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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?

Lucifer Holmes
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  • 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. – Andy aka Nov 04 '22 at 10:51
  • @Andyaka I added more details now. – Lucifer Holmes Nov 04 '22 at 14:15

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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.

user324996
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  • Okay, that's understandable. Thank you for the answer. – Lucifer Holmes Nov 08 '22 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.) – Lucifer Holmes Nov 08 '22 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. – user324996 Nov 08 '22 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. – Lucifer Holmes Nov 08 '22 at 18:12
  • Have you had a talk with your friend about this? I could use some answers still. – Lucifer Holmes Nov 12 '22 at 18:30
  • I was not able to talk to him this week but plan to next week. I will let you know what he has to say. – user324996 Nov 12 '22 at 22:34
  • I don't want to ask you to divulge anything that you consider confidential but more information would be helpful. What kinds of focal distance are you trying to achieve? Are you using an array? What dielectric are you using? How are you constructing this antenna? What are you using for a source? Is it connected via a coax? Distance? Basically... any details that would help discussion and that you are comfortable sharing. – user324996 Nov 12 '22 at 22:45
  • One more question... have you constructed a simulation model? – user324996 Nov 12 '22 at 22:46
  • Have you measured the antenna's impedance? Do you have any details of those measurements? What have you done to tune the antenna for 1GHz operation? – user324996 Nov 12 '22 at 23:32
  • I am not very concerned about focal distance since the antenna is not an array type. I just need enough field in the near field range (around 30cm). I've been using FR4 and Rogers 6006, but I'm thinking of ceramics now. I'm okay with the antenna being fed with a coaxial or microstrip source. I'm analyzing this using HFSS, which is where I tried the different methods. The antenna's impedance was set to 50 ohms for simulation, but I didn't check those measurements since I was focused more on bringing the design to 1 GHz operating before manipulating anything else. – Lucifer Holmes Nov 14 '22 at 10:30
  • I'm working with NFC at 13.56 MHz so 1GHz is a different animal. My antennas are FR4 spirals which are tuned with additional series and shut elements to resonate at 13.56 MHz. One could theoretically design a spiral antenna with the right inductance and capacitance to resonate at the desired frequency without additional tuning but that is not what I have done. – user324996 Nov 14 '22 at 18:21
  • I see. How large are these antennas? – Lucifer Holmes Nov 14 '22 at 20:38
  • The antennas I am working with are about 42 mm diameter. – user324996 Nov 15 '22 at 02:08
  • Wow, that's a very small antenna for its frequency. Looks like I ought to look into tuning via additional elements. – Lucifer Holmes Nov 15 '22 at 16:56
  • Well... I'm not certain but you may be thinking of the antenna in terms of wavelength. I think of the near-field antenna as an inductive coil that generates the magnetic field. – user324996 Nov 15 '22 at 23:00
  • Hmm, I see. That's something. – Lucifer Holmes Nov 16 '22 at 10:13
  • It just registered in my mind that you are looking for 30cm distance. For NFC (13.56MHz) effective distance is more like 4cm typically. Granted, that is for communication and energy harvesting for the passive NFC tag. It is not clear to me what you are trying to do beyond generating a magnetic field at 1GHz. I'm not sure what impact operation at 1 GHz will have on effective distance. – user324996 Nov 16 '22 at 16:25
  • A quick LC resonance calculation for 1 GHz indicates that even with a very small inductance of 10 nH you need to have a very low capacitance of 2.53 pF. A 10 nH coil is probably a very small diameter with few turns so I'm not sure what kind of a magnetic field you can generate from something like that. The low capacitance might be hard to achieve as well. – user324996 Nov 16 '22 at 16:44
  • Well, that antenna is for imaging of the brain for biomedical use. I know for a fact that many design antennas for this purpose, but they are rarely patch-type, especially at 1 GHz. Hence, it is a challenge to get the size and design parameters right. – Lucifer Holmes Nov 17 '22 at 11:10