"The Adafruit shield was designed by RF engineers using the best test equipment to create a layout and antenna with 10cm (4 inch) range, the maximum range possible using the 13.56MHz technology."
Where is it said that the 13.56MHz technology max range is 10cm (4 inch)?
What would need to happen to this shield that the distance would be longer? Like 20cm or 30 cm?
To quote the page indicated: (quote) NFC (Near Field Communications) is a way for two devices very close to each other to communicate. Sort of like a very short range bluetooth that doesn't require authentication. It is an extension of RFID, so anything you can do with RFID you can do with NFC.(end quote).
This device uses the magnetic field of the RF signal to communicate, not the electric field. Think of two transformer windings places adjacent to each other. The powered one induces current into the other magnetically. What you see on the board is not an "antenna", but one winding of a transformer.
The only way to increase the effective distance is to increase the power. To double the distance, you need to X4 the power. To double that distance, you need to X4, that power, again. You see the problem.
With the added complication that the RFID device is powered by, as well as communicated with by the signal from the Tx/Rx coil. So as the RFID gets further away, it has less power to transmit back with, so it probably falls off even quicker than inverse square. I was hoping to track the movement of our cats through a "cat flap" but could not position the coil in an optimum place for the sensor on the cat's collar to communicate with the coil. I was thinking of winding a coil that goes around the cat flap hole, so at least the RFID device would have to pass through it, but how to get the coil windings right so it still has the right resonant frequenscy is a bit befond me.
Cheers, R
robwlakes:
With the added complication that the RFID device is powered by, as well as communicated with by the signal from the Tx/Rx coil. So as the RFID gets further away, it has less power to transmit back with, so it probably falls off even quicker than inverse square. I was hoping to track the movement of our cats through a "cat flap" but could not position the coil in an optimum place for the sensor on the cat's collar to communicate with the coil. I was thinking of winding a coil that goes around the cat flap hole, so at least the RFID device would have to pass through it, but how to get the coil windings right so it still has the right resonant frequenscy is a bit befond me.
Cheers, R
Find what is called a "grid dip meter". It is a variable frequency oscillator that uses plug-in coils to cover a large frequency range. A meter on the devices will show a "dip" when the dip meter is tuned across the frequency of the other coil. Set the meter to 13.56MHz and adjust the coil on the cat door to get the meter to dip. You may need to add a capacitor to the cat coil or possible add/remove turns of wire.
You can use the dip meter to determine if the cat coil is resonant above or below 13.56MHz .