Hello, Chris: I think your colleague made a mistake when converting units of the=20 variables. To solve this problem, we could replace the variables when changing the= =20 units and here it goes: V(dB-V/m) =3D Vu (dB-uV/m) / 10^6 R(dB-Ohms) =3D Ru (dB-uOhms) / 10^6 Then units can be omitted and the equation could be changed to: 20*log I =3D 20*log V - 20* log R 20*log I =3D [20*log Vu - 120] - [20*log Ru - 120] Now we may understand that Ru is not R anymore and 20*log Ru is 171.53=20 instead of 51.53. In this way the two 120 can be cancelled and the final=20 result is 20*log I =3D 20*log uV - 171.53 In fact you could directly use R without converting it to Ru / 10^6. Sometimes I feel that including units into the equations may cause more= =20 confusions and that's why I don't use the units in MathCAD, or I should say= =20 I haven't been able to master the feature by far. Regards. At 11:39 01-8-13 -0700, Chris Padilla wrote: >Perhaps some of you can help me out here: > >In EMC/EMI, we often deal with units of dBuV/m (dB-microVolts per meter). > >What if I wish to convert the dBuV/m to A/m? > >For simplicity, use 120*pi Ohms or 377 Ohms as the impedance of free-space >(assume a plane wave, in the far-field, etc.) so Ohm's law in logarithms= is: > >20*log I (dB-A/m) =3D 20*log V (dB-V/m) - 20* log R (dB-Ohms) (1) > >since our meter reads in dB-uV/m, we need to convert the voltage units: > >20*log V (dB-V/m) =3D [20*log V (dB-uV/m) - 120 dB] (2) > >next, we need to convert the impedance: > >20*log R (dB-Ohms) =3D [20*log R (dB-uOhm) - 120 dB] (3) > >Put (2,3) into (1) and: > >20*log I (dB-A/m) =3D [20*log V (dB-uV/m) - 120 dB] - [20*log R (dB-uOhm) - >120 dB] (4) > > >Now here is where my colleague and I differ: > >He simply cancels out the "120s" and gets as a final answer: > >20*log I (dB-A/m) =3D 20*log V (dB-uV/m) - 20*log R (dB-uOhm) or > >20*log I (dB-A/m) =3D meter reading (dB-uV/m) - 51.53 (dB-uOhm) (A) > >(recall that R =3D 377) > > >I don't think one can simply cancel the "120s" as that changes the >equation. The 120 in (2) must stay or the fact that it is (dB-uV/m) no >longer is true. Once the 120 is "cancelled" the units go back to >(dB-V/m)! Same thing with (3), the units are now (dB-Ohm). > >Therefore, I say that the final answer is: > >20*log I (dB-A/m) =3D meter reading (dB-uV/m) - 171.53 (dB-uOhm) > > >How can I mathematically validate either answer...I am just not seeing it >to make it elegant nor rigorous. Anyone clear me up here? I believe I am >right (I have done some simple circuits to prove it to myself). > >BTW, to be really rigorous, I should be doing this with E & H's but it >doesn't matter as that isn't the argument here I am trying to >understand--we cold drop the /m part on I and V to be more proper but that >doesn't change the problem. > > >Thanks----->Chris > > > >------------------------------------------------------------------ >To unsubscribe from si-list: >si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field >For help: >si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > >List archives are viewable at: > //www.freelists.org/archives/si-list >or at our remote archives: > http://groups.yahoo.com/group/si-list/messages >Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > -- Wang Xiao-yun Shanghai, China, http://go.163.com/philharmania =C9=C6=CA=D8=D5=DF=B2=D8=D3=DA=BE=C5=B5=D8=D6=AE=CF=C2=A3=AC=C9=C6=B9=A5=D5= =DF=B6=AF=D3=DA=BE=C5=CC=EC=D6=AE=C9=CF=A3=AC=B9=CA=C4=DC=D7=D4=B1=A3=B6=F8= =C8=AB=CA=A4=D2=B2=A1=A3 --=A1=B6=CB=EF=D7=D3=B1=F8=B7=A8 =BE=FC=D0=CE=B5=DA=CB=C4=A1=B7 ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu