# Recommendations for an active, directional RFID sensor

I have been using two Loc8tor devices (an active, highly directional RFID consumer device) on a personal robot to try to locate an RFID tag.

While the Loc8tor has served its purpose of showing an active, directional RFID sensor with high resolution can be used for locating an RFID tag, the Loc8tor itself is not very hacker friendly. I am looking for an alternative reader and tag that can be integrated into my robot.

Ideally, it would meet the following specifications:

• Each reader would be less than $100, cheaper is definitely better • The reader should be around the size of a credit card to a dollar bill • The tag will be placed in ranges of 0.5 to 3 meters from the reader • The reader should run on some voltage less than 5v, ideally 3.3v or less • The higher the resolution of the reader, the better. Say 3 cm resolution at a 1 meter distance. • The (RSSI?) value from the sensor is easily accessible. Meaning I could wire that into an Arduino input pin and use the value. • Could buy in small (1 to 2) or large quantities (100+) These specifications are ideal, but not all are required (though the price is the most important). I should hopefully be able to list better specifications after I see what alternatives are out there. • How does the Loc8tor work? – tyblu Jan 10 '11 at 16:04 • The Loc8tor is an active RFID reader running at the 2.45GHz ISM frequency. It comes with two powered RFID tags that you put on an object, like a cat's collar. When you turn the Loc8tor on, it gives you an idea of how close you are to the tag by lighting up LEDs and emitting a sound. When you face the direction of the tag, the sounds and lights get stronger. – baalexander Jan 10 '11 at 19:27 • I mean, how does it know the tag's direction? It must use either a directional antenna or two omni's with either phase or power comparison. Sounds very interesting. – tyblu Jan 11 '11 at 7:30 • I would like to know too, tyblu. I had assumed it was a directional antenna. – baalexander Jan 11 '11 at 15:56 ## 2 Answers What about using the difference between the signals from two antennas? In theory, if two antennas are exactly the same distance from the target, then the two signals should cancel each other out if subtracted. If they are slightly off, then they would be slightly out of phase, and they wouldn't cancel. You could only use this technique to find the angle to the target. You would have to rotate the antennas to actually read the RFID (because their would be no signal when you were pointing directly at it. (This technique is used in Ham Radio circles as a poor man's way to find people who interfere with radio repeaters so they can be reported to the FCC.) • Thank you Eric. Do you have any examples of components/projects - preferably for distances ranging from 0.5 to 3 meters - you could point me in the direction on? – baalexander Jan 12 '11 at 20:30 Let me show some skepticism here - RFID frequencies are so low, that any directional antennas would completely fail (on small robots). I would recommend to do the following: 1 usual RFID reader forcing your tag to reply + 3-4 simple(dumb) manual RF receivers tuned on your RFID frequency. Then you detect the signal on each of receivers, average it with low-pass filter & measure with precision ADC (16-bit should be enough). Then you'll be able to calculate (more-or-less) exact direction to the tag. This will be definitely cheaper than 100$.

• I'm definitely open to other RF solutions. What I'm not understanding from your recommendation is how the simple RF receivers will determine the location. The robot is about 40 cm wide, if I have an RF receiver on each end, would they be sensitive enough to pick up different signal strengths? Do you have examples of RF receivers that could do that? – baalexander Jan 10 '11 at 19:05
• And to be clear, I was able to get the direction from the loc8tor with a set up similar to youtube.com/watch?v=eJyDqn6rQKE. Now, I am trying to use two readers that don't move instead of a servo. – baalexander Jan 10 '11 at 19:18