By Wolfgang Buescher, DL4YHF
Last modified: October 16, 2015.
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In 2015, german radio amateurs were asked to participate in a study
of migrating bats by the Arbeitskreis Fledermäuse Sachsen-Anhalt e.V.
(a group of german bat enthusiasts and scientists). In autumn 2015, the full text was available here,
along with reception reports, maps, and audio recordings of the bat's tracking signal.
Four individual bats of a migrating species (Pipistrellus Nathusii, in German: Rauhautfledermaus) were equipped with low-power VHF transmitters shortly before they left Eastern Germany, heading for their winter quarters in southwestern Europe.
Each of the four bats (nicknamed Betti, Kalli, Lotti, and Netti) transmitted short pulses ('S' in Morse code) on a different frequency, with an estimated ERP of a few hundred microwatts.
The author has always been fascinated by bats and the reception of weak radio signals, furthermore a suitable site for a VHF receiver on a hilltop was available. So a 'semi-automatic' VHF monitoring station was set up there, which could listen 'day and night' for the bat telemetry signals to increase the chance of reception (instead of just 'listening in' occasionally).
The four bat's frequencies were widely spaced (150.09, 150.125, 150.165, and 150.2 MHz), so the SSB VHF receiver had to be switched cyclically between the four frequencies. Because the remote VHF receiver site has no internet connection, and is out of reach for a WLAN link, a low-power, half-duplex, analog UHF FM link was set up between the computer (inhouse) and the remote receiver.
For this application, the receiver was remotely controlled by Spectrum Lab (via the FM link), using DTMF tones to switch the VFO frequency every few seconds.
Here is a short clip of Netti's 400 microwatt 'bat-transmitter', received in Spenge near Bielefeld with an IC-706 with a narrow CW filter, and a narrow-band preselector for 150 MHz (details on request) :
This document contains some info about the principle - hoping to inspire fellow hams and bat enthusiasts to try something similar during the next campain (check www.fledermaus-aksa.de).
The configuration file 'Bat_Radio_Recorder.usr' (contained in the SL installer) can be loaded into Spectrum Lab. With a suitable hardware, it will switch the bat frequencies, generate spectrograms, and record different audio files for each bat, including the bat's name, date and time in the filename (e.g. 'Netti_20150910_2211.ogg). The audio can be recorded as a wave- or Ogg/Vorbis file to save disk space.
Spectrum Lab's frequency analyser was configured for 'multi-strip' mode (details in the next chapter), with each horizontal strip showing the signal of one of the four bats (more on that later). At the end of each strip, a command was sent to the remote VHF radio via DTMF (using this homebrew adapter) to switch to the next frequency in the 'four bat cycle'.
At the same time, for each of the strips in the spectrogram, an audio file was recorded for later analysis. A few days after starting, the spectrogram showed a weak trace on one of the bat frequencies, which was later confirmed by listening to the audio file. The successful reception of 'Netti the Bat' on her fast overnight flight across Germany was even rewarded with a 'bat diploma' :o).
Of course, the principle is not limited to monitor bats equipped with VHF transmitters !
Also, if the logging PC can be directly connected to the 150 MHz receiver via USB or serial port (for example, using the CI-V protocol), the system would be simpler, and the frequency-switching without the 'DTMF telemetry' would have been much faster.
The next chapters describe the operation of the 'monitor', and how to modify the 'bat radio recorder' for other purposes.
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Spectrum Lab's main frequency analyser was configured for 'multi-strip' display mode.
At the begin of each strip, a bunch of 'conditional actions'
print the date and time into the spectrogram, send a DTMF sequence ("touchtones") or a CI-V command
to switch the VHF receiver to the new frequency.
At the end of each strip, the 'channel name' (in this case, Betti, Kalli, Lotti, Netti) is printed.
Each strip was approximately 2.5 minutes long (depends on the available screen size),
thus the 'four-bat-cycle' repeats after about 10 minutes, and a new screenshot of the spectrogram
is saved every 10 minutes on the harddisk.
At the begin of each strip in the screenshot, the DTMF command sequence can be seen. With a direct CI-V connection between PC and receiver, this would not be necessary, and less time would be wasted to switch the frequencies.
Dozens of spectrograms were recorded over night, along with hundreds of timestamped audio files (each with the name of the bat recorded in that file). The spectrograms could be quickly browsed with an image viewer (like IrfanView) to find activity on a certain frequency. The timestamps on the left side of the images helped to identify the audio file (with the matching timestamp), which were used to verify the presence of the 'wanted' signal by simply listening to the recording. This turned out to be more effective than letting the computer 'listen' to the weak and fading 'S' in Morse code.
The display- and record cycle (print date and time, print name of the bat, change frequency, wait for the end of a sweep of the spectrogram, ...) was implemented using Spectrum Lab's conditional actions (a kind of simplified script).
The control instructions used in the 'bat recorder' are shown and explained in the SL manual.
They can be easily adapted for similar purposes, for example for direct control of a suitable VHF receiver without DTMF tones (using the radio's remote control commands.
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