13.56MHz RFID tag problem

Hello everybody:
Recently I have designed a RFID Tag antenna on the pcb which work at 13.56MHz, I measure the inductor value of the pcb wire about 400nH 600nH, the capacitance value of the tag ic, it is about 70 90 pF, so I place a parallel capacitance next to the tag ic. Through my testing, I found the 160pF value make the tag work well, the reader can write and read the tag at the 1.5cm distance(because the antenna is very small so the distance is very short, the short distance is I need), when I reduce the distance and place the tag next to the reader slowly, it does't work, the RFID reader can't communicate with the tag, can somebody help me ? Sorry about my english.
The attachment is my pcb design.
Attachment:
Antenna All.pdf

I have no experience at RFID, but it seems your tag IC is overloading. Please check if the rectified voltage is within the manufacturer specs. If too high, try with fewer turns and more capacitance, and check if at 1.5cm distance is still working well. Other possibility: too much coupling distorts the transference between transmitter and receiver, you could be experiencing k*Q much greater than 1 and mistuning. Here, too, less turns would help.

If doesn't work when distance is greater than 1.5cm, and also doesn't work when touching the two antennas, means the dynamic range of the RFID receiver is very poor, which is unusual for an ASK system.
Most probably the antenna circuit gets detuned.

The reader has an antenna too? check out if the polarisation of both are same?match..

Thanks erveybody!
Maybe I agree with Vfone. Yes, the dynamic range of the RFID receiver is pool, but this system is FSK modulation, I think FSK system have more anti-jamming capbility than ASK system. So if the tag's antenna circuit gets detuned, then how to explain the tag is worked well at the 1.5cm distance,except something is make the tag worked in a detuned condition. Somebody can ell me what's that?
Now I put my test step in the attachment, it is a very particular account of the test.
Attachment:
The Problem of 13.56M tag antenna.pdf

If the Q of the resonant antenna circuit is not high enough, the device cannot rectifies the RF voltage to develop the specific DC operating voltage, and making the circuit functional.
If is possible, go for a conducted test. Verify the input power necessarily to get the DC voltage, and also check the dynamic range of the receiver which most probably should be somewhere between -40dBm and +10dBm
The maximum magnetic flux that is passing through the tag coil is obtained when the two coils (reader coil and tag coil) are placed in parallel with respect to each other. This condition results in maximum induced voltage in the tag coil and also maximum read range.
From your picture can be seen that when you lift the tag, this actually goes more in parallel with the reader coil, making the circuit to work.
I think you have to replace the reader antenna, with a smaller one, and remove the iron board from the back, which I do not understand its meaning near the loop.

Since the source impedance of RFID system is a complex value, its impedance matching is indeed different from the common 50-ohm system. The odd phenomenon you encountered actually is not odd in HF RFID systems. First you have to know that in HF RFID systems, the transfer of data and power is based on the near-field coupling between the reader coil and the tag coil. In a simplified way, we can model this near-field coupling by a transformer. The key is that the mutual inductance not only is used to transfer data and power but also to tune the impedance/resonance both at the tag side and the reader side. Therefore there is possiblity that even the mutual coupling becomes larger due to shorter distance however the resonance is detuned at either side, and finally turns out a shorter read range. For more information, a paper is attached for reference. With respect to the different results on area A and area B, I suggest you to evoke some full-wave simulator to study the magnetic field distribution along the surface of the reader coil, and/or the coupling between the reader coil and the tag coil. There is possibility that the magnetic field distribution of the reader coil is not uniform. Also two documents are attached for reference.
UHF Near-field RFID Reader Antenna_APMC 2009.pdf (257.1 KB)
Energy Scavenging for Inductively Coupled Passive RFID Systems.pdf (1.08 MB)
RFID_Systems_with_Maxwell_Software.pdf (173.9 KB)

Thanks Streamlet! I have read the article > & > , and I know that the tag does't get more power when it is too close to the reader, so it maybe can explain my second test.
About my first test, today I ask the tag company,they said :"if the tag get the same power in diffrent area A & B, maybe the tag is worked well, but the reader's receiving sensitivity is not strong, so they can't communicate with each other ". I don't know is their words right?
Next, according to your(everybody) suggestions, I will adjust the reader's antenna and make it can read the tag in my needed area.Maybe I will do another bigger tag pcb, but this way may wait a long time, because the tag's size is limited by the product.
Thanks everybody for help me!

I think the reader "antenna" (coupler coil) with underneath metal plate is producing an irregular field geometry where it's hard to predict an operation volume.
You're giving numbers for tag coupling at two different positions and ask, why the real operation is different for these. I guess, the coupling has been measured with a cable, while the tag operation has been tested without it? This may be one of several possible explanations.
You also didn't tell, what's the actual field strength respectively rectified receiver voltage for the tested situation.
You said the modulation is "FSK". Are you implementing a known RFID standard or designing your own communication scheme?

Hello FvM!
The truth is the reader communicate with the tag use ISO/IEC15693 standard, I use the logic analyser to capture these data between two device, so I know the tag is FSK modulation.
About the receiver's rectified voltage,I think I can't measure it, you know the tag is a intergrated IC, I just can measure the power use the spectrum analyzer, all the measure data are in the attachment "The Problem of 13.56M tag antenna.pdf ", that is all.

Thanks fo clarifying about the implemented standard. According to the specification, ISO/IEC 15693 is using ASK with pulse position encoding for the reader to tag direction. For tag to reader, one or two subcarriers of fs/32 (423 kHz) and optionally fs/28 (484 kHz) are used. The two subcarrier variant actually implements FSK, the one subcarrier OOK.
ISO 15693 is intended as a vincinity RFID technique, using low data rates and in some cases reader powers up to a 10 W range to achieve several m reach.
For specified operation, the field strength in the operation volume is between 150 mA/m and 5 A/m.

hi,i have a question about this antenna,when you simulate this antenna using HFSS,what is the size about the air box?

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