Re: [PSES] CISPR11 Rad Magnetic Field Emissions Limits

Thu, 17 Jan 2019 19:53:39 -0800

The bottom line here is that the usual emission standards don't take into account that the non-radiating near H-field falls off as the cube of distance (an issue with BPL, remember) and the wave impedance in this area is nowhere near 120 Ohms.   Measuring at 3 meters using 51.5 dB as a conversion factor just doesn't  hack it.
Some years ago, I had occasion to look for incremental fall-off with distance from a wheel mounted tire pressure sending unit. EVERYTHING was in the non-radiating near field, but I could find no way to take that into account in the standard.    I needed an H-field antenna with curves for that distance not derived from E-fields much farther off. 

Cortland Richmond
Semi retired at Belcan,
Grand Rapids MI


On 1/17/19 6:22 PM, John Woodgate wrote:


Well, no it isn't. The definition of the decibel is based on power, 
and is only extended to voltage and current by adding the assumption 
that the resistance is constant.  There is no formally-correct way of 
extending it to resistance and even less to impedance. There is a 
fatal flaw:  Consider 10*log(W) = 20*log(V) - 10*log(R), but 20*log(V) 
= 20*log(I) + 20*log(R). The multiplier is 10 sometimes and 20 at 
other times. It gets worse rapidly if you try to accommodate complex 
impedances.



You can calculate 20*log(Z1/Z2) but you shouldn't call it 'decibels', 
any more than if you calculated 20*log(your height/my height). Of 
course, if you did a similar calculation for your income/my income, it 
would have to be 10*log(yi/mi), because 'money is power'.


Best wishes
John Woodgate OOO-Own Opinions Only
J M Woodgate and Associateswww.woodjohn.uk
Rayleigh, Essex UK
On 2019-01-17 22:17, Ken Javor wrote:

Re: [PSES] CISPR11 Rad Magnetic Field Emissions Limits It would be 
completely correct to say that the 51.5 dB factor is dB above one 
ohm, which is the difference in magnetic and electric field units.


Ken Javor
Phone: (256) 650-5261


------------------------------------------------------------------------
*From: *John Woodgate <j...@woodjohn.uk>
*Reply-To: *John Woodgate <j...@woodjohn.uk>
*Date: *Thu, 17 Jan 2019 22:11:50 +0000
*To: *<EMC-PSTC@LISTSERV.IEEE.ORG>
*Subject: *Re: [PSES] CISPR11 Rad Magnetic Field Emissions Limits




To fill in, that 51.5 dB  = 375.84 numerically and comes from the 
impedance of free space, 120*π = 376.99 ohms. which is actually 
51.5266 expressed in 'decibels'. Actually, expressing an impedance in 
'decibels' is a illegal operation 'but it works, so why bother?'.



Best wishes
John Woodgate OOO-Own Opinions Only

J M Woodgate and Associates www.woodjohn.uk <http://www.woodjohn.uk> 
<http://www.woodjohn.uk>

Rayleigh, Essex UK

On 2019-01-17 21:57, Ken Javor wrote:


     Re: [PSES] CISPR11 Rad Magnetic Field Emissions Limits You are
    making too much of it.  If the limit is in terms of dBuA/m, you
    use the loop magnetic field antenna factor, which is 51.5 dB less
    efficient than the loop electric field antenna factor.  Of
    course, this is based on a far field assumption, and that is all
    you have available.  But the magnetic field antenna factor is
    fundamental; it is the loop electric field antenna factor that is
    based on the FF assumption, so you need not worry at all.

     No worries!

     Ken Javor
     Phone: (256) 650-5261



    ------------------------------------------------------------------------
    *From: *"Kunde, Brian" <brian_ku...@lecotc.com>
    <mailto:brian_ku...@lecotc.com> <mailto:brian_ku...@lecotc.com>
    *Reply-To: *"Kunde, Brian" <brian_ku...@lecotc.com>
    <mailto:brian_ku...@lecotc.com> <mailto:brian_ku...@lecotc.com>
    *Date: *Thu, 17 Jan 2019 21:27:15 +0000
    *To: *<EMC-PSTC@LISTSERV.IEEE.ORG>
    <mailto:EMC-PSTC@LISTSERV.IEEE.ORG>
    <mailto:EMC-PSTC@LISTSERV.IEEE.ORG>
    *Conversation: *CISPR11 Rad Magnetic Field Emissions Limits
    *Subject: *[PSES] CISPR11 Rad Magnetic Field Emissions Limits

    Reference CISPR11/EN55011:2016 version.

     If you are an expert at the Radiated Emissions test from 150Khz
    -30Mhz for Class A Group 2 equipment, I could really use your help.

     I haven’t done this test in over 25 years. I refurbished our
    Active Loop antenna and placed it 10 meters from the EUT.  I
    believe the data values read by our receiver is dBuV/m.  However,
    the limits as it appears in Table 10 are in dBuA/m.  How do I
    convert?

     I have an old 1999 copy of this standard that shows the limits
    with the exact same frequency ranges but the limits are in
    dBuV/m.  Do I just use these limits or do I have to somehow
    convert the receiver data to dBuA/m.

     I know a straight conversion between dBuV/m and dBuA/m can be
    difficult because above 4Mhz we are in the far-field, but below
    that we are getting into the near field. And the impedance
    calculation can be difficult to obtain.

     Am I making too much of this or just take a simple conversion
    based on the differences between the two versions of the standard?

     Thanks for the help.

     The Other Brian

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