I'm attempting to build a system using an RFID reader and tag. I need about 2-3 feet so I'm planning on using an active tag. Here is my first problem. All active tags seem to have ranges in excess of 100 feet and I can't go passive due to power limitations and antenna size restrictions. Is there an active tag out there designed for low range. Also is there a specific antenna I have to use for an active chip or will the parallax antenna in this link work fine? http://www.trossenrobotics.com/store/p/5378-Parallax-RFID-Card-Reader-Serial.aspx.
I'm obviously new to RFID so any help is much appreciated!
Hoi,
AFAIK are all ACTIVE tags for wide range (also AFAIK min. 50 meters).
These Tags are designed for i.e. robot cars,freight parcel etc., means they has to identify the car, parcel, container on a longer range.
Not really 50 or 100 meters needed, but in case if there are i.e. walls between the Tag and the reader, they have these "power".
Doei
Trixi
No special antenna needed with active tags but you will need a special reader, that one won't work with active tags.
Thank you for your replies. Any recommended combinations to achieve 2-3 foot ranges? I don't want the RFID signal to be picked up any further than 3 feet...
The secret of all RFID-Readers is the coil (called antenna).
It must have a specific size and a specific number of turn of the cu-wire coil and the wire a specific diameter.
here some results of self-brewed coils:
I am using on my model railrout this passive reader 
to identify locos and wagons and have tested a long time with size and turns to get the best result - but if you do not have any machine for that the solution could be bad.
Best is, you buy such a coil, its not as expensive as you may thing.
i.e. here:
http://www.google.de/imgres?imgurl=http://i01.i.aliimg.com/img/pb/288/739/275/275739288_893.JPG&imgrefurl=http://german.alibaba.com/product-gs/rfid-coil-356904087.html&h=600&w=800&sz=83&tbnid=SEpF-0m3KCNtJM:&tbnh=90&tbnw=120&zoom=1&usg=__WRiZwWeFWapHjmHvrLL3XoE7Qg8=&docid=Ej9w4-w2gMAPKM&sa=X&ei=loExUs20FYHTtQaC5oGgAg&ved=0CG4Q9QEwBg&dur=1194
Also have a look on the different frequencies of tags and readers - there are different for different solutions.
Doei
Trixi
Trixi:
It must have a specific size and a specific number of turn of the cu-wire coil and the wire a specific diameter.
No .... the thing that matters is the resonant frequency. The large the coil the bigger the range and you will be able to resonate it by adding capacitors. However the coil driver must have the power to drive the coil.
You can get a range of 2 - 3 feet with a large coil and passive 125KHz tags but you will have to build it yourself. Normally RFID applications don't need a long range. When I designed them I used to get about 18" range with a 4" by 2" coil. Aim to get it to resonate with about 10nF across it, and tune it to 5% either side of resonance for the maximum reading distance.
Is this now anything other than i wrote ?
Trixi:
Is this now anything other than i wrote ?
Yes I thought so.
You seemed to imply that there was something very much more specific about the design of a coil than there actually is.
I did not notice any information that you gave as the the resonance either.