Improving antenna for 13.56 RFID reader/writer programmer

Hi I realized a reader/writer RFID programmer with a microcontroller 18F with usb that allows me to read and write RFID tags, the programmer works fine but the antenna is very weak ... my project uses a component of ST and I have considered as an example of the simplest antenna pattern, which uses a capacitor of 100pF in parallel and the signal is directed to the antenna, how can I fix this? what can I change the antenna?

Did you review the ST application notes for the chip, available on the CRX14 product page? They calculate a maximum operation distance of 4.5 cm for the reference design. For larger operation distance, they suggest a one transistor PA and more sophisticated matching circuit. I didn't check, if the simple matching circuit of reference design actually achieves maximum performance available without an additional power stage, but most likely, you can't get much more. But without the series capacitor needed for impedance transformation, it can't work.

I used a capacitor in series with the antenna and then usually 10pF capacitor as in the scheme but the problem is that the tag is not read even to 1cm away, maybe something is wrong in the calculation of the antenna? as an antenna must be to be effective? How many turns?

Hi,
You must have/calculate a resonant frequency at 13.56MHz...
K.

How do I calculate? I did not understand much from pdf ... you know give me some specific information?

Here's a description that might help you:
First, the RFID "antenna" loop is EM simulated to find out the loop inductance. Next, the capacitor value for resonance at 13.56MHz is calculated.
http://www.sonnetsoftware.com/suppor...ign%200602.pdf
Files available here:
http://www.sonnetsoftware.com/resources/

I downloaded the FREE version of the program but is very difficult to use ... so I decided to try a few examples already.I calculated the exact measurements of the antenna are in the picture, the antenna is good with those values calculated?

Apart what you have possibly calculated for your antenna, you should check the actual resonant frequency and tune the circuit if needed. This should be more easy than fighting with confusing antenna calculations... And even if you're sure about your calculation, why not simply test if the parts are O.K.
P.S.: In my opinion, a 2.6 cm loop isn't reasonable for more than a few cm distance operation, even if perfectly tuned, otherwise, the field strength directly at the coil would be too high. You can review the coil dimensions of commercial readers related to the specified operation distance to learn about suitable reader coil design. May be different, if you're targetting to a restricted "operation volume" with a minimum distance to the coil.

What are the dimensions and number of turns? If you can provide these numbers, I can easily calculate the inductance for you, so that we can check the resonance frequency with your capacitors.
Another resource for 13.56MHz RFID antenna design and calculation would be this ST appplication note:
http://eu.st.com/stonline/products/l...e/an/15284.pdf
Online inductance calculator to evaluate the equations given in the app note:
http://smirc.stanford.edu/spiralCalc.html

the number of turns completed is 2

Hi,
Reader antennas have usually 5-6 turns & ca. check card the measures(50x80mm outside), but similar turns are typical at miniature versions (1/3t, 1/4th of 5x8cm)too...
Your problem will be the readibility/work distance _ its in allday business similar as the diameter of the reader coil...
K.

I read the tag has a size of 1.5X1.5cm, so what do you advise me to do?

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