interesting rescue reflector

[coppercone2]

I found this interesting. https://recco.com/shop-attachable-recco-reflector-scandinavia/

[ataradov]

This is interesting. The claim is that "The handheld detector can locate RECCO reflectors within a range up to 80 meters through air and 20 meters through packed snow".

It would be great to see someone with proper equipment try to go into technical details. Passive detector doing 20 meters though the snow seems crazy.

[Emo]

Hi These devices probably are based on the avalanche transceivers operating on 457Khz. They will need an energy puls on some frequency in order to obtain the range mentioned. The Pieps brand transceivers do have 40-80 m range.

Eric

[berke]

This is interesting. The claim is that "The handheld detector can locate RECCO reflectors within a range up to 80 meters through air and 20 meters through packed snow".

It would be great to see someone with proper equipment try to go into technical details. Passive detector doing 20 meters though the snow seems crazy.

The better 58kHz store alarm (EAS) systems can detect tags at 4-5 m range to allow concealed installation at higher-end stores for example, and they have to respond very quickly with a low false alarm rate because you can't expect customers to stay still for many seconds while the detector gets a reading, and you don't want to unnecessarily bother a customer who just bought a $5k handkerchief.  The tags are just passive high-Q resonators, some are coiled, some simple strips of magnetostrictive material.

Unlike shoplifters, avalanche victims can be expected to stay still, so you can realistically extend the detection time by one or maybe two orders of magnitude.

It might be impressive if they have miniaturized the reader and/or if it has a fast response time.  Otherwise shouldn't be too hard if you allow yourself ginormous coils and a read time of say 1 minute.

[ataradov]

But still there is a huge jump from 5 m in open air to 20 meters in a packed snow. I have no doubt that it works, I'd just like to see more details on the tech.

[fourfathom]

The [store security] tags are just passive high-Q resonators, some are coiled, some simple strips of magnetostrictive material.

Is this a different type than those I've seen?  I've taken a few apart for fun, and they appear to have a diode across a foil dipole structure, where the stimulus gets somewhat rectified by the diode and the resulting third-harmonics are radiated by the dipole.  There may be filter or other resonant structures along with the antenna.

I would think it would take a pretty strong signal to cause diode rectification at a distance, even with a low-threshold device...

[berke]

The [store security] tags are just passive high-Q resonators, some are coiled, some simple strips of magnetostrictive material.

Is this a different type than those I've seen?  I've taken a few apart for fun, and they appear to have a diode across a foil dipole structure, where the stimulus gets somewhat rectified by the diode and the resulting third-harmonics are radiated by the dipole.  There may be filter or other resonant structures along with the antenna.

I would think it would take a pretty strong signal to cause diode rectification at a distance, even with a low-threshold device...

Yes those are different, the ones you are talking about are the UHF kind, which work like you say although I haven't seen any myself.  Most common "dumb" alarm systems (exclusing RFID) are the 58 kHz "acousto-magnetic" ones (Sensormatic et al., usually small white rectangular plastic boxes with metal strips in them) and the 8.2 MHz resonant "RF" ones (the stickers with the spiral foil pattern).  The older, magnetic strip-only systems aren't used anymore but I've seen them in libraries.

[MarkT]

Passive detector doing 20 meters though the snow seems crazy.

Snow is mostly air and ice, which are both dielectrics and support RF propagation, for instance ice-penetrating radar is used to map the bedrock of Antarctica...