How should I use characteristic mode analysis for antennas that are needed for near field imaging?

Question

I am designing an electrically small microstrip patch antenna, and its "radiative" near field (r = wavelength) reaches quite long enough for the whole imaging region. Basically, I don't need its power to be radiated to the far field.

According to CMA (characteristic mode analysis) theory, we ought to minimize its equation for best power radiation to the far field. So, doesn't it make sense to maximize the function for maximizing the near field instead?

I basically came up with this from reading the HFSS reference guide on CMA, "Where R and X are the real and imaginary parts of the EFIE impedance matrix, Z = R + jX. The real part of P is the power radiated and the imaginary part is the power stored in the near field. For antenna design, we want to maximize the power radiated, or equivalently, minimize the power stored in the near field. The solution to this minimization problem is the generalized eigenvalue problem ..."

So yeah, I thought I can just reverse the optimization to get more near-field storage of power, thinking it would maximize power for the imaginary part.

If this does not work, what does? Should I just design the antenna based on normal principles, and add some sort of radiation absorber around the setup in the real world?

Answer 1

Hmm... interesting question. I know nothing about CMA but have been working extensively with near-field (NFC) antennas during the past year. Generally, working with RF was new to me even though I did understand real vs. imaginary impedance, etc.

Disclaimer - Anything I write here isn't necessarily correct. It just reflects what came to my mind when reading the question.

My understanding (thus far) is that the imaginary part represents the stored energy. But, I'm not sure it equates to the power transmitted in an antenna's near field as the quotation led you to believe.

When tuning an antenna, maximum power is obtained by matching source and load impedance at the resonant frequency. At the resonant frequency the impedance of the antenna becomes purely real... i.e. the imaginary impedance is zero (in the ideal case).

For near field antennas the power is transmitted via the magnetic field. The mutual coupling of the antennas (in the case of NFC) affects the impedance of the overall antenna system. Achieving resonance with matched source and load impedance maximizes the current conducted by the inductance of the antenna coil thus maximizing the magnetic (near field). This includes the coupling effects of the NFC antennas (mutual inductance of the reader and tag antennas).

I don't understand near-field imaging but it seems to me that the near field will be heavily influenced by the environment and that as that environment changes the resonant point of the antenna will also change significantly. This leads to significant changes in the efficiency of the transmitted power via the magnetic field.

I've got a variety of random thoughts about this but I think that would be a departure from the original question.