I am searching to design magnetic antenna with high directivity in order to maximize RFID range in low frequencies. My question is : it is really possible to design such antenna? with which materials and which shape?
The values of low frequencies that interest me are : 125Khz and 134.2Khz. What about directivity of such antenna? Have they always the behavior of a magnetic dipole antenna? Thank you.
The wavelength at 125kHz is 2.4km so this is a fairly pure near field magnetic problem. It is not so much of an antenna directivity problem as maximizing the coupling between 2 resonant magnetic loops or perhaps ferrite rod antennas. (Usually the term antenna is used when the size of the antenna is close to or much larger than the wavelength.) Size is important (loop diameter or rod length) High Q is also important to maximize range by increasing current/magnetic field strength. Ferrites work well at 125kHz. When the loop is small compared to the distance between the antennas the field is essentially a magnetic dipole. Often it is also a challenge to keep the loops aligned well enough. Simulating the coupled system often gives a good idea of what is going on by just looking at the field lines when moving the coils around.
Hello,
I was slightly irritated by your question (you will see, why) and had to check some things ...
The term "magnetic dipole" is quite unusual. 'Dipole' is used for antennas where the capacitor has been 'stretched' (see https://en.wikipedia.org/wiki/Dipole_antenna for some details), for magnetic antennas the term dipole is unusual.
If you want(ed) to ask whether these magnetic antennas are resonant: yes, they are.
According to my own experiences directivity is good. Sometimes too good - forcing you to meticulously align reader and tag antenna. Though a bit of ferrite or other medium capable to concentrate the magnetic field helps.
For these frequencies you typically have multi-turn loop antennas with some capacitor to tune the frequency.
Overall, this is the 'old and proven' technology. Working very well with antennas with a diameter of about 20 cm on the reader side. The whole thing is slightly 'tricky' when you have antennas the size of credit cards on both sides. Don't know exactly why, but I did resort to directing the field by means of passive 'field concentrators' (ferrites or - during development - a simple screw driver).
Hope this helps.
There is one more thing to watch out for compared to ordinary antenna systems. Ordinary far field directive antennas work best when they point at each other. Magnetic coupling is more complicated since You eventually have to turn the receiving antenna 90 deg at some offsets and distances. This is easy to understand looking at the field lines, but can become confusing and complicated in practice. The solution can sometimes be to use 3 receiver coils and add the signals.
I have seen the term magnetic dipole used, but agree that it can be confusing. It seems to make some sense to me for rod antennas but not so for more complex structures like folded dipoles and such. An electric dipole has a critical well defined electrical length determining wavelength. This is harder to define for magnetically fed antenna since there is no obvious length measurement determining the wavelength. The wavelength is usually set by the resonator capacitor, or perhaps a cavity size.
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