The usual parameters given with TEM cells are field uniformity and input impedance (or VSWR). Although they may appear to be very good up to the declared frequencies of operation, which depend on the transversal size of the cell, they are only good for empty cells. Presence of any conductor in the cell significantly reduces the upper frequency limit, by introducing strong resonances and/or change of propagation mode. Vendors usually won't tell you that. That is true even if the size is smaller than usually stated max useful dimansion of 1/3 of the height between the septum and the floor.
Measuring from the outside, say S11 or S21, will usually not show large change of reflection or transmission of the cell. However, you can easily see the peking of the field if you want to use the cell for calibrating a field probe. The probe will measure strong peaking (+ or -) of the field, depending on the relative orientation and size within the cell and type of the probe (E or H). I've extensively studied it as a part of a research project about ten years ago in Austrian Research Center Seibersdorf (the report might still be available somewhere there). This is the largest limitation on use of TEM cells. Unless you have large mismatch of impedance at the tapered sections, which you can easily check with TDR (a network analyzer in TDR mode can also tell you that), I suspect that you see what I have described. Neven > > Hello all, > > I have a problem regarding E and H field generation inside a TEM cell. I > have built a standard cell with Zw of 150 Ohms, which I need to generate > a known magnetic field strength around 90 MHz. The separation of the top > and bottom conductor is 25 cm, the overall length is about 130 cm. > > But I don't get the expected results, only at lower frequencies up to 20 > MHz the results are usable. At higher frequencies I suspect that I get a > standing wave inside my room, or that the cell dimensions in itself are > so large compared to the wavelength, that I get an unwanted resonance > inside the cell. > > I have been looking for material about this in the 'net, but so far I > have not found pay dirt..... > > What am I missing here??? > > Greets > Jochen Feldhaar > Telejet GmbH > > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ > > To cancel your subscription, send mail to: > [email protected] > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Ron Pickard: [email protected] > Dave Heald: [email protected] > > For policy questions, send mail to: > Richard Nute: [email protected] > Jim Bacher: [email protected] > > All emc-pstc postings are archived and searchable on the web at: > http://ieeepstc.mindcruiser.com/ > Click on "browse" and then "emc-pstc mailing list" ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
