Long range RFID detection

I have a question about detecting RFID tags at long range. I understand there are LF, HF,and UHF RFID frequencies.

I'm looking to detect a tag from a minimum distance of 2 meters (actually, 5 meters if possible). I would really like to us HF chips, but I'm not sure if it's possible to detect those from that distance.

I was thinking of using passive tags, but if it's necessary to use an active one I wouldn't be so against it.

Answer

Passive tags are powered from a antenna in the environment. The tag then puts a varying load on the RF field, which the transmitter detects. This varying load is a digital bit stream with the tag's ID, checksum, and sometimes additional information.

This system inherently works at close distances. To get enough power to a tag for it to run 5 meters away would take a large antenna and lots of power being sent out by the antenna. If your installation can support a coil of wire a few meters in diameter, then maybe the size of the antenna is OK in your case.

However, the amount of power it must dump into the near field will be very large. As distances go further, the volume over which the antenna has to provide enough power to run a tag goes up. In fact, it goes up with the cube of the distance. This in itself isn't a issue since this is near field and the energy returns to the antenna each cycle. The problem is that the amount of power the tag can absorb as a fraction of the total the antenna sends out gets smaller. Put another way, for the same power at the tag, the signal to noise ratio at the receiver goes down rapidly with distance. Eventually it becomes impractical, which I think will be well before 5 meters.

Another problem is the RF noise (from the rest of the world's point of view) a large antenna dumping lots of power will cause. No matter how carefully you try to arrange the antenna and objects in its near field, some of the power will escape the near field and propagate outwards. Not only are there legal limits to what you are allowed to radiate, but it can cause problems with your own equipment. There will also be unknown effects on humans in the field. There is still much study and disagreement about how much RF energy a human can safely absorb at various wavelengths without long term effects.

We may be able to suggest alternatives if you explain what you are really trying to accomplish instead of asking about a supposed solution. There are such things as semi-passive RF tags. They contain a battery, but don't transmit until they see a particular RF signature.

There are also fully active tags that contain a battery and transmit occasionally on their own. These are true propagating RF transmissions, so can be picked up a good distance away. I worked on such a system that used 434 MHz carrier. The tags transmitted every 10 seconds, a single 2032 coin cell would last 1-2 years, and they could be received up to 60 feet away in a open environment.