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What is near field antenna?

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xcy_antenna

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reactance reactive near field antenna

Hi,
can anyone kindly tell me what is near field antenna ?Thanks in advance.



Best Regards
Xcy_antenna
 

is it "near-field antenna" or "near-field of antenna"?
 

Near field is a region where EM field interact stongly with each of other. Mathematically the input impdedance is defiend as Rin+-Xin
The imaginary part of the input impedance, Xin is associated with the energy stored in the near field region. The sign of the input reactance depends on the near field predominance of field type: electric (capacitive) or magnetic (inductive)
 

The boundary between the near and far fields is often taken as 2*D²/λ where D is the largest dimension of the antenna. I view this as the distance where the radiation pattern characteristics are relatively well solidified and no longer changing with distance.
 

Actually in the near field the field quantities are interactive thats the very reason to measure pattern in the far field region. However it is still possible to measure them in near filed.
 

Above all answers are assuming that the original question is "What is the near field of an antenna?". However, I try to answer the original one.

I'd like to regard some antennas which we take advantage of their near field to transmit power and signal as near field antennas. Someone may refuse such kind of "definition". Try to review the definition of "antenna" on the first page of C.A.Banasis' "Antenna Theory" and think about the electrically small antenna, we will find exciting issues.
 

the near field is a region (also called the fresnel region) the other two regions making up the space around an antenna are the reactive near field (closest0 and the far field (fraunhoffer region)
 

Actually as i understood, Question is 'what is near field antenna'. So Some RFid systems work on near field effects of antennas. and on this type of RFid systems you can use small tags(cos just inner loop of asic can excite the chip).

exp: at uhf rfid systems you can find some near field antennas. But beware of the dead zones above the antennas. And the tags of this rfid system can be 10 mm diameter or less. Also you can read the tags with this type of antennas max. 10 cm.
 

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