Hi Lucian, I'm assuming you're talking about the wide-band pyramidal absorbers usually found in anechoic chambers. Their attenuation characteristics are typically measured one of three ways, depending on the frequency range of interest: a. coaxial waveguide: for very low frequencies, from DC to the cutoff frequency of the first TM mode (depends on dimensions, ~up to 200 MHz), in TEM mode b. square waveguide: for "mid" frequencies, around 300-500MHz, in TEM mode c. plane wave methods: two horns suspended on an arch positioned above the absorber at a height at least equal to the far-field distance, >1 GHz. All three methods try to emulate free-space far-field conditions in which a plane wave (E and H vectors perpendicular to the propagation vector) impinges upon the absorber, after which the backscattered reflection is measured. There are some other methods under development but those are not widely practised (yet).
In all setups, a carefully calibrated vector network analyzer is used to measure the reflection coefficient of the system. Other instrumentation can be used as well but can be cumbersome to use, inaccurate and typically does not allow for the use of a gate to isolate the absorber response. The basic procedure is to measure the reflection of the test setup without the absorber as a reference (i.e. the response of the back of the coax or square waveguide, or a metal reference plate in the case of the plane wave method) and measuring the reflection levels with the absorber in place. The difference between the two is the absorption of the absorber. Although the basic principles are simple, obtaining valid measurements with these setups is very tricky. There are a lot of measurement and setup issues to consider. There is not an official standard for performance measurements of absorbers that I'm aware of. The de facto absorber test standard referenced by most absorber manufacturers is a university of Michigan report describing the arch method (plane wave). Hope this helps. -Robert Robert Bonsen Principal Consultant Orion Scientific email: rbon...@orionscientific.com URL: http://www.orionscientific.com phone: (512) 347 7393; FAX: (512) 328 9240 --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
