Maybe I was lucky, to start in EMI at Wang Labs, a company that made
minicomputers. With equipment comparable in size to the distance between EUT
and antenna, I quickly became familiar with the lovely effects of mutual
coupling. It was no great leap to understand that current flowing on a hard
drive bigger than an office desk might in some directions provide gain over
isotropic! (FWIW, my spell checker said this should be spelled "satiric" and
I am tempted not to argue).
This is not rocket science, to repeat the phrase. Even when it IS! (A good
majority of the EMC scr*wups I've seen have be from ignoring High School
physics.)
To the subject: YES.
DO use your comb generator to perform site comparisons. It's important that
techs, engineers, managers and executives understand one cannot plop equipment
down in any chamber and test to within 1 dB (hah!) of a fixed limit to achieve
a "passing" result. One needs evidence, and this will do it. I recently
bought a surplus "50 MHz" comb generator -- two, in fact, one for ME -- on
ebay. Shakes confidence, it does.
We might profitably discuss the deplorable habit of using chambers as storage
rooms... Never fear; the comb generator can provide undeniable evidence why
this is a bad habit.
One further and slightly off topic comment: a paper some years ago suggested
AF measurements done in the chamber where antennas are used, thus compensating
for differences in chambers. This is particularly applicable to EN14, DO-160,
MIL-STD-461 tests where chambers are not even semi-anechoic and the
measurement distance is only 1 meter. One might even look at using a test
object consisting of a typical UUT and its cables.
Cortland Richmond
KA5S
----- Original Message -----
From: Bill Owsley <mailto:[email protected]>
To: Bill Stumpf <mailto:[email protected]> ;Derek Walton
<mailto:[email protected]>
Cc: Grace Lin <mailto:[email protected]> ; [email protected]
Sent: 11/20/2009 5:54:46 PM
Subject: Re: 3-meter Chamber Site Comparison
Ah.... But back in the day when mini-towers were first coming out.we found we
could pass that 3 meters and fail at 10 meters. And then discovered that
these amazing technical marvels were launching plane waves that did not roll
off the usual expected 10.5 dB. Around about that time I think there was IEEE
EMC article about electromagnetic missles or some such fanfare. Now the
military actually has a microwave crowd control device that gives the target a
sensation of being set on fire. It's amazing what you can do with the flat
side of a mini-tower when energized just right. ps. another pound of spring
fingers solved that problem.
- Bill
In the event of a national emergency, click on the following links to provide
directions to your duly elected mis-representative.
http://www.usa.gov/Contact/Elected.shtml
or...
https://writerep.house.gov/writerep/welcome.shtml
http://www.senate.gov/general/contact_inf! ormation/senators_cfm.cfm
--- On Fri, 11/20/09, Derek Walton <[email protected]> wrote:
From: Derek Walton <[email protected]>
Subject: Re: 3-meter Chamber Site Comparison
To: "Bill Stumpf" <[email protected]>
Cc: "Grace Lin" <[email protected]>, [email protected]
Date: Friday, November 20, 2009, 12:38 PM
Hi Bill,
I take exception when you say data taken at 3m will often show
surprising
difference if measured at 10m even if set up identically
My observations, with typical small objects not great big things, is
that
emissions are very close at both distances apples to apples.... What does
change is that the mast height at which the emission maximizes changes..
Also, 10 metre testing MISSES emissions from EUTs at higher mast
heights.
IMHO it reaffirms my opinion that the current tests are regulatory, and not
oriented towards problem mitigation. It's time we got back to just doing whats
necessary to stop practically observed problems.
Darn, that turned into a soapbox fast....
Happy holidays,
Derek.
Bill Stumpf wrote:
> Grace,
> Any good Calibrated Noise Emitter at the correct frequency range is a
good
source for checking your OATS or chamber. There are "Round Robin" programs
offered by a coup! le groups that can anonymously show results of your lab
compared to other labs participating. When testing actual products what must
be remembered is that every EUT will look different from one site to the next
due to setup and test distance, among other variables. Also, EUT's are
generally "distributed" RF sources and data taken at 3m will often show
surprising difference if measured at 10m even if set up identically. Bill
>
>
------------------------------------------------------------------------
> *From:* [email protected] <http://us
mc396.mail.yahoo.com/mc/[email protected]>
[mailto:[email protected] <http://us.mc
96.mail.yahoo.com/mc/[email protected]> ] *On Behalf Of *Grace Lin
> *Sent:* Friday, November 20, 2009 6:54 AM
> *To:* [email protected] <http://us.m
396.mail.yahoo.com/mc/[email protected]>
> *Subject:* 3-meter Chamber Site Comparison
>
> Dear Members,
> I am asked to perform site comparison and would like to hear your
comments.
> Should I purchase a signal source? If yes, do you have any
reference? I
do have a COM-POWER comb generator and don't know if it is good enough for the
purpose.
> Should I use an EUT for site comparison?
> Thank you.
> Best regards,
> Grace Lin
-
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