The tag antenna is very inefficient. The reader antenna has to make up for this by being as big as possible. Read some application notes for tag reader systems where most of the equations are spelled out clearly. If you put them in a spreadsheet you can see what happens when you vary the reader coil diameter.

For sure small antenna wont be efficient and low directivity and small traveling distance.

But, my question , how can it communicate(tx and Rx) with reader antenna? If it works as a tag antenna then why not as a reader antenna because As an antenna is a reciprocal device.

well, Now a days, people are looking for small reader as well. Now, i need to design a reader antenna where my space limitation is just 30 mm for 865 MHz. That is why it comes on my mind.

The wavelength at 865 MHz is about 340 mm. An antenna 30 mm across is less than a tenth of a wavelength across and counts as a small antenna (less than a third of a wavelength across). It may have efficiency and bandwidth issues, but you might be able to use it. Read about Small Antennas (Wheeler, or Hansen and Collin).

the NFC reader needs to deliver the power to the NFC tag because NFC tag doesn't have a battery. So, NFC reader transmits the high power RF signal to the tag. If the NFC tag doesn't have enough efficiency, it's hard to deliver enough power to the tag which required NFC tag operation such as load modulation. Even if the antenna RX/TX operation is reciprocal but not this because NFC reader has power.

There are several reasons. First, the gain and total efficiency of UHF RFID tag antenna are very low as the tags are designed to have a small footprint and dimensions. UHF RFID read range is directly proportional to the product of reader and tag antenna so if you use a tag antenna as a reader antenna the read range, which is a main UHF RFID system parameter, will be very short and your system becomes unpractical. Second, UHF RFID tag antenna is designed to be matched with the IC chip complex impedance (with a large capacitance-type reactance). All commercially available UHF readers have 50 Ohm output ports to be connected with the 50 Ohm reader antennas. Thus, if you try to connect a tag antenna with the UHF reader, you'd have quite large impedance mismatch and return loss and again a very limited read range. Third, in many applications a CP reader antenna is required to be able to communicate with linearly polarized tags arbitrary oriented in the plane normal to the direction of propagation from the reader antenna. Most of tag antennas are dipole-types linearly polarized so they don't satisfy that condition.

Dear Prof. Andrey S. Andrenko ,

Thanks for yours detailed answer. I agree with your every points. But still, I have few doubts on my mind on the basis of your answers.

First, what you said about '' UHF RFID read range'', I am agree. But, my again it comes on my mind that ...then how Tag antenna transmits the signal to the reader. We may assume that there are certain distances in between reader and antenna. That is why we need a decent gain and efficience level for reader antenna. But with low efficience and with minus gain (dB), how a tag antenna can communicate with reader ????

Secondly, yes it will be a problem. But in that case we can use impedance transformer or any other matching technology. For example, Loop antenna has also very high impedance which is used for near field antenna for bad reflection coefficient.

Thirdly, We may think about the spiral antenna (printed inductive coil) as a RFID Tag so we can overcome the linear polarized issue.

Debraj

When the two antennas communicate, the received signal is proportional to the product of their gains. If one is terrible the other has to be good. They cannot both be terrible. A similar principle holds with inductive coupled tags. Read the application notes, put the equations together and work out the answers, in a spreadsheet, for instance. That is what I did. Then you can try using a small read antenna and see what happens.

The main factor of the limitation of the RFID system is the sensitivity of the RFID tag chip. Currently for Passive UHF RFID chips you will find it (-20 dB) and it differs in both reading and writing processes.

In addition, for sure it would be perfect if you are using the reader's antenna at the tag size but this will let the tag to be very bulky and thus you will find very limited applications for such a system.

If you are using the the tag Antenna at the reader side so the following will happen:

by regulations, The UHF RFID Reader power shouldn't exceed 2W ERP in ETSI standard and 4W in the FCC standard, thus you will need a Power Amplifier that supports these power levels and conserve the out of band emission to be less than -30 dBm (by means of several filters) - because the PA is a nonlinear component that emits a power not only in the fundamental but also in the harmonics. Accordingly, you will have a system with low efficiency.