From: "Tony (remove \"_\" from email address)"
Subject: Cheap, home made TEM cell?
Date: Fri, 22 Nov 2002 14:11:14 +1000
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For a long time I've contemplated making a TEM cell for precompliance immunity
testing (or maybe that should be pre-precompliance testing). For my own use I
didn't need to worry about infringing patents etc, so I was relatively free to
copy any commercial product.
I liked the GTEM design with its simple 50 ohm feed and generally cheap
construction, but it seemed that the 50 ohm impedance came from the high
capacitance from septum to shell, whereas I'd get more V/m from a higher
impedance design and a step-up transformer.
Enter the Eurotem 2 - compact, balanced 200 ohm load and a superior test volume.
But the enclosure needs to be fully lined with expensive ferrite tiles.
Another approach seems to be to combine the two - a long cell similar to the
GTEM (with resistive septum loading and absorber across the mouth), but
1 more slowly tapered (both to reduce problem reflections if I don't get it
exactly right, and to maximise the test space for a given mouth area),
2 containg four striplines like the Eurotem; narrow and further in from the
shell, to get the impedance back to 200 ohms, to get the most from a small
3 Also like the Eurotem, line the mouth with ferrite tiles instead of pyramids,
to maximise the test volume (still expensive, but much cheaper than lining the
whole thing) and to extend the bottom frequency limit, albeit with slightly
Which leads to the questions - can anyone direct me to shareware or software
that can calculate the impedance of a twinaxial transmission line (the Eurotem's
quad arrangement is actually symmetric) in which the septa are striplines at 45
Also, does anyone know where I can buy cheap ferrite tiles?
Finally, have I got it all wrong?