Tuesday, November 22, 2011

Experimental Parabolic Microphone

I've always wondered how well a parabolic mic works. Here is my experimental setup for testing such DIY parabolic microphone. It is a great weekend project and will let me experiment with high-gain / low-noise audio amplifiers. The heavily wooded area off my back yard is plentiful of singing birds.

Photographic tripod is used as mount for the parabolic dish. I had to construct a simple mounting bracket and a "focusing" contraption that will let me use different size and shape microphone elements.

I got the actual parabolic reflector from "the place where you can find anything" - eBay. The parabolic reflector is made out of polyethylene plastic. The diameter is 21 inches with focus point 4 inches from the bottom. It came as one solid reflector - I had to drill the mounting hole. I had a few ideas for mounting the dish - I wanted to be compact and simple so I decided on a single hole in the center.

The microphone mounting frame is made from semi-rigid coax (RG-402) and small PCB board for attaching the mic element. I used a threaded cable feed-thru to both - attach the dish to the bracket and mount the RG-402 frame with the microphone. A "sandwich" of metal and rubber washers - including two large and thick rubber washers provides "shock-mount" for the dish. The mic frame is fed through the threaded feed-thru using silicone cemented inserts.

The "focusing" rig lets me move the microphone element to the exact focus point of the dish. The focusing range is about 2 inches. Normally, the mic can be fixed in the focus of the parabola, but I am planing to experiment with different mic elements and they all vary in size and shape so I wanted to be able to adjust the mic frame with no hassle. Two spring-tensioned wing nuts fine-tune the mic frame.

Small plastic container is holding the battery pack, microphone preamp and 900 MHz FM transmitter (a.k.a Baby Monitor - For the initial testing and to validate the concept I just modified a baby monitor and then used my IC-R20 scanner to listen and record). Currently, I am working on my low-noise/high-gain preamp. The radio-channel link sort of works but the noise levels are way too high. Proper mic pre-amp and better (broadcast grade) FM transmitter are planned for the next stage.

Hand-held mode. Large plastic handle salvaged from old angle-cutter provides comfortable grip.
A word of warning - the surface reflectivity of the polyethylene dish is high enough to produce smoke from the mic's wind guard while I was playing with the dish and decided to verify the focus by pointing it at the sun. It took less than a second! I was able to act quickly and saved the guard from catching on fire :) (stupid move but the damn thing looks transparent :-)