Date: Sun, 17 Nov 2002 00:04:21 +1100
From: Andrew Tweddle
X-Mailer: Mozilla 4.79 [en] (Windows NT 5.0; U)
Subject: Re: Cheap, home made TEM cell?
Organization: Client of Alphalink Australia Pty. Ltd.
"Tony (remove \"_\" from email address)" wrote:
> 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
> linear amplifier.
> 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
> lower performance.
> 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
> degree angles?
> Also, does anyone know where I can buy cheap ferrite tiles?
> Finally, have I got it all wrong?
You are probably applying overkill to your precompliance problem.
A good open space is quite OK to do a frequency scan, I do them in my
back yard with an EMC antenna.
Testing of immunity is best done with the relevant "zapper".
Ferrite tiles are great but they are meant for low frequency absorption.
An open range does a better job at less cost, unless you have a high
If you do then one old trick is to line under the plaster of your lab
with "chicken wire" for the high frequency shielding.
As for your transformer try AppCAD it does the calculation for
transmission line impedance.
If you have a military size budget, disregard everything I have said and
buy yourself an EMC company :)