magnetic near field

Hi. I am currently designing an antenna that needs to have a high magnetic field strength. Is there any specific features or golden rule that i need to follow on to get something out? Thanks in advance.

Usually, loop antenna is a good way to realize near coupling. You can search some RFID antennas as examples. It can be realized by multilayer PCB, Flex Printed Circuit, or just wrap a wire to form a coil.

To be specific, was actually looking into a loop form antenna at 900MHz. But there is actually a size contraint of around 4cm by 3cm in my case.

Theoretically, the magnetic field strength of a antenna depends very much on its size and Q factor. Was actually wondering under a 4cm by 3cm constraint, is it even possible.

The dissipative resistance and the magnetic field strength in the Loop Antenna depends by loop perimeter, conductor width/thickness, magnetic permeability, conductivity, dielectric loss and by frequency.
The circumference of your Loop Antenna is less than λ/2, the radiation resistance is small, so the radiated field is low.

Since the loop antenna for your size constraint is much smaller than the half wavelength, it is only working for near field coupling, and the radiating fields are not important as traditional antenna that working on far field.

So one of the important factors is the distance between your transmitting and receiving antennas. In this case, I believe you should simulate/design the system based on considering both the Tx/Rx antenna with a certain distance. The mutual coupling impedance in this case is more important, and it is a function of Tx/Rx antennas distance, orientation.

There are some commercial near-field products, typical ones are the RFID systems in 30MHz ranges. Also, some of the wireless mouse/keyboard are using the near-field antennas. One of the loop antenna is at the USB dongle, so I don't think that is a problem for your design. They usually can work for one or a couple meters.

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Just found that your operation frequency is 900MHz. So you can even try to design the far-field antennas by using meander line, PIFA or other structure. Usually far-field antenna has better performance than near-field coupling antennas.

the UHF Rfid tags couple with the interrogator Electrostatically rather than Electromagnetically

UHF Rfid tags will perform much better with a patch antenna rather than a loop antenna.