Making a directional receiver

jtaylor408

I am trying to make the RF receiver (433 MHz) directional. My goal is to place it on a board attached to a servo and the servo rotates. Basically it is sweeping out an area looking for a transmission signal.. I want it to stop rotating once it finds the signal. Any ideas on how to make it directional, as in it receives from a small angle at a time in stead of in all directions.

Jonathan

Welcome to the whacky world of antennas. Part science, part art. Look up Yagi antennas. You can also use loop antenna for direction finding. Loops have a sharp null perpendicular to the signal. In this case you would actually be looking for where the signal disappears. Do a search for directional antennas.

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Phil Pilgrim (PhiPi)

Here is an article about constructing your own yagi antennas cheaply. I built one once from PVC pipe and welding rod, and it worked as advertised. However, if I were going to embark on a project like yours, I'd use 900 Mhz instead of 433 Mhz, since the antenna will be less than half the size at the higher frequency.

-Phil

Addendum: I found that antenna. Attached is a photo.

Post Edited (Phil Pilgrim (PhiPi)) : 2/27/2009 7:17:45 AM GMT

Beau Schwabe

jtaylor408,
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As Jonathan·mentions this is mostly going to be a phenomenon related to the structure of your antenna.· There are a couple of methods that mechanically work without much in the way of sophisticated timing, electronics, etc.. so·technically a Basic Stamp·could do what you want.· However there are·a few other things·to consider.· A coil antenna works because·if the "open end" of the coil is perpendicular to the·source of the radiated signal, the signal through the coil will cancel out resulting in a substantial dip·in signal strength.· This is because the signal reaches "both" open ends of the coil at the same time and electrically cancels out the incoming signal to the receiver.· Other methods include two identical antennas that combine their signal in the exact center.· The result of this combining or mixing produces similar results to the coil method, except the "open end" or area between the two antennas will face the signal source.· There are other methods to detect the direction of a signal, but they mostly all rely on a differential mixing approach, where the "difficult" part·is taken care of mechanically rather than electronically.· In terms of electronics for practically ALL methods, a good tuner capable of detecting the signal strength is all that is necessary.· In other words a "tuned" LC tank to your desired frequency with a simple filter to an ADC (Analog to Digital Converter) chip is enough.
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Beau Schwabe

IC Layout Engineer
Parallax, Inc.