Winding an RFID Antenna

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AndyG
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Posted by AndyG: Wed. Sep 17, 2008 - 03:32 PM
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Hi Folks

We are designing a project at the moment that will need to read 134.2 KHz RFID devices using the TI chipset.

Having got off first base using the evaluation kit, we are now into prototyping and it appears (I may be wrong) that TI do not supply the 80mm disk antenna other than as a part of the evaluation kit.

Furthermore the antennae they DO supply appear to cost from $100 upwards.

So it looks like we may have to wind our own.

We have some instructions from TI that suggest we use 15 turns of 0.2mm enamelled copper wire at a diameter of 75mm. This suggests that we will end up with a 47 microHenry coil, with a Q factor of 17 or 18.

However, if we use any of the inductance calculation formulae that we find by Googling, we cannot get 47uH as an answer. We get an answer around 40uH. (The reason we are doing the calculation is that we are trying to find alternative coil dimensions that are better suited to our application, but have the same inductance, series resistance and Q factor.)

Can anybody explain to a humble software engineer why we may be getting this discrepancy?

Thanks in advance.
Andy G.

If we are not supposed to eat animals, why are they made out of meat?

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Learning and Information, General Electronics
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ossi
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Posted by ossi: Wed. Sep 17, 2008 - 03:35 PM
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And if you measure you will probably get another
answer !

Predictions with good accuracy (better 10%)
for that type of coils are, I think, not easy.
Many formulas are kind of "guesswork"
derived under certain assumptions.

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AndyG
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Posted by AndyG: Wed. Sep 17, 2008 - 03:49 PM
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Thanks, Ossi.

We have multimeters and oscilloscopes. Could you suggest HOW we could actually measure the inductance?

Sorry, but this is not even my area of knowledge, never mind expertise.

Andy

If we are not supposed to eat animals, why are they made out of meat?

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ossi
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Posted by ossi: Wed. Sep 17, 2008 - 03:51 PM
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Do you have a frequency counter and a signal
generator at hand ?

If not, wind the coil, attach it to
the RFID reader and adjust by using
capacitors in the resonant circuit
for maximum amplitude or best
detection distance.

Or built this one :

http://www.aplomb.nl/TechStuff/E...

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ka7ehk
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Posted by ka7ehk: Wed. Sep 17, 2008 - 04:52 PM
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One of the problems with inductance calculation is that the coil is generally assumed to take the shape of a solenoid that is longer than its diameter. Accuracy drops when the "length" is short compared to diameter. That is why you have the discrepancy.

As for how to solve the problem, ossi has some good suggestions. But, certainly, if you are in an Engineering College, you SHOULD be able to find something to measure inductance. A Network Analyzer perhaps. Even a Function Generator with a resistor to measure voltage amplitudes and phases to determine reactance and, thus, inductance. Or same with a capacitor to determine resonant frequency. For a one-off project like this, building an inductance meter seems a bit much!

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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ossi
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Posted by ossi: Wed. Sep 17, 2008 - 05:11 PM
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You may use an AVR to generate short pulses
(1 us or so) with long pauses (1ms or so).
With these pulses "trigger" an LC series
circuit using your coil and a well known capacitor.

Capacitor in the range to have the resonance
near your working point.

Measure voltage across capacitor to determine resonance
frequency of LC circuit making damped oscillations.
From known f and C compute L
(don't forget 2*Pi in Thomson formula).

Simple, not very accurate.

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ford2go
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Posted by ford2go: Thu. Sep 18, 2008 - 06:51 AM
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Might do better to make a parallel LC circuit and drive it through a transistor for isolation. Use a simple common emitter circuit with probably a 10 K from the drive source to the base and a 1 k in the collector. Pulse duration and frequency are probably OK. Actually, you can use a considerably shorter rep time than one ms. 135KHz has a cycle time of about 8 us, and the tank won't ring very long.

If it doesn't produce enough signal, you could make the pulse width a little wider.

And, despite the forum, you can probably make what you need with a 555 timer. Better yet, you may have a pulse generator laying around.

Measure carefully, and your results should be decent.
(1/(2*pi*L*C)

For extra credit, you can measure the decay rate of the wafeform ( it will be a damped sinusoid). Do some searching, and you should come up with a formula to determine the Q -- most of which will be related to the coil,

You could also do some further digging and just use your coil and a resistor. Drive it with a sine wave
of some reasonable frequency and measure the voltage and phases across the either the coil or the resistor -- whichever one is connected to ground. Should be able to find the calculations in a basic ac circuit text.

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AndyG
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Posted by AndyG: Thu. Sep 18, 2008 - 09:14 AM
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Thanks, Folks.

It sounds like a project that is going to get handed over to the hardware guy when he gets home from the States. :)

@Jim: You think I am still in engineering college! As a 50 year old, that's nearly as flattering as getting "ID'd" when I go to buy beer. :D

Cheers,
A

If we are not supposed to eat animals, why are they made out of meat?

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ford2go
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Posted by ford2go: Fri. Sep 19, 2008 - 10:12 PM
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Well, good luck to you. Incidentally, if you do want to try something sooner, I realized that I was over complicating everythig in my last post. (Senior moment, or possibly senior year!)

Anyway, you could just use your LC network and hook it up to an adjustable sine wave generator through a resistor.

Adjust the frequency and measure the coltage across the tank. Voltage will peak at resonance, and you can calculate the L. Also, the points 3db down will show the bandwidth -- which also can be used to calcualte the Q.

Otherwise, let the hardware guy do it!

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jayjay1974
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Posted by jayjay1974: Fri. Sep 19, 2008 - 10:16 PM
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Is the actual L really important? Isn't it more important that the resonance peak occurs at 134.2 KHz?

By adjusting the number of turns while watching the tank voltage you should be able to center the antenna to this frequency.

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icarus1
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Posted by icarus1: Sat. Sep 20, 2008 - 06:27 PM
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As i said in my 2 days previous post I am trying to finish a project using the EM Marine chipset at 125KHz. The antenna that I use is designed on my 2 sides pcb's top and bottom. The diameter of the antenna is ~65 - 75 mm, 30 turns, 0.2mm thicness and 0.2mm gap between two turns. The L = 104uH with a 16 Q.

The reading distance results are perfect (~13cm), by having the peak antenna current at 150mA.

Michael.

Michael.

User of:
IAR Embedded Workbench C/C++ Compiler
Altium Designer

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AndyG
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Posted by AndyG: Thu. Oct 2, 2008 - 02:55 PM
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As an addendum to the above thread...

Having found a TI supplier who is both helpful and knowledgeable (RFID Components Ltd. in the UK), they proposed buying a $1 47 uH coil from RS and putting a 1 Ohm 1/4 W resistor in series.

Works like a storm. Problem solved. :)

A.

If we are not supposed to eat animals, why are they made out of meat?

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