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Would it be possible to make a small device which can be easily detected with a metal detector (or similar) from several feet away?

It would need to operate kind of like a shop-security tag but it would need to be much smaller. The idea we have would require the device to fit in a 5mm diameter hollow straw/tube. It could be several inches long (ie could easily have a long antenna) but can't exceed ~5mm in the other two dimensions and it must be very light.

I'm not a physicist and certainly don't know much about radio, but could a device with a monopole or dipole antenna be set up to resonate at a frequency close to what the metal detector scans with (approx 100Khz) and perhaps cause the detector to react in a way which is out of proportion to the actual amount of metal used to construct the antenna?

No data exchange is needed (unlike real RFID tags), nor any way to disable the device (as in a security tag) - just the ability to detect it from as far away as possible without the device requiring any power source of it's own. Any ideas would enable me to make a prototype or even just test the concept would be appreciated. I've tried to find pre-existing devices which would work like this, but they all seem too complicated (active RFID) or devices are too big in at least one dimension to fit in to the tube which is the case with security tags and anything which requires a coil type inductor/aerial.

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If the metal detector scans at 100 Khz, that's a wavelength of 3 kilometers. You're going to have a tough time getting any reasonable sized object to resonate for that frequency. The ski tag mentioned below is designed to resonate at 915 MHz, for comparison. –  user1631 Jun 21 '12 at 18:15

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Generally those sort of passive rectifying tags need antenna which are long in one dimension. They have to a be a reasonable fraction of a wavelength - and you need relatively long wavelengths for long range at reasonable power.

For example the Recco tags for ski clothing would work and are about 2" long. One issue may be the angle of them relative to your search - you need the antennae to be perpendicular to your detector for a strong signal.

I don't know their exact range but they do helicopter based searches for skiers buried under snow so a few 100m in air should be possible

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And of course an advantage of an off-the-shelf standard is you can easily get a tag to test it. If you have a ski resort or a mountain rescue unit near you they will have a Recco detector you migth be able to try –  Martin Beckett Jun 21 '12 at 16:24

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