charlie@oakhill.UUCP (Charlie Thompson) (04/24/89)
This weekend I decided to scrounge through my junkbox and build a BATRADIO. I took a 25 Khz narrowband ultrasonic transducer and preamp'd it and passed the amplified signal to a homebrew direct conversion 25 KHz receiver. The receiver was a homebrew mixer and a Wavetek 111B (B as in battery powered). The output of the mixer was lowpass filtered and sent a jambox for listening. Austin has a rather unique downtown bat population that lives under the Congress street bridge. At sundown the bats begin stirring and then, all at once, about 500,000 bats stop hanging around and start flying in one giant cloud. Bazzare. The BATRADIO worked great and I learned a lot about bat transmissions. Later, after the bats had left the bridge, I drove home and set up the BATRADIO again. I detected one lone bat, somewhere in the dark chirping away. Bat transmissions are FM/AM. That is, they are swept pulses, like a chirp...which is what they sound like when you downconvert to the 'human band'. Sometimes the chirping is fast ... sometimes slow. The bat on my street was chirping slowly until a car drove by and it suddenly went to double the chip rate...presumably it had 'locked on' to the car. When the car went away the bat resumed chirping at the slower rate. From what I could tell the bats must have a 'deviation' of 5 to 10 KHz. Since my receiver was only about 2 KHz wide at 25 KHz I couldn't know exactly what the deviation was. The transducer that I used was resonant so it was not very useful to tune my local oscillator since the sensitivity dropped off abruptly outside the range of the transducer's center frequency. Anyway, it was an educational experience... and the best VLF'ing I've done in a long time! Happy bat hunting. -Charlie Thompson WB4HVD Austin, Texas