Electromagnetic Radiation Effect on Foraging Bats - Recent Advances and Issues in Environmental Science - page 65

As in most radar systems, the antenna of the radar usually swept through 360
degrees. For the current experiment this would reduce the extent of exposure along
any radius. Therefore the experiment was repeated with the antenna of the radar fi xed
such that the radar signal was orientated directly towards the area of highest bat ac-
tivity. Similarly the duration of exposure to the radar signal is dependent on the duty
cycle of the radar transmitter (pulse length×pulse repetition frequency). Therefore the
experiment was repeated at each site using two different pulse length/pulse repetition
rates (0.08 μs/2100 Hz, 0.3 μs/1200 Hz,) with the radar antenna fi xed to maximize
exposure. A portable electromagnetic fi eld meter (PMM 8053-Accelonix Ltd.) and
isotropic fi eld probe (EP-330 Isotropic E-Field probe-Accelonix Ltd.) were used to
measure the maximum value (peak hold) of the electromagnetic fi eld strength (EMF)
of the radar in volts per meter (v/m) at three distances from the radar antenna (10, 20,
30 m) for each of the two radar settings implemented throughout the study.
Bat Activity Recording
At each foraging site bat activity was recorded at three distances from the radar antenna
(10, 20, 30 m) using automatic bat-recording stations [26]. Each automatic station con-
sisted of a Batbox III heterodyne bat detector (Stag Electronics, Sussex, UK) linked to
a count data logger (Gemini Data Loggers, UK Ltd, Chichester, UK) via an analogue
to digital signal converter (Skye instruments, Ltd). The signal converter converts ana-
logue signals from the bat detector into digital signals that can be recorded by the data
logger. Every 0.5 sec a positive or negative signal is sent to the data logger indicating
the presence or absence of ultrasound respectively. Therefore, the recorded number of
bat active half seconds referred to as “bat counts” over a 30-minute trial provides a
quantitative index of bat activity during that period. Most narrowband detectors will
detect a range of frequencies centered on the value shown on the tuning dial. For the
Batbox III this window is ±8 kHz of the tuned frequency, therefore the frequency was
set to 50 khz in order to effectively detect each of the five breeding species of bat in
Scotland ( Pipistrellus pipistrellus, Pipistrellus pygmaeus, Myotis daubentonii, Myotis
nattereri, and Plecotus auritus ). The component parts of the system were housed in
large plastic boxes with a hole cut for the bat detector microphones. Automatic re-
cording stations were positioned on platforms 1.5 m above the ground and orientated
perpendicular to the radar signal (Figure 1).
In conjunction with the automatic recording stations bat activity was recorded
continuously during each trial using a frequency division bat detector (S-25, Ultra-
sound Advice, London). This method of ultrasound transformation allows calls to be
recorded in real time on audiocassettes and the number of recorded passes provides
a quantitative assessment of bat activity Bat detectors were linked to a tape recorder
(Sony Professional Walkman, Tokyo, WMD6C) containing metal-tape cassettes. At
each site the bat detector was placed at a distance of 20 m from the radar antenna and
the height and direction remained constant at 70 cm. The 60 min recording at each
site were analyzed using BatSound software (BatSound Pro, Pettersson Elektronic
AB, Uppsala Sweden). In addition to the total number of bat passes, terminal feeding
buzzes at each site were counted. These characteristic sounds are produced by aerial
hunting and trawling vespertilionid bats when prey capture is attempted [27] and can