Nuts and Volts - December 2008

for a 10-32 screw that holds the board to the coupling nut at the center of the reflector. The smaller hole is for 6-32 threaded rod. Since the parabolic dish we used has a three inch focal distance, we used a three inch 6-32 screw instead of threaded rod.

Adjusting Microphone Position

If you know the focal length of your dish, then use a ruler to position the mic at the focal point. If you don’t know the focal length, there are two empirical ways to find it.

If your dish is shiny and reflects light, expose it to a bright light and use a piece of paper to find the focal point. Move the paper towards the bottom of the dish and find the point where the spot of light on the paper reaches its smallest diameter. That’s the focal point. (If you use sun light, be careful not to burn a hole in the paper.)

If you can’t use light, use sound. You’ll need something that generates a steady audio tone at a frequency around 1 kHz. Point the dish towards the sound and adjust the position of the mic while you listen to the output of the amplifier. The sound will be loudest at the focal point.

Headphones

Inexpensive headphones usually have ear pieces that sit on the ears letting some sound leak out. You can get feedback if the mic picks up the leaked sound. Better headphones have ear pieces that fit over the ears and don’t leak sound. However, better headphones often use a 1/4 inch plug. You can get a 1/4 inch to 1/8 inch adapter at RadioShack (part number 274-875). Since 3. 5 mm is slightly larger than 1/8 inch, the adapter will fit. Or, if you plan to use only those headphones, you can replace the 3. 5 mm jack with a 1/4 inch jack.