TemCell Woes

Dec 20, 2008 12 Replies

In our company we make stuff that must meet EMC requirements. Usually the radiated emissions are the most challenging. We have these checked in accredited test houses, which are expensive.



So we try to pre-check the products to see if there is fair chance of success.



We have no OATS or anechoic room, so we make do with a logper antenna in an empty office room. The ambient radio smog is making this virtually impossible nowadays.



I recently bought a secondhand 3m TemCell, intending to use it the wrong way around (not immunity, but unwanted radiation).



After reading up on the matter I became convinced that it should be possible to get at least a fair -albeit uncalibrated- impression of the radiated spurious of the EUT.



Not so however. If a EUT with known radiation characteristics is placed in the TemCell, all I see is a Flat line on the spectrum analyzer. I might as well have connected a dummy load. Even with a preamp I see nothing.



Connecting the same spec on the same setting on the antenna, the screen lights up. And the EUT does radiate..



Question:



Anybody else out there doing what I do? Am I expecting something that is not there? Does a TemCell not work both ways (reciprocal)?



Or is the cell just broken...and do I need to tear it open for repairs?



Thanks in advance!



Best Regards, Rene


According to this site

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Yes, you should be able to pick up any radiated emissions (RF) from a DUT. Did you try rotating the DUT? otherwise it sounds like bad connectors.

Cheers

Any chance the previous owner left an isolator or attenuator screwed on the port you are expecting signal out of??

Joe Leikhim K4SAT "The RFI-EMI-GUY"© "Treason doth never prosper: what\'s the reason? For if it prosper, none dare call it treason." "Follow The Money" ;-P

Also is the TEM Cell good for the frequency band you are concerned with? May be operating below cutoff..

Joe Leikhim K4SAT "The RFI-EMI-GUY"© "Treason doth never prosper: what\'s the reason? For if it prosper, none dare call it treason." "Follow The Money" ;-P

Thanks for all answers so far...The Cell is rated up to 300 MHz, and the EUT I selected (because it was emasured in a testhouse before, and has known radiations) - happily radiates in the 100MHz .. 1 GHz range.

I would not be amazed by a dropoff above 300 MHz, but a flat line..

@ Martin: Yes, I did rotate the EUT any witch way.. Had representative wiring connected. In free air (using the logper antenna) it (the EUT) even "outradiates" the considerable local radio smog.

I think I indeed need to check the connectors and such.

Thanks! - Rene

A common problem fro N connectors is someone trying to attach a PL259 and splaying out the fingers on the center conductor.

Joe Leikhim K4SAT "The RFI-EMI-GUY"© "Treason doth never prosper: what\'s the reason? For if it prosper, none dare call it treason." "Follow The Money" ;-P

Or using the wrong impedance 'N' connector.

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...but it's a BNC...

After the Xmas holidays I will check if the septum has broken loose from the connector - should have done that in the first place I guess..

The poster I replied to mentioned the 'N' connector. There are also differences in 50 & 75 ohm BNC connectors.

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... but, unlike N-type connectors, the mating diameter of the pin is the same for 50 ohm and 75 ohm BNCs (see

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and then the technical drawings provided by companies such as Amphenol).

If it's a conventional TEM cell with a connector at each end, a quick check would be to feed a signal generator to a power meter, directly and then through the cell. If it's a GTEM cell or some other type with a built-in termination, a network analyser should show what's happening.

Chris

Older BNC connectors weren't compatible. I've replaced enough of the over the years.

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-snip-

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'50? and

75? connectors are intermateable'

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'All versions are fully intermateable'

Evidently, they are now.

Chris

So, all the older BNC connectors have disappeared from the face of the earth? How naive.

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