RFID performance with small antenna?

[Mike Harrison]

RFID manufacturers seem to be a bit cagey about the operating range of their products - I wonder if
anyone who has direct experience could help....

I'm looking at an application that needs to read a few tens of bytes from a read-only RFID tag.
The choice of tag type and frequency is open at the moment, but needs to be passive, no bigger than
a car-alarm keyfob, reasonably cheap and probably off-the-shelf rather than custom.
It needs to be writeable in production but not necessarily by the same type of reader.

Problem is that we have limitations on the size of reader antenna, and power available.
The antenna needs to fit in a space of about 40 x 40 x 5mm - from what I've seen so far this would
suggest that if we go for a 125KHz tag, a ferrite antenna would be appropriate, but for 13.56MHz, an
open coil may be possible, but may suffer detuning from stray capacitance.
(Are ferrite antennas viable at 13.56MHz ?)
Available power is about 7V minimum, but this will have something like 8 ohms in series with it due
to other requirements. Overall power draw is not a problem as it will only read when commanded to,
which is not very often - the main issue is the series resistance,

So the question is, what sort of range could we expect with the above limitations ?
100mm has been suggested as desirable but I'm skeptical that this is possible from what I've seen
from some 125K tags I've played with.

 

[Rene Tschaggelar]

RFID manufacturers seem to be a bit cagey about the operating range of their products - I wonder if
anyone who has direct experience could help....

I'm looking at an application that needs to read a few tens of bytes from a read-only RFID tag.
The choice of tag type and frequency is open at the moment, but needs to be passive, no bigger than
a car-alarm keyfob, reasonably cheap and probably off-the-shelf rather than custom.
It needs to be writeable in production but not necessarily by the same type of reader.

Problem is that we have limitations on the size of reader antenna, and power available.
The antenna needs to fit in a space of about 40 x 40 x 5mm - from what I've seen so far this would
suggest that if we go for a 125KHz tag, a ferrite antenna would be appropriate, but for 13.56MHz, an
open coil may be possible, but may suffer detuning from stray capacitance.
(Are ferrite antennas viable at 13.56MHz ?)
Available power is about 7V minimum, but this will have something like 8 ohms in series with it due
to other requirements. Overall power draw is not a problem as it will only read when commanded to,
which is not very often - the main issue is the series resistance,

So the question is, what sort of range could we expect with the above limitations ?
100mm has been suggested as desirable but I'm skeptical that this is possible from what I've seen
from some 125K tags I've played with.

My experience with passive 13.56MHz RFID antennas is
such that the sender antenna defines the range. Roughly
the diameter of the sender antenna. So your proposed
antenna will be good for a hand thickness apart.

Note that the power is limited by law, you cannot use
this antenna and drive it with 1kW.

The whole looks different when the tag has a battery,
then the range is greatly enhanced.

Rene

 

[Jim Thompson]

RFID manufacturers seem to be a bit cagey about the operating range of their products - I wonder if
anyone who has direct experience could help....

I'm looking at an application that needs to read a few tens of bytes from a read-only RFID tag.
The choice of tag type and frequency is open at the moment, but needs to be passive, no bigger than
a car-alarm keyfob, reasonably cheap and probably off-the-shelf rather than custom.
It needs to be writeable in production but not necessarily by the same type of reader.

Problem is that we have limitations on the size of reader antenna, and power available.
The antenna needs to fit in a space of about 40 x 40 x 5mm - from what I've seen so far this would
suggest that if we go for a 125KHz tag, a ferrite antenna would be appropriate, but for 13.56MHz, an
open coil may be possible, but may suffer detuning from stray capacitance.
(Are ferrite antennas viable at 13.56MHz ?)
Available power is about 7V minimum, but this will have something like 8 ohms in series with it due
to other requirements. Overall power draw is not a problem as it will only read when commanded to,
which is not very often - the main issue is the series resistance,

So the question is, what sort of range could we expect with the above limitations ?
100mm has been suggested as desirable but I'm skeptical that this is possible from what I've seen
from some 125K tags I've played with.

http://www.savi.com/index.shtml

...Jim Thompson