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Ignore first message!!
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This has turned into an interesting discussion.
1. On the question of Class A vs Class B etc.. I vaguely remember that the
genesis of the FCC limits for Class A & Class B were indeed derived from the
concerns of installation. As I recall, extensive research went into
examination of the sensitivities of television receivers and apatment
complexes and as a result the 3M test distance and Class B limits were born
for residential applications. Of course this was back in the 70s when 300 ohm
cabling for TVs was in vogue!!.
Naturally the FCC recognized that not all EMI problems would be resolved by
design/testing alone. Chris is correct in his statement that the emissions
test bears little resemblelance to reality. The FCC ( & others) insist on
warning statements etc.
(I believe the VDE has the honor of having the FIRST legal EMI requirements.
In the original 0871 standards they were more concerned with conducted limits
- hence their severity. The Class A radiated limits had relief in certain
bands that allowed for very high emissions indeed.)
2. Mutual coupling (?)
Reading the e-mails on enhanced emissions at 3M vertically polarized
generated some thoughts.
2.1 I have discussed the issue of calibrating an antenna using ANSI C63.5
(horizontal only) and testing using ANSI C63.4 (v&h) with one of the authors
involved in BOTH standards. The answer I get consistently is that:
a) we need to calibrate in "free space" ( or close to it)
b) calibrating an antenna with V& H makes the test look like the
NSA and
hence site anomilies dominate the vertical readings.
2.2 For my type of products (floor standing), Vert 1M seems to be worst case.
This has more to do with the install than mutual coupling - I test at 10M.
My humble opinion only.
Charles Grasso
EMC Engineer
