STUDY OF FREE-RANGING SHARKS 471 



Receiver/Antenna System 



A standard terrestrial-wildlife tracking system is utilized, in this case an 

 AVM model LA-12 receiver (12 frequency channels), 8 and a hand-held 

 yagi directional antenna (3-element, 112-cm long) (Figure 25). The higher 

 gain, 4.3-m, 8-element yagi provides maximum range under conditions where 

 it can be handled, e.g., larger boat, land-based vehicle. Best directionality is 

 obtained by using a null-peak, twin yagi or loop antenna system. 



For relatively short-range reception, such as from a boat, a hand-held or 

 mast-mounted antenna suffices. For maximum range detection, the antenna 

 should be elevated higher than is practical from a small boat; airplane track- 

 ing is desirable. Another method to achieve long ranges is to use receiving 

 stations on land at high elevations, such as on the mountains of Santa 

 Catalina Island, Calif. The ultimate tracking system would involve satellites, 

 the feasibility of which has been investigated by Goodman et al. (1973) for 

 both marine and terrestrial species. 



Signal-Reception Range 



Under conditions of good propagation, a radio signal in air diminishes in 

 strength with distance primarily because of spreading. There is nothing com- 

 parable to the large through -water absorptive losses of ultrasonic transmis- 

 sion, thus ranges obtained by radio units through air are much greater than 

 those of USTs of the same power. Once in the far field of the radiating 

 antenna (beyond about one wavelength), the field strength can be considered 

 to diminish as the inverse first power of the distance, i.e., spherical spreading 

 at 6 dB /double distance. Thus, a radio signal 1 juV/m at 1 km should 

 diminish only to 0.5 uV/m at 2 km and so on. 



In practice, signal ranges over water depend strongly on the height of the 

 receiving antenna. Generally, the higher the antenna, the greater the range, 

 unless a phenomenon called ducting occurs (Mackay 1970); it can result in 

 unexpectedly long ranges near the water's surface. For the higher frequencies 

 such as 151 MHz (which are virtually line-of-sight), the earth's curvature is 

 mainly responsible for the need to raise the antenna at long ranges. A general 

 indication of the range capabilities of transmitters of the types discussed are 

 provided in Tables 10 and 11. 



APPLICATIONS IN BIOLOGICAL STUDIES 



A complex telemetry system is justifiable for a biological research program 

 only if it can yield the desired biological information better than other, less 

 costly methods. Telemetry is particularly applicable to gathering behavioral 

 and physiological information on certain relatively large animals that are 

 both difficult to observe in the wild and difficult or impossible to keep in 

 captivity. Such is the case with many species of large, wide-ranging sharks. 



8 AVM Instrument Co., 810 Dennison Dr., Champaign, 111. 61820. 



