[RTTY] RF-Interface-soundcard feedback

[Bill, W6WRT]

ORIGINAL MESSAGE:

On Fri, 20 Feb 2009 17:13:48 -0500, Curt Nixon <cptcurt at flash.net>
wrote:

>I took my linear out of mothballs today in the hopes of gaining a few dB 
>on rtty.  The soundcard and interface are creating feedback from the 
>linear so it all becomes unstable and not usable. 

REPLY:

I had similar problems here and three things fixed them:  

First, have the transceiver and the computer sitting next to each
other so you can run a short ground wire between the two chassis. All
of your audio cables act as a short antenna with the transceiver at
one end and the computer at the other. Adding the ground wire shorts
the ends of the "antenna" together.

Second, install a powdered iron toroid on each cable going into the
computer - keyboard, mouse and audio cables in particular and possibly
the AC cord. I originally tried ferrite clamp-on chokes but they did
no good at all. Powdered iron toroids cured the remainder of the RFI.
Use ones large enough that you can wrap a half-dozen or more turns
around each one. I used mix #6 from Palomar in the T-130 size for the
smaller cables. You will need a larger size for the AC cord.

Third, if you are using coax to feed your antenna directly you will
almost certainly have RF on the outside of the coax. Install a
choke-type balun somewhere in the coax, preferably at the junction of
antenna and coax, but if that is not practical, break the coax
somewhere before it comes in the shack and install the balun there.
The idea is to keep the RF current on the outside of the coax out of
the shack.

With all three measures in place I can run 1500 watts on any band and
have  no RFI at all. All my antennas are within 50-100 feet of the
shack.

RFI can be very vexing. Good luck.

73, Bill W6WRT