RFID reader antenna with 2 series capacitors - what do they stand for?

Question

I have found a RFID antenna "ready to use with enclosure". However, it has a coil in series with 2 3.3nf 50V capacitors.

It doesn't work with my cheap RFID reader circuit (HZ-1050); it only works if I remove the capacitors.

^{simulate this circuit – Schematic created using CircuitLab}

I have three questions:

1. What is the purpose of these capacitors?
2. Why doesn't this antenna work with my reader?
3. Can I make it work? How?

Answer 1

If I look carefully at the photos of the HZ-1050 it appears to have at least one 3.3 nF capacitor already. Also I see that the antenna that is used does not have any capacitors. Maybe your antenna coil has a higher inductance than most antennas and it needs 2 3.3 nF caps in series. If you can try shorting one of the caps on the antenna and see if that helps. If not, try shorting both. You could also short the capacitor on the PCB but make sure there is at least one capacitor in series with the antenna coil at all times. It does not matter if the capacitor is on the PCB or in the antenna as long as there is a cap.

1) The caps make a series resonant circuit in combination with the antenna coil. It seems they needed 1.65 nF which is two 3.3nF in series.

2) There's also a cap on the board and now there are too many caps making the system oscillate at the wrong (too high probably) frequency.

3) see above

Answer 2

1. The most obvious answer is to tune the antenna to:

$$ f = \frac{1}{2 \pi\text{ } \sqrt{LC}} $$

2. Possibly because it's tuned to the wrong frequency or because it's supposed to look like an inductor and shouldn't be tuned at all.

3. You've found the answer to that one already; just remove the capacitors.