user1711: Why is a toroidal inductor being used in the RFID reader described in DIY FSK RFID Reader?
I feel that a toroidal inductor wouldn't be very effective in grasping the changes in electromagnetic field changes.

user3219: Yes, Chris Stratton is right. A toroidal inductor keeps flux within the core, reduces the external flux to near zero and have near zero ability to grasp/convert external flux/magnetic field into electrical signal. Apparently, the word 'toroidal' was used in loose sense referring to a circularly shaped coil.
In an real-life commercial product, a 125 kHz coil typically has many turns on a very thin wire on an open-air core (no core).
An example profession factory made a 125 kHz RFID with a GitHub Arduino library and photos of an antenna coil:

125 kHz RFID module - UART
Seeed-Studio/RFID_Library
125Khz RFID module - UART (wiki)

Also, see Building RFID Card Reader using PIC Microcontroller.
Note, for higher frequency RFID, not shown in above examples, coils have fewer turns than for 125 kHz.

1711: <p>Why is a toroidal inductor being used in the RFID reader described in <em><a href="http://playground.arduino.cc/Main/DIYRFIDReader" rel="nofollow">DIY FSK RFID Reader</a></em>?</p>

<p>I feel that a toroidal inductor wouldn't be very effective in grasping the changes in electromagnetic field changes.</p>
 3219: <p>Yes, Chris Stratton is right. A toroidal inductor keeps flux within the core, reduces the external flux to near zero and have near zero ability to grasp/convert external flux/magnetic field into electrical signal. Apparently, the word 'toroidal' was used in loose sense referring to a circularly shaped coil.</p>

<p>In an real-life commercial product, a 125&nbsp;kHz coil typically has many turns on a very thin wire on an open-air core (no core).</p>

<p>An example profession factory made a 125&nbsp;kHz RFID with a <a href="http://en.wikipedia.org/wiki/GitHub" rel="nofollow">GitHub</a> Arduino library and photos of an antenna coil:</p>

<ul>
<li><p><em><a href="http://www.seeedstudio.com/depot/125Khz-RFID-module-UART-p-171.html" rel="nofollow">125 kHz RFID module - UART</a></em></p></li>
<li><p><em><a href="https://github.com/Seeed-Studio/RFID_Library" rel="nofollow">Seeed-Studio/RFID_Library</a></em></p></li>
<li><p><em><a href="http://www.seeedstudio.com/wiki/index.php?title=125Khz_RFID_module_-_UART" rel="nofollow">125Khz RFID module - UART</a></em> (wiki)</p></li>
</ul>

<p>Also, see <em><a href="http://www.8051projects.net/news-i140-5-building-rfid-card-reader-using-pic-microcontroller.html" rel="nofollow">Building RFID Card Reader using PIC Microcontroller</a></em>.</p>

<p>Note, for higher frequency RFID, not shown in above examples, coils have fewer turns than for 125&nbsp;kHz.</p>
