[Elecraft] Re: RF Coupleing

[Jim Brown]

On Wed, 16 Jul 2008 13:12:22 -0400, Don Wilhelm wrote:

>The K3 uses only a single wire to connect to the SO-239 jacks.  The 
>return path is through the chassis metal.  If one adds a direct return 
>path from the SO-239, that eliminates the potential for a haphazard path 
>for the return current.  While a chassis return may be good enough in 
>one situation, 

One of the first things they teach you in EMC 101 (they have those courses 
in some of the better EE schools) is that at RF, current will follow coax 
and ignore a much better DC path, while at DC and low audio, it follows 
Ohm's Law.  

Those single wire jumpers are simply bad RF design, and are a major cause 
of the isolation deficiency noted. The connection back through the chassis 
is only good at and near DC, because it adds the inductance (and loop 
area) of that wire jumper and the chassis return to the RF path. Those 
jumpers should be coax. Period. 50/75 ohms doesn't matter much, it simply 
needs to be sufficinet for the voltage and current at those points. 

73,

Jim Brown K9YC