Great dialog, just the path that I was hoping would develop.
One thing I have learned since the question was first asked, all
biconical antennas are not made equal. The original antenna calibrated
at an outside test organization, exhibited a 5 dB difference between the
vertical and horizontal polarizations at 3 meters in the frequency range
of 30 - 50 MHz.
Another antenna subsequently calibrated at the same organization had
less than 1 dB difference between v and h, 1m and 10 m. This outcome
was more in line with the expected outcome of the calibration per C63.5
which stated "minor variations with polarizations and geometries" where
geometries is understood to mean test distances.
Don Umbdenstock
Sensormatic
----------
From: chasgra...@aol.com
To: 72146....@compuserve.com; chris_dup...@compuserve.com;
emc-p...@ieee.org
Subject: Re: Antenna Calibration/Site Attenuation
List-Post: emc-pstc@listserv.ieee.org
Date: Tuesday, August 26, 1997 5:20AM
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
