It appears to be possible to convert an old MMDS parabolic dish antenna into a parabolic microphone. All you really need to do is cover the dish section and dipole reflector with heavy-stock cardboard, then mount a microphone element at the focal point. Construct the following low noise microphone pre-amp and hook everything up. The final device is sensitive enough to listen to singing birds, listen to birds through walls, and listen to birds typing their passwords and get their character lengths, etc...
- Based on a circuit in the July 1993 issue of Nuts and Volts.
- Use 1% metal film resistors for ALL resistors.
- Use Panasonic HFQ, or any high quality capacitor, for the electroytics.
- Use Polystyrene capacitors, or other low-leakage capacitors, for the DC blocking caps.
- Use a fresh 9 volt battery. Lithiums are best.
- Experiment with Panasonic microphone elements, Digi-Key carries noise cancelling and directional models.
- Use large ground plane and metal shield.
- The 4.7M / 100 ohm resistors control the amplifier's gain (47,000 currently). You can make the 100 Ohm resistor variable to control the gain.
- The ALC circuit keeps sound level constant.
- Try to use an aluminum MMDS dish antenna as they are the lightest. You may want to cover the corners with rubber tape to keep them from vibrating when bumped.
- Shock mount all the metal parts if possible, especially the handle.
- For a quick 300-5000 Hz bandpass filter, use the 1:1 isolation transformer from Radio Shack, part number 273-1374. Add it right after the current amp.
- LM387 Op-Amp Datasheet (124k PDF)
