[sdiy] EM radiation -- current or power ?

Magnus Danielson magnus at rubidium.dyndns.org
Sun Dec 19 09:46:50 CET 2010


On 12/19/2010 09:31 AM, Andre Majorel wrote:
> On 2010-12-18 17:47 +0100, Magnus Danielson wrote:
>
>> 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.
>
> Thanks for the replies everyone, that was instructive. It seems
> that the answer is that it'll induce less hum into guitar
> pickups but more into line level cables. And using twisted pair
> shielded cable for power would help (twisted for line, shielded
> for guitar pickups).

You do want the guitar cable to be twisted too. :)

> The same cable will carry two 18 V secondaries but they'll
> obviously be in phase and I don't know how much cancellation
> will occur. A new application for star quad ?

Notice that star quad is "constellation" which just in the cross-section 
analysis beat a normal pair of cables, but both would gain additionally 
from being twisted. For star quad to have the intended effect, you want 
the same amount of current in each of the wires as well as same voltage. 
If you don't do that you just created two pairs of wires with larger 
cross-section area and some minor cancelation.

> Leaving the rectifier bridge and smoothing caps close to the
> voltage regulator is attractive because AC is easier on the
> switches. I also have a vague notion that AC is more flexible
> than DC but I'm not sure exactly how.
>

The first line of defence caps usually sits near the rectifier. If you 
are picky you also want to shunt them with smaller caps to save the 
large caps as well as reducing the cross-section area for those higher 
frequencies being generated by the diodes.

Cheers,
Magnus



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