Along the same line of discussion, what antenna factor is used when
making radiated emissions measurements below 80 MHz in accordance with
CISPR documents when the dipole antenna is tuned to 80 MHz? For
example, if you are measuring an emission at 40 MHz with an antenna
tuned to 80 MHz, do you apply the 40 MHz or 80 MHz antenna factor? It
seems to me under these conditions of measuring one frequency while
tuned to another that neither antenna factor is correct and that a new
set of antenna factors needs to be determined for the 30 MHz to 80 MHz
range with the antenna continuously tuned to 80 MHz.
______________________________________________________________________
Jim Hulbert Tel: 203-924-3621
Senior Engineer - EMC Fax: 203-924-3352
Pitney Bowes email: [email protected]
P.O. Box 3000
35 Waterview Drive
Shelton, CT 06484-8000 U.S.A.
______________________________ Reply Separator _________________________________
Subject: Antenna Correlation
Author: "TDonnelly" <[email protected]> at SMTPGWY
List-Post: [email protected]
Date: 2/18/98 3:02 PM
I think there may be a little confusion here, let me see if I can clear it
up. In your e-mail you refer to calibration of a site. My original message was
related to measurement technique for radiated emissions measurements. This is a
separate issue from site attenuation measurements which are often called site
calibrations.
When making measurements to CFR 47, Part 15, and using ANSI C63.4-1992 there
are 2 requirements that conflict for measurements at lower frequencies. The
first allows broadband antenna use as long as the results can be correlated to a
1/2 wave tuned dipole. The second requires a 1 to 4 meter height scan. At lower
frequencies where the tuned length of the dipole exceeds that which would allow
use of a 1 meter lower limit a conflict occurs as the dipole cannot go down to 1
meter. When broadband antennas are used at these lower frequencies, and a 1
meter lower limit is utilized, the result cannot be correlated to the dipole.
The only way to establish the required correlation is to adjust the lower limit
of the height scan range to that which would be utilized by a dipole. The key
here is that both antenna must be electrically centered near the same elevation.
This will typically produce a lower measured level as you noted, however I do
not believe this will cause the regulatory evaluation problems as you pointed
out. Since the date which the original message was posted I had a detailed
conversation with Joe McNulte at the FCC's Office of Engineering and Technology.
He concurred with the need to establish correlation and restrict the height scan
of broadband antennas accordingly.
Keep in mind there is an underlying premise to the FCC/ANSI standards that
everything should correlate back to a predictable set of conditions
(measurements on an OATS with dipole antennas). You are allowed to test in an
anechoic chamber of GTEM as long as you can correlate your results to an OATS,
you can use broadband antennas as long as you can correlate your results to 1/2
wave tuned dipoles.
When testing to the CISPR family of documents (including the harmonized
versions) an 80 MHz tuned dipole is called out for testing at frequencies 80 MHz
of below. Again a broadband antenna is allowed but now the results must be
correlated to the 80 MHz tuned dipole at these frequencies. Using a 80 MHz tuned
dipole the need to restrict the lower limit of the height scan is not required.
As with the 1/2 wave tuned dipole I have found the best correlation occurs when
the electrical centers of the antennas are near the same elevation.
Tom Donnelly
EMC Engineer
Lucent Technologies
[email protected]
