> Eugen Leitl[SMTP:[EMAIL PROTECTED]]
>
>
> On Wed, 10 Apr 2002, Tim May wrote:
>
> > How come? Because I am assuming the transponders are in the same
> > position on each bill. If you want to posit some "spatial diversity"
> > model, that helps, but not but a huge amount. This sounds too science
> > fictionish to actually deploy (transponders are not the same as
> > letters, and cannot be moved around on a random basis).
>
> The RFIDs are small enough to be able to positioned randomly within the
> fabric of the bill, but it still doesn't help if they yell in unison
> (something much less problematic with ultrawideband, admittedly). Phased
> array is sure nice, but homing in on a cloud of weak emitters with cm
> spatial jitter doesn't sound viable. Doorway antennas should be able to
> pickup a RFID-tagged cash signature relatively easily, though, though it
> won't be able to pick up the ID of each individual bill in a stack, unless
>
> you're willing to stand in that (shielded) doorway for a long time.
>
[...]
[1. Agreed, this thread has lost steam.
2. It always amazes me how often people on this list will handwave and
speculate on subjects which a few minutes with Google will settle. Too
often, we're like the medieval academics who debated endlessly over how
many teeth a horse had, while refusing to go down to the stable to
conduct an actual count. ]
I refer you to the TI RFID FAQ, at
http://www.ti.com/tiris/docs/customerService/faq.htm
to quote:-----------------
4) What is the minimum separation between Tags?
There is no minimum separation between tags. Using the SID
(anti-collision algorithm), multiple tags close to the antenna can be
readily identified but if they are at the extreme reading range, they will
require some separation (5 cm) to prevent mutual de-tuning. If
individual tags are passing an antenna, some separation will need to
be maintained if the anti-collision algorithm is not being used. The
separation is related to the size of the reading zone and should be
sufficient that only one tag is in the field at one time.
5) How many Tags can be identified?
Using anti-collision techniques, there is theoretically no limit to the
number of tags that could be inventoried at one time. Although, in the
real world this is dependant on a number of factors:
1. The size of the reader's antenna
2. RF Power
3. How closely packed the tags are
4. How long you are prepared to wait for the data
It requires a strong RF field for simultaneous identification, as tightly
packed tags can 'soak up' the power and possibly de-tune each other
if they are too close together. Smaller antennas have stronger RF
fields close to the antenna, whereas a larger antenna, capable of
reading more tags, and may have a less dense RF field.
Using simultaneous identification, around 30 tags/ sec can be
inventoried. If you wish to read extra data, the time required will
increase, e.g. to inventory 20 tags and return one page of information
from each, takes 1 second.
Without simultaneous ID, up to 60 individual tags can be read in one
second.
-end of quote -----------
> Righto. By the time EU will be tagging banknotes (and only bigger ones)
> I'm very ready to crush and/or nuke the RFID tags in the microwave.
>
... and run into trouble from salesdroids who won't accept them.
[...]
Peter Trei
