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]
