# Visual understanding of EM fields within a rectangular metal container

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. I would like to know how the EM field behaves inside the container, such as where there will exit destructive interference and constructive interference?

In terms of interference from the other antennas, this will not be an issue due to only one antenna will be on for a given period. Basically the antennas are switched on and off in a periodic manner.

• You might gain a feel for modes from acoustic properties of rooms: resource.isvr.soton.ac.uk/spcg/tutorial/tutorial/Tutorial_files/… You might also find some applets to find room modes. – glen_geek Sep 13 '19 at 19:27
• @glen_geek considering I am not designing a cavity resonator and from the answer below there will exist a variety of modes in this container, what will be the consequences of having a variety of modes CO-EXISTING in this container practically? – Joey Sep 13 '19 at 19:37
• Nothing is good or bad of itself, only in the context of what you are trying to do. What are you trying to do? What do you want to happen at your red dots? Are you trying not to overheat them? In which case limiting the drive strength emperically will work. Are you trying to talk to them so need a certain minimum signal? In which case some are likely to fall below your minimum for any given antenna, but probably be OK for at least one of the antennae. – Neil_UK Sep 14 '19 at 6:16

915MHz has a wavelength of about 300mm, or one foot. The container has internal dimensions of many many times this. This means the standing wave pattern, resulting from the inevitable interference, is multimode. Many different modes can be excited.

While you can guarrantee that there will be zero tranverse electric fields adjacent to the conducting walls, you can guarrantee very little else. In theory, accurately known dimensions would allow you to compute what modes would be excited. In practice, the slightest irregularity in the walls will cause all your predictions to be wrong. The other 'off' antennae will complicate the geometry no end. Points of high field and low field will move around as the container walls flex, as the contents moves, as the frequency shifts slightly.

With a reasonable degree of certainty, you can say that a low field spot with one antenna running is unlikely to still be low with another one in use. If your red dots/balls are transceivers to be communicated with, then you're likely to be able to talk to all of them, just not necessarily all at the same time, pick the right antennae for each transceiver.

As you don't know for certain which combination of the many modes possible you are driving, the antenna impedance will be unpredictable.

For industrial heating, it is common to choose a frequency and box dimension that supports only a single mode. This way, the field strength within the box can be accurately predicted.

• your other 3 antennas, whether overtly resonant or not, will upset or alter the fields. And whether those 3 have external resistive/reactive terminations, also affects the fields. – analogsystemsrf Sep 13 '19 at 16:59
• @neil_uk - is there software available that I can use to simulate the this. So I can see a 3D simulation of the em behavior. – Joey Sep 13 '19 at 17:02
• @analogsystemsrf do you know if multimode interference is good or bad? – Joey Sep 13 '19 at 21:04
• If you have no other circuitry inside the box, then there is no problem with ANY type of modes, correct? Thus you need to define the volts/meter field intensity that may degrade your signal-noise-ratio, or may cause bit-errors. – analogsystemsrf Sep 14 '19 at 2:55

I cannot give you a complete answer and you would need some kind of 3D electromagnetic simulator. I have used HFSS (a FEM simulator) and CST (a FDTD simulator), but both of these are extremely expensive. Based o a quick googling, there are some free alternatives, but I have no experience on any.

The device you are looking at is called a Cavity resonator. The cavity resonator is a conductive box (like yours) where the waves transmitted by your antennas "bounce" of the walls. At certain frequencies, the length of the box wall is an exact multiple of half of the wavelength. At those frequencies, the waves can bounce back and forth in the resonator very well. That is called a resonance. The list below is a list of frequencies where the resonance happens. The subscript numbers correspond to the number of half wavelengths that fit inside your box at that frequency. The lowest resonance mode is called $$\TE_{101}\$$ and that mode can fit (exactly) one wavelength in two directions and none in the third direction. In your resonator, the TE101 resonance happens at 64 MHz. See Compact cavity resonators using high impedance surfaces for field plots of a TE101 resonance

I used the formula for resonance frequencies of a rectangular cavity and listed below all modes that are within 1 MHz of your design frequency. As you can see, there are multiple different resonance modes near your frequency and it is not easy to tell which mode you are exciting. You might even end up exciting several modes at the same time.

$$f_{16-14-2}: 915.0 MHz$$ $$f_{6-14-3 }: 914.5 MHz$$ $$f_{7-14-3 }: 915.6 MHz$$ $$f_{6-13-5 }: 914.5 MHz$$ $$f_{7-13-5 }: 915.6 MHz$$ $$f_{14-12-6}: 914.2 MHz$$ $$f_{15-11-7}: 914.4 MHz$$ $$f_{11-10-8}: 914.8 MHz$$ $$f_{14-6-10}: 914.2 MHz$$ $$f_{1-3-11 }: 915.7 MHz$$ $$f_{2-3-11 }: 916.0 MHz$$ $$f_{11-2-11}: 915.2 MHz$$ $$f_{14-1-11}: 915.1 MHz$$ $$f_{15-0-11}: 915.4 MHz$$

• what do these modes mean practically. What modes from the list you gave me the best scenario. I am sorry if these questions seem common sense to you, but I know little on this particular topic. – Joey Sep 13 '19 at 17:06
• @Joey RF modes are a very common and basic concept in RF engineering. They are sometimes said to be just standing waves in cavities or conductors, but this is only a single aspect of that idea. I recommend reading some wikipedia articles or RF tutorials first to get a grasp, and ask some more specific questions here later on, as it takes books with thousands of pages to encompass modes. – Ariser Sep 13 '19 at 17:28
• considering I am not designing a cavity resonator and from the answer there will exist a variety of modes in this container, what will be the consequences of having a variety of modes CO-EXISTING in this container practically? – Joey Sep 13 '19 at 19:37
• The consequence will be that you don't know how the field will behave. What do you want to accomplish? – user24368 Sep 13 '19 at 19:54
• @Joey: Your calculation is correct. – user24368 Sep 14 '19 at 2:40

If anyone wants to get assistance regarding visualizing an em field in any environment. Please use gprmax. It's an open source em simulator which means it's free to download. The only issue is installing it, as it is not straightforward. Hope this helps someone as it has helped me.