Author Topic: TekBox TEM cell – how is it constructed ?  (Read 1128 times)

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Offline tomudTopic starter

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TekBox TEM cell – how is it constructed ?
« on: April 16, 2024, 09:01:38 pm »
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.html

Quote
Tekbox 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/#msg4068439

Great, thanks for the design file, I will see what I messed up :D

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.


« Last Edit: April 16, 2024, 09:08:58 pm by tomud »
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Offline tomudTopic starter

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Re: TekBox TEM cell – how is it constructed ?
« Reply #1 on: April 19, 2024, 10:58:10 pm »
For those interested, I found a scientific article that explains this problem a bit (attached).

Personally, I'm waiting for the PCB I ordered, so I'm not able to write more at the moment. The PCB is modeled after the TekBox septum - I based it on the photos I found on the Internet (we'll see what the results will be).

For every complex problem, there is a solution that is simple neat and wrong...
 
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Offline tomudTopic starter

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Re: TekBox TEM cell – how is it constructed ?
« Reply #2 on: April 29, 2024, 09:53:17 am »
Well, we're slowly starting to start building the TEM cell. Let's see what the results of the test version will be...  :popcorn:
For every complex problem, there is a solution that is simple neat and wrong...
 
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Offline tomudTopic starter

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Re: TekBox TEM cell – how is it constructed ?
« Reply #3 on: May 01, 2024, 04:36:06 pm »
And the TEM cell is assembled ;) The effect is as in the attached photos ;)

When it comes to components, I tested resistors in the 40R - 820R range. They improve the lower frequency band, which is somewhat spoiled by the notches on the Septum PCB. I ended up using 100R - it's possible the choice could have been better, but I didn't have many resistor values in stock.


« Last Edit: May 01, 2024, 04:38:00 pm by tomud »
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