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

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?

• Maximizing the near-field will also increase the far field unless you take steps to really hammer the H-field properties of your antenna (assumed to be a short dipole or monopole). Please also note that your question requires a yes/no answer <-- you might want to re-phrase it to attract answers that are more than one word. Commented Nov 4, 2022 at 10:51
• @Andyaka I added more details now. Commented Nov 4, 2022 at 14:15

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.

• Okay, that's understandable. Thank you for the answer. Commented Nov 8, 2022 at 13:52
• I have a question for you since you work in near-field antennas (I hope you would answer): What's the easiest way in which you achieve miniaturization for the low frequencies? Apart from changing the substrate's dielectric value unreasonably high? I am having too much trouble with miniaturizing a microstrip antenna for 1 GHz to the size of a normal 4 GHz antenna. (I've spent days iterating, so any good help is much needed.) Commented Nov 8, 2022 at 13:59
• I began wondering about the construction of miniature antennas after your initial post. Unfortunately, I don't have any specific knowledge or experience to share, just speculation. How small (physical dimensions) does your antenna need to be? I will be asking some questions of a friend that is an expert and if I learn anything of interest I will follow up. Commented Nov 8, 2022 at 16:10
• I want the antenna to be ideally less than 6 cm for both length and width. Normally, a 1 GHz microstrip antenna would have 14-15 cm sides. I have tried shorting wall and increasing the dielectric with some success, but meandering, slotting, pins, fractal, etc. seem to be very iterative and I'm not able to land on 1 GHz resonant frequency after I start with the necessary dimensions for the antenna. Commented Nov 8, 2022 at 18:12
• Have you had a talk with your friend about this? I could use some answers still. Commented Nov 12, 2022 at 18:30