[Elecraft] RF issues with logging computer / internal keyer
Jim Brown
jim at audiosystemsgroup.com
Wed Jul 1 11:21:07 EDT 2009
On Tue, 30 Jun 2009 11:15:50 -0700 (PDT), Julius Fazekas n2wn wrote:
>My first attempts were OK, but laced with problems. Often I had to drop from
>5 watts to 2 to keep my logging computer from locking up in a key down
The fundamental cause of this problem is the improperly designed serial cable
for the K2. It uses parallel wires, and is shielded. A proper cable designed
for RF immunity requires TWISTED PAIRS.
I ran into exactly the same problem you did in Chicago, when my antenna for
80M and 160M was a long wire that ended in the shack and was fed against a
counterpoise. That put a current maxima in the station, right next to the
computer, and the associated magnetic field coupled into the serial cable and
locked up the computer (in my case, at about 8 watts).
The fix was quite simple. I replaced the parallel wire cable between the K2
and the computer with CAT5, using one pair for each signal circuit. I also
terminated the return of each pair to the SHELL of the DB9 rather than pin 5,
fixing a pin 1 problem that also couples RFI. With that simple fix, I could
run my K2 to my Ten Tec Titan at full power with no RFI.
The wiring of a conventional serial cable is documented in
http://audiosystemsgroup.com/HamInterfacing.pdf
Note that the K2 serial cable is NOT a standard serial cable, so the wiring in
the applications note applies only the wiring between the K2 and the computer.
No change is required for the wiring between the K2 and KPA100.
Note also that a cable shield provides NO magnetic shielding, only ELECTRIC
shielding. Twisting is required to prevent magnetic coupling, and also greatly
reduces electric field coupling. Nearly all coupling in the near field of an
antenna is magnetic, except at higher HF frequencies. It would only be
necessary to shield this cable if you are using high power above 15MHz with an
antenna that is within a few feet of the rig.
73,
Jim Brown K9YC
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