how to create a strong magnetic field

Dear fellows,

I am trying to design an antenna for near-field magnetic coupling (UHF RFID). I know that it is easy to design such an antenna in HF. A multi-turn loop works great. However, in UHF, we all know that it is quite different due to the radiation resistance, parasitic capacitance, and so on. Therefore, I have several questions on the transmitting antenna:

1) what kind of antenna is good for this application? Will the loop still do its job, or other potential shapes?
2) Theoritically, higher current will create a higher H field. Should we persue an antenna with lower resistance? We can match it to 50 ohm later. Here, I believe that we don't need to care much on the radiation resistance.
3) Is there an easy PCB-based balun that we can implement? So we can connect it with the coax cable and the loop.

Thank you for help.

Dear Fellows,

I hope that someone can help me on this issue. I need to point out that I am talking about the near field. And I have some updates on it regarding my questions in the last post:

1) a loop (small) antenna or a slot antenna may work.

2) Impedance matching by using lumped elements may not be a good solution, since it results in a narrow bandwidth

I also found that the matched antenna (self resonant) didn't gurantee the highest H field, since H is relevant with I, and I may not be the largest in this case.

Thanks,

I hear that a small loop driven by a current source is the solution. But I am still trying to figure out how to do that.

Hi bingjiang99,

The tags for UHF RFID are almost all dipole. In other words
the mechanism for coupling is the electric field. And they are
all designed for far field, meaning the two antennas are not
close to each other.
However, your question is about a near field. I assume what
you want to do is to use a "coupler" or some sore of antenna
that can trasnfer the information using a short distance.
Am I right?...Lets discuss that.....I would think that the best
way to transfer energy is not the H field but the E field.

Hello jallem,

Thank you very much for your reply. Yes, you are right-- "most UHF RFID tag antennas are dipole". Impinj has new types of tags in small loop styles, which use magnetic field to communicate. The surrouding environment has less effect on H field than E field in the near field. This the advantage. I want to design an antenna which has very strong H field in the near field, which can be used to read tags with loop antennas.

Thanks a lot,



Added after 3 minutes:

Hello biff44,

Thanks a lot for sharing your idea. That is a good point. However, I have no control on the driving circuits. I can only work on the antenna side.