Forgot!
E Fields from a true dipole do drop off at the inverse cube
rate also, It's just that the fields almost always get
referenced to a ground or ground plane which makes them
drop off for all practical purposes at the rate of the
inverse square.
- Robert -
> From: Y W Leung <leungderek2...@yahoo.com.hk>
> Date: Tue, 21 Jun 2005 13:56:25 +0800 (CST)
> To: emc-p...@ieee.org
> Subject: Near field H-field measurement.
>
> 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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