Keying Interface & RF?

David Palmer
Wed, 07 Aug 1996 09:04:42 -0600

Eric, WD9GGY writes:
> My problem concerns the homemade keying interface that I put
> together: It works great on 80, 15, & 10m, BUT when I try to use it on
> either 40 or 20m it seems to get some kind of RF (?) interference....

Hi Eric, 
I recently built up the parallel printer interface described in the 
appendix of the TRLOG manual.  At first I too had 
problems in that when the Icom 735 was putting out greater than about 
50 watts on 40 or 15, the Icom would lock in transmit mode. 
Guess the 2N2222 had reached its FCC-mandated RF exposure limits, eh?

My interface circuit was constructed right on the pins of the DB25-M
connector--no box--no shielding--no shielded cable--so I wasn't too
surprised at this outcome.  To fix it I just replaced the twisted pair
wire going from the DB25 connector to the radio with some shielded 
microphone cable, and just at the point where this cable leaves the 
DB-25 connector, I wrapped it several times around a ferrite core.
This core is one salvaged from an old IBM PC Convertible's LCD adapter
so I don't know anything about its characteristics nor do I know whether
it or the shielded cable did the trick, but my suggestion would be 
to find some ferrite and try wrapping your cable around it nearest the
PC and see if it helps. 

Cheers es 73, 
Dave Palmer, N6KL/0, in geopolitically challenged Boulder, Colorado

I'd rather be surfing the ionosphere!