I am wondering about the design of the TEM cell made by TekBox. They used an interesting solution by dividing the septum into several transmission lines to reduce resonances and suppress perpendicular electromagnetic fields.
They used some electronic components to connect the transmission lines. However, they are covered with something, probably to make their identification difficult.
Unfortunately, I don't understand how it works and what these elements may be. I found two scientific articles on a similar topic on the Internet (attached). Resistors were used there, but it was not explained why and how the values of these resistors were selected.
There are also some speculations about it on the Internet.
The first statement talks about using resistors or ferrites
https://essentialscrap.com/tem_cell/temcell.htmlTekbox uses slots in the septum copper, bridged by some kind of ferrite or resistive material. This dampens any perpendicular electromagnetic fields and reduces high frequency resonances. A similar technique is explained in "Expanding the Bandwidth of TEM Cells for EMC Measurements" by Myron L. Crawford, 1978 (DOI: 10.1109/TEMC.1978.303664), but instead of slotting the copper it uses absorbent material near the shield. Again I do not know if these methods will effect sensitivity of the measurements.
The second statement from this forum talks about using capacitors (topic Parametric TEM cell design):
https://www.eevblog.com/forum/projects/parametric-tem-cell-design/msg4068439/#msg4068439Great, thanks for the design file, I will see what I messed up
Also a tip for getting less noise - you can create a "closed cell" with just cardboard cut to fit on the tem sides, and aluminium foil glued to the outside and connected to tem enclosure. The measured emissions from the device are similar to what you get without the shielded walls, but the difference in noise from the environment is massive.
As for the issue with slots - tekbox cells (at least TBTC3 that we have at work) also use splits in the septum lengthwise, but the sections are connected to each other using SMD capacitors.
Maybe there is someone smarter than me here and can explain how it works. Alternatively, point me to some scientific articles that will explain the principle of operation and allow me to select such elements when building my own chamber for hobby purposes. Of course, if a working TEM cell is built, the project will be made public for free, including on this forum.
Attached is a photo of the septum PCB from the TekBox chamber (from a video on YouTube) and scientific articles I found.