From my previous posts Insight into cavity resonators, Visual understanding of EM fields within a rectangular metal container I have slowly gained a better insight into cavity resonators and thereof. I have the following scenario setup inside a rectangular metal container. The container size is 12.03m by 2.39m by 1.84m (lxbxh). Additionally, inside the container I have placed 4 antennas equally spaced, this is shown using the green squares (ignore the red dots/balls). The frequency of these antennas is 915MHz.

In terms of the multimode aspect, I am not too sure if this is necessary to take into account since my aim is to not make a resonator but just to see the interference within the container created when the antenna radiates its energy in the container.

In the event I do need to take into account the multimode aspect, I also know that an example of a multimode cavity is a microwave oven. How can I then test the EM field inside the container for the EM fields but not for the application of heating the contents?

I want to then use this information to see if when I place RFID tags on pallets of goods inside the container, which regions will result in the RFID tags receiving no power due to destructive interference and which regions will result in there been power. I know when the goods are loaded on to the contiainer the EM field will change but that is a topic for another day, For now I am assuming the contianer been empty and place this tags in it. So first I need "to see the interference created within the container when the antenna radiates its energy in the container"

  • \$\begingroup\$ What is your design actually trying to accomplish? What are the performance metrics for the design? Without knowing this it's hard for us to understand what aspects of the "EM environment" are important to you. \$\endgroup\$ – The Photon Sep 20 '19 at 16:29
  • \$\begingroup\$ given the long pieces of metal in your antenna will upset the modes, how do you expect to predict any accurate fields/modes? \$\endgroup\$ – analogsystemsrf Sep 20 '19 at 16:35
  • \$\begingroup\$ @analogsystemsrf the antenna will be recessed into the container. My plan is not to make a cavity resonator so I am not sure if the modes are really important. What will this mean in terms of simulating the EM field if there exist no modes? \$\endgroup\$ – Joey Sep 20 '19 at 16:41
  • \$\begingroup\$ Hi @Joey, we have asked this in previous threads as well. What is the end goal you are aiming for? The resonances are already too complicated to be predicted and the measurement antenna (and cable) will distort them even further. \$\endgroup\$ – user24368 Sep 20 '19 at 17:02
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    \$\begingroup\$ Great! That question already makes a lot more sense. \$\endgroup\$ – user24368 Sep 20 '19 at 17:19

As has been mentioned in the comments, the resonance modes in your box will not be predictable and will be different for an empty container than for the container filled with pallets and will vary when you insert your goods on the pallets. Even if the goods would always be the (nominally) same, the modes probably will vary from shipment to shipment.

There are several ways you can figure out the solution: 1) simulations, 2) measurement with a probe, 3) building your final setup with goods and everything and trying it out.

I have only used CTS and HFSS, but I found some possible tools that you might want to try: MaxFEM, emGine, deal.II, Hypre. I have no experience with any of these, they are just what seemed promising after some googling. Even if the simulations aren't perfect, they might help you visualize a lot while varying only one input parameter.

The measurements will be difficult because your measurement setup will disturb the setup. So one route might be to minimize the disturbance as much as possible and hope that it would be enough. I have sometimes needed to measure currents that are induced in conductors in the near field. That measurement has similar challenges as yours: if I make a probe and connect it to the measurement equipment, the current in the cable completely messes up the current that I try to measure. One solution that I have been thinking about would be a small loop as a probe, possibly tune it near your frequency with a capacitor, use Schottky diodes as amplitude detectors, add an op-amp as a current driver, modulate LED intensity with the current, steer the LED light through optical fibre, and de-modulate outside your container with a photo diode. After that, you can move your probe inside the container while transmitting with the antennas and see how the intensity varies.

Full disclosure: I didn't build the system and there might be some issue that makes the idea completely nonsense. Good luck, let us know how it went?

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  • \$\begingroup\$ I do apologize for my previous statement regarding the sarcasm comment. I have been trying for months to acquire some form of direction and have been let down by many individuals, so I did not think you were been serious. However, I do appreciate your guidance. Will definitely upload the answer if I find it out. I do believe it will benefit other users as well. \$\endgroup\$ – Joey Sep 20 '19 at 20:58
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    \$\begingroup\$ It's good to take baby steps in when approaching your solution (like you have done in past five questions, but for us, it's not easy to help you if we don't understand where you are aiming at. After answering "Mitigation of dead zones when applying circular polarized antenna" I thought that your container might be too small to sustain any resonance, then "Visual understanding of EM fields within a rectangular metal container" made it clear that the container is so big that the term "resonance" becomes almost meaningless. \$\endgroup\$ – user24368 Sep 20 '19 at 21:12
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    \$\begingroup\$ And that's why most of the questions have comments like "What are you trying to do?". We aren't questioning your intentions, we just wish to understand what you want to do. The answer is different if you want to make an RFID tag reader or a shipping container-sized microwave oven. \$\endgroup\$ – user24368 Sep 20 '19 at 21:17

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