as a medium for radiotransmission
requires shorter antennas. The usual antenna length is 0.25 wavelength.
Wavelength size depends on the velocity of the generated radiowave, which
decreases with increasing dielectric constant of the medium. Because the
dielectric constant for water is about 80, the length of the transmitting
antenna may be reduced from that required for air (dielectric constant about
1.0) (Weeks et al. 1977). The velocity of radiowaves in water is about 0.11
that of air; therefore, 0.25 wavelength antennas of commonly used frequencies
of 15 to 50 MHz may be reduced from- 1.5 to 0.5 m in air and from 0.17 to
0.06 m in water. These lengths (0.17 to 0.06 m) are acceptable sizes for
transmitters used in most fish studies.
It is theoretically possible to increase the efficiency of a transmitting
whip antenna to increase the strength of the received signal. Because the
resistance of the media (water) to radio transmission cannot be controlled by
the investigator, the only other variable relative to the antenna is antenna
length. An investigator might gain efficiency by making a whip antenna longer
than-0.25 wavelength and as straight as possible. Unfortunately, this increase
in length may not be desirable for transmitting antennas used in fish work,
especially if the transmitter is to be used with smaller fish species.
The use of a loop or coiled transmitting antenna is highly desirable for
modules surgically implanted in fish because of the necessity for compactness.
It may be possible to increase the radiation resistance (antenna efficiency)
of some types of coiled antennas by increasing the length of the coil. How-
ever, fora "tuned" inductor type, the coil is part of a circuit and resonates
at a certain mode. The tuned inductor type antenna, therefore, cannot be made
more efficient simply by increasing the length (Kolz, pers. comm.). Amore
detailed discussion of loop antennas is beyond the scope of this report;
however, these antennas have considerable application in fish research.
RADIO RECEIVING SYSTEMS
The receiving system for radiotelemetry consists of an antenna, a
receiver, and the interconnecting transmission line (Fig. 2). The size of an
antenna in air must be nine times larger than in water. Size considerations
are very important if an aircraft is to be used for tracking because of wind
resistance. Sma11 antenna size is also desirable if tracking is done from
shore or a small boat. Receiving antennas work best when elevated; however,
there can be constraints on heights due to field conditions.
Transmitting antennas must radiate radio signals in all directions.
Receiving antennas should have a capability for receiving from ail directions
as well as functioning unidirectionally. The only practical approach to this
problem with existing technology is to employ two separate antennas. For fish
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