[Elecraft] Re: RF Coupleing

[Don Wilhelm]

Vic,

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, let's suppose that the builder did not tighten the SO-239 
mounting screws adequately, or the mounting hardware has begun to 
oxidize, then the return path becomes more 'iffy'.  Attaching a return 
wire from a lug on the SO-239 to the board ground provides a fixed 
return path that is not dependent on the chassis.  Note that the return 
currents for the two antenna connections can possibly interfere with 
each other if there is any small voltage differential on the chassis 
(there always is a differential, the question is just how small a 
differential is tolerable).

73,
Don W3FPR

Vic K2VCO wrote:
> I was told by Elecraft that one of the reasons for the poor isolation 
> between the antenna ports is this: if you don't have the subreceiver 
> installed, then the KAT3 subreceiver port is unterminated. So either 
> install the KRX3 :-) or plug a 50-ohm resistor into the port -- it's 
> on the top rear corner of the KAT3.
>
> I haven't tried this (I have a KRX3).
>
> I will add that I am not terribly happy with the isolation between the 
> ports even with the subreceiver installed.
>
> Don Wilhelm wrote:
>> As an alternative to coax, try using twisted pair - pull a couple 
>> pairs out of a short length of CAT-5 cable.  One of the pair would be 
>> grounded at each end (will require adding a solder lug at the SO-239 
>> jack).
>>
>> 73,
>> Don W3FPR
>