longe range readers and tags

PS, definitely read through this post:

http://www.rfidtoys.net/forum/forum_posts.asp?TID=26

Basically, with the two readers you have, you can point antennas in opposite directions and be able to compare RSSI signals and see if a tag is on the left or right, and approximately how far away it is on the left or right based on the signal strength. If you had four antennas, you could theoretically be able to tell if a tag was on the left, right, in front, or behind you. This assumes you have a stationary set up and tags that move around in free space.

If you have a fixed area like a room, and you feel you can get highly fine grain/sensitive and accurate RSSI readings, you could technically set up your two antennas to get X and Y coordinates. You could use three antennas to get an approximate 3D location within the fixed space. Check out this post for some ideas on tracking tags within a fixed space.

amal wrote: " I think you’re on the right track… but with one reader and one antenna, at best you’ll only get a rough guess as to the distance a tag is from the reader… and that’s going to be in a controlled area like an open gym or something. As soon as you put things like walls or other objects that might cause interference, then the RSSI value will change depending on distance AND obstacles between tag and reader antenna, which means forming a linear relationship between RSSI signal strength readings and physical distance very difficult.   Hope that makes sense.   Amal"

Amal, Thanks a lot for all your help... I really appreciate it.
so you think I should use more than one reader to get an accurate location?....
This is what I bought: 2 small 2.4Ghz readers.. each one will have a 2.4Ghz antenna... and 2 active tags of 2.4GHz
Please advise.. Thanks.

I just got to reading your other message about this project. I hope you don't mind that I post it and my response up here:

gandalfblack wrote: Amal, I bought your book because I want to create a project based on the RFID technology...The book is great... However, I noticed that you don't have a lot of info about long range active tags/readers.... I've been trying to do some research on it, and I noticed that it;s pretty hard to find vendors of active rfids... many of them does not even show the prices of the products... I've been also trying to find IC/chips... but I noticed that most of them are alredy encapsulated..so it's not possible to see how they did it... I really need some help on this.. here is what I;m trying to do... I'm trying to implement a RFID based tracking/locate solution as a student project. The main objective of the project is to locate/track objects. is something similar to the loc8tor.... It would be great if you could give me some information on where I can find long range RFID kits with a well documented SDK for using in my project. for what I read in your book it seems that the rfids I need are the active ones... a) what frequency should I use? b) This sites seem to have what I need.. but I'm not sure if I should buy any of those.. I don't want to buy something that it's not going to work for my project.. specially that all these readers/tags are very expensive.. This are the sites I found: www.savi.com www.rfcode.com www.ela.fr www.synometrix.com Any help you can provide me I would really appreciate it. Thanks. James..

Hi James, I’ve been plowing through emails ever since I got my office put back together, so please forgive me for the late reply.

To answer your questions:

a) I don't think frequency will really matter when dealing with active tags. It's not as big of an issue as it is with passive tags.

b) I'm pretty sure the loc8tor product is an impressive hack sitting on top of a basic RFID reader, but it’s a difficult hack with patented intellectual property attached to it I'm sure. The point here is that creating something like this will take knowledge in electrical engineering and probably some knowledge of advanced signal processing techniques.

That being said, if you want to keep this project low cost and the results don't have to be accurate enough to aim missiles with, I'd look for a cheap active system that returns RSSI (returned signal strength indicator) data for the tags being read. You're not going to find a cheap active system that will be designed to do this kind of object location... so I'd say your best bet is to lead the hunt for a reader and tags by price alone (as long as they return RSSI). There are systems like rfid-radar and even "active rfid" location finding systems based on simple wifi network access points, but they are both very spendy.

You can do a basic range-finder (distance from reader, but no direction) by using the RSSI of each tag and doing a rough guess as to the distance based on signal strength. There are lots of posts about RSSI and the issues surrounding them on the forum: http://rfidtoys.net/forum/search.asp?KW=RSSI&SM=1&SI =PT&FM=0&OB=1

Another thing you could do is mix RSSI with several directional antennas either attached to an antenna switcher (like the one in the book) or you could use multiple readers. That would give you basic distance and direction by comparing RSSI levels coming in from the different antennas.

I'm trying to implement a RFID based tracking/locate solution as a student project. It would be great if you could give me some information on where I can find long range RFID kits with a well documented SDK for using in my project. Any help you can provide me I would really appreciate it.

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