Near field?! You're way inside near field below 200KHz.
At those fundamental frequencies, you are correct.
Magnetic fields drop at the rate of inverse cube from a
dipole. You didn't say the size of the EUT. However, if it
is smaller than 1/5 the distance to the antenna, you can
correctly assume the "nonradiating" magnetic field is
dropping at inverse cube.
I'm reluctant to rely on any accuracy using a loop to
measure E-Field at that low a frequency. Should use a
field probe. The E Field should drop off as the inverse
square function.
In the range of 50-200KHz, measure E-Field and H-Field at
two distances and you should see those ratios.
- Robert -
On Tue, 21 Jun 2005 13:56:25 +0800 (CST)
Y W Leung <[email protected]> wrote:
> 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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