Could you please tell me which kind of this antenna is? I supposed it is a modified dipole, but not quite sure. Is there any available equation for calculating the original dimensions of this type of antenna? Thank you.
As mentioned in the comment (why not make that an answer? I would have upvoted it), this looks like an RFID tag antenna.
These antennas usually provide three functions:
- to receive RF at a specific frequency and allow it to be decoded
- to receive RF energy and convert to DC, to power the IC in the middle
- to allow the IC to transmit back a signal that can be picked up by the tag reader
To satisfy these purposes, it has to operate on at least two frequencies. Possibly three frequencies, if the power to drive it is separated from the frequency it receives on - and possibly one, if it listens then transmits on the same frequency, although that would require the IC to be more complicated, and the antenna would not need to be as complicated as it is.
As also mentioned in the comment (that should have been an answer), these antennas are usually created by experimentation, tweaking the shapes and sizes of the different parts of the antenna to get the best overall performance.
Honestly, it looks like some kind of folded dipole with an additional dipole attached to the sides. This would give it fairly good characteristics on several different frequencies, and it has probably been optimized through experimentation for the frequencies it is intended to operate on.
If you have an antenna analyser (or a dip-meter) you can work out what those frequencies are by using a coil of wire and putting it around the antenna, and seeing where the impedance changes.