[Elecraft] DX Doubler/K3 issue

[Jim Brown]

On Mon, 15 Mar 2010 19:44:31 -0400, Joe Subich, W4TV wrote:

>This is a common problem with DXDoubler due to differences 
>in chassis potential between transceivers.  TopTen Systems 
>have even published an application note concerning the issue: 
>http://www.qth.com/topten/apnote9.htm 

>Their solution, as you have already discovered is to add 
>two transformers in the headphone line between one of the 
>transceivers and the DXDoubler. 

They also suggest a low resistance bond between one transceiver 
chassis and the other BEFORE resorting to transformers. That's 
my fundamental recommendation as well. It also helps to get 
power for both radios from the same AC outlet, or from outlets 
whose green wires are bonded together (for example, multiple 
outlets in the same steel backbox).

There are two common causes of this problem. One is the 
potential difference between the two radios. Bonding and using 
the same outlet will fix that.  

The second common cause is a magnetic field that couples into 
the loop between the two radios and the DXDoubler. That field is 
most often is established by a big power transformer very close 
to the rig (either the linear supply for the rig(s) or the power 
transformer for the power amp. The field is coupled to the audio 
two ways. First as the induced voltage, and second as current 
flowing into a pin 1 problem at either (or both) end(s).  

Magnetic field coupling can be minimized by reducing the loop 
area. In this case, the loop consists of the audio cable shields 
and the radio bonding path. This may sound counter-intuitive, 
but that means making one of those cables longer so that the 
cable for radio two follows a path from the DXDoubler to radio 
one and then along the bonding conductor to radio two, and 
lacing the cables together with ty-wraps. 

Adding transformers can also break that magnetic loop, but if 
the transformers are unshielded (and most inexpensive 
transformers are unshielded) the transformers may pick up the 
magnetic field and couple it as a differential signal.  

73,

Jim Brown K9YC