Derek,
The FCC specifies using a loop antenna for measuring field strength below
30 MHz. Extrapolation can be used if the field strength is measured at two
distances and the factor calculated (it will be close to 1/R^3), otherwise
the default of 40 dB/decade (1/R^2) must be used.
We normally use 30 m and 10 m for the two distances. We have taken
measurements at 300 m, 30 m, and 10 m and found the extrapolation factor to
be consistent. At these frequencies, there is negligible difference with
or without a ground plane.
Regards,
Frank de Vall
Sr. Engineering Manager - Compliance
Assa Abloy ITG and HID Corporation
Y W Leung
<leungderek2000@y
ahoo.com.hk> To
Sent by: emc-p...@ieee.org
owner-emc-pstc@ie cc
ee.org
Subject
Near field H-field measurement.
06/20/2005 11:56
PM
Dear experts,
I am measuring the fundamental and spurious emissions from an EUT ( with
interanl loop antenna) of fundamental frequencies of about 50kHz to 200kHz.
The measuring range of spurious emission is up to 1GHz.
The limits are according to a local standard from a local regulatory body.
The emission limits for both fundamental and spurious emissions are quoted
in electric field ( dBuV/m) and magnetic field (dBuA/m), means
the emissions from the EUT must not exceed the limits of both E field AND
H-field. The spectrum analyzer display (dBuV/m) because it already included
the transducer factor ( magnetic field antenna factor:-55dB/m and the cable
loss).
I am using EMCO 6502 loop antenna for the radiated emission measurement at
10 measuring distance. Since the limits are quoted by the regulatory body
are at 30 and 300 meters distance, so I use extrapolation factor (value of
3) i.e. 20 log (300/10)^3 to determine the corresponding limit at 10
meters from a 300 meters limit which equals to 88.3dB mark up for the
H-field.
My questions are :
1. From theory, if both Tx and Rx antennas are loops antenna, under near
field conditions, H-fields is predominant, so the H-field is inversely
proportional to D^3, so I use the extrapolation factor 3. For H-field
measurement , I just use the reading from the spectrum analyzer plus the
cable loss and magnetic antenna factor (-5dB), and use this calculated
value to compare with the extrapolation value. please correct me if I am
wrong. ( I know there is another approach of extrapolate the measured value
and compare with the limits). Please correct me the method and
extrapolation factor if I am wrong.
2. Since the requirement of the limits are in both H-field and E-field.
Which extrapolation factor (2 or 3) should be used to determine the limit
>from i.e. 300 meters to 10 meters? The reason of supporting if use 2 is
because under near field coditions, the E-field is inversely proportional
to D^2. But now both Tx and Rx antennas are loop antennas, so finally I use
3 to extrapolate the limit of E-field ( i.e. from 300 to 10 meters) and
compare with the spectrum analyzer reading, please comment and/or correct
me if I am wrong.
Thanks advance for your any comment or opinion.
Regards,
Derek Leung.
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