[sdiy] EM radiation -- current or power ?

[Magnus Danielson]

On 12/18/2010 08:31 AM, Andre Majorel wrote:
> Does the amount of EM radiation depend on current or power ?
>
> In other words, will two wires carrying 1 W at 18 VAC induce
> more 50 Hz hum in neighbouring audio cables and circuits than
> two wires carrying 1 W at 230 VAC ?
>
> In other other words, are there any drawbacks to putting the
> power transformer and the rest of the power supply in separate
> boxes ?
>
> Thanks in advance.
>

EM radiation can be both of E and H field type in the near field, where 
as in the far-feild the characteristic impedance of 377 Ohm for air will 
sett the relationship. For 50 Hz the near-field range is huge.

So, from an EMC perspective, reduce the loop area of those cables to 
reduce the H-field emission and shield it to reduce the E-field 
emission... and likewise for the receiving end. Lowering or Raisign the 
receving impedance can help in separation of mainly E-field or H-field 
sensitivity of the receiver. Lower impedance reduces E-field 
sensitivity, higher impedance reduces the H-field sensitivity. Since 
this is a reciprocal aspect, the same will be true for the transmission 
side. Matching the impedance to that of air helps to improve transmission.

Essentially, it is always both, but depending on the particulars one 
will dominate. As you increase the effect for a particular voltage, you 
move from H-field dominant to E-field dominant.

Twisting of cables helps for lower frequencues (it has an inherent 
high-pass behaviour and above a cut-off frequencies, in the range of 
tens of MHz, the twisting fails to have this effect) as the far field 
exposure the sum of inversed and non-inversed loops cancels. 
Reciprocally will apply so twisted cables will also have less pickup.

Cheers,
Magnus