NiPah said:
Heronblade said:
They do, by their name they are transmitting a radio frequency, the EMF field is just another name for the field of radio waves being sent out passively by the device. These are pocket based RFID cards meaning that they will have a pretty high powered signal to talk with the receivers. Also those "bulky receivers" are about 400$, smaller then a cell phone, and can pick up a signal from about 600 feet away outside or 250 inside. They also have most costly versions that put the signal on a GPS device, and thats just the civilian model.
While they are intimately related, there's a wee bit of a difference between a magnetic field and a radio signal, please learn it.
The tag/receiver combination you are thinking of is the wrong type. RFID tags designed to be detected from more than 1-2 meters require their own internal power supply and emit their own signal rather than "echoing" one, they also don't rely on an external EMF.
The kind placed in identification cards however have no internal power supply, produce no signal at all until powered by an external magnetic field produced by the sensor, and tend to have a maximum effective range of approximately 1.5 meters, at least using standard receiver units. The limitation is mainly in terms of field strength, EMF fields are limited by the inverse square law, meaning they die off very quickly with distance.
You're tying to compare a cellphone sized powered electronic device to an unpowered chip approximately the size of a grain of rice.
A completely passive RFID tag can in theory be triggered from any distance, but in order to do so, you have to put more and more juice into the magnetic field. If 1.5 meters is the normal max, going out to three meters requires four times the energy, Twelve meters requires sixty-four times the original input, etc, etc. By the time the field is strong enough to effect the chip from any real distance, you have another problem aside from a skyrocketing electric bill, you're sitting next to an electromagnet more than strong enough to f*** with your receiver or any other electronics you have on your person. In addition, the triggered signal output by these chips is typically quite weak. I don't have specific data on the actual strength of the return signal, but I do know that line of sight is nearly critical, even from short range. (The human body, clothing, etc. don't reduce a radio signal by enough to be counted.)