# RFID coil antenna design question

Discussion in 'Electronic Basics' started by Ben, Oct 23, 2003.

1. ### BenGuest

Hi all,

I am going to use some RFID (radio frequency identification) tags to
track some birds as they move around our aviary. If you don't know how
these work, basically the idea is that you have a coil antenna which
creates a field which is somehow modulated in a unique manner by any tag
which might come into its range, in a way which allows the antenna
driver circuitry to identify the tag.

You can only put small tags onto birds, which means read range can be a
problem. Most easily available antenna are loop shaped, and the area
where the tag can be read is mostly inside the loop. This works well
with animals if you can get the animal to pass inside the loop, but I
would like to try something different and I wonder if anyone can offer
me some advice. I want to build a perch antenna, but I'm not certain how
to go about it. Even if you don't know much about how RFID works, but
you know the shape of a coil determines its field you might be able to
help me because I know nothing!

Assume a perch would be a cylinder around 10cm long and 4cm diameter. I
have two ideas. One would be to just wind the coil around the cylinder
with each turn going around the cylinder like a solenoid or something.
But I have no idea if that will produce much field outside the coil? My
other idea is to wind it with each turn going around the whole perch
from end to end, to produce a coil shaped like a very flattened
rectangle. Would either of these ideas work? By the way I do know how to
use a formula to tell me how many turns I need to tune the antenna to
the right frequency. I need 1600 LuH so for the first idea around 366
turns.

Thanks very much for any advice.

Cheers,

Ben Kenward

2. ### TerryGuest

I vaguely recall; Inductance (L henries etc.) to be something
about 4pi x Number of turns (squared) divided by the inductive
constant of the core (air being 1) and the area within the coil
or summat?
So with that in mind;
Could you wind a big coil/s around the whole cage, or a
section/zone of it? Thereby identifying bird is in a certain
'Zone' at a certain point in time etc.? Idea.
Regarding winding around the perch; I guess that would give you a
field with greatest intensity at right angles to the perch? With
least intensity off the ends of the coil/perch, just like a ferro
resonant antenna in a portable radio etc., which can be used for
direction finding the signal being strongest when coming in from
broadside to the coil antenna? What frequency are you using?
This make any sense?
Terry.
PS. Four (4) cm. diameter perch! That's a big bird? I guess you
are not looking at budgerigars?

3. ### BenGuest

Crossposting this now to sci.electronics.design to see if anyone there

Thanks:

Yeah, I could do that. Although, to get a system like that working you
either need a more powerful reader than I have, or bigger tags than you
can put on these birds.
The frequency is 125kHz.
Yeah, kind of. So you are saying with winding the coil with turns going
round the perch, the strongest field would be broadside to it. That's
exactly what I need for the tags to be read when the bird lands on the
perch. Looks like I should give this method a try.
Well, they're bigger than budgies, but not huge... About the size of a
pigeon. Here's our research page:

http://users.ox.ac.uk/~kgroup/tools/tools_main.html

Ben

4. ### John PopelishGuest

Ben wrote:
(snip)
The strongest coupling will happen when the two coils are on the same
axis, with the small one inside the larger one. It may be helpful to
extract the pickup coil from an RFID tag, and connect it to an
oscilloscope, while waving it around a few different energized perch
designs, to make sure your assumptions are real.

5. ### Wim TonGuest

The strongest coupling will happen when the two coils are on the same
Temic (ex Telefunken) has an application note about antenna design for their