[Elecraft] Capicator Size for filtering across battery power

[Jim Brown]

On 10/7/2012 7:06 PM, David Moes wrote:
> That is interesting.    I suspect that they are reacting to the
> elecromagnetic field created by rapid current fluctuations in the DC
> lines rather than picking up RF.

NOT electromagnetic fields, just simple Ohm's Law.  There's IR drop on 
the power cable between the battery (or power supply) and the rig, and 
if the rig is running full power, peaks of at least 20A, with an 
envelope that follows SSB modulation.  If you are powering accessories 
from that same battery and have audio connections between those 
accessories and the rig, the IR drop (modulated SSB envelope) will 
appear between the chassis of the accessory and the chassis of the K3, 
and will be added to the audio.  That will SOUND like RF feedback, but 
it is NOT.  W8JI told me about this several years ago.

There are at least four good fixes for this issue.  I've set up and 
tested all of these, and all of them work.

1) Don't power accessories from the same supply that feeds your rig.  
Instead use a separate wall wart for them.

2) Power accessories from the same supply, but use a simple power 
splitter (Y-cord) at the RIG end of the power cable to get the accessory 
power.

3) Power the accessories from the battery or big power supply, but bond 
from chassis to chassis of all interconnected equipment with short, fat 
copper.

4) Power the accessories from the battery or big supply, but use a 
monster capacitor across the terminals right at the rig.  When I say 
monster cap, I'm talking at least 10,000 uF. That's big, and it ain't 
cheap.

73, Jim K9YC