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The Case of the Morse Code Oven Latch - RF Cafe Forums

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Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement


Kirt Blattenberger
 Post subject: The Case of the Morse Code Oven Latch
Posted: Sun Jun 24, 2012 10:22 pm 
 
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In the May 2012 edition of QST, "Hands-On Radio" column author H. Ward Silver has an article titled, "RFI Hunt." It is a very interesting saga of discovering, then troubleshooting, then correcting a very strange and unlikely issue. In a nutshell, Mr. Silver installed a new 105-foot dipole antenna about 30 feet over his house and, unbelievably, when he operated CW at 30 or 40 meters (at 25 W or higher), the door safety latch on his self-cleaning oven would energize during the dot or dash transmission. He works his way through many iterations of line chokes, bypass capacitors, and shielding within the oven to finally stop the interference. His assumption throughout the process was that RF energy was being coupled into the oven electronic controller and triggering the solenoid driver circuit.
 
The illustration he provided implies that his "shack" is in the next room. Since with a good match the ladder line emits very little radiation, and the dipole is so far above the house, is just seems unlikely to me that the problem is RF induction. Maybe it really is.
 
I wonder whether the problem instead is a spike of just the right characteristic being sent on the AC house wiring. No doubt he has a very good ground bonding system so the signal would have to be conducted on the neutral or even the hot wire each time the transmitter is keyed. The kitchen wiring, per modern electrical code, would not share an AC circuit with another room, but it is possible that the two Romex cables run parallel to each other in the wall or ceiling. If the house is older, and/or rewiring has been done, it's anyone's guess how the wiring is run.
 
One sure test would be to place a dummy load at the transmitter and see if the problem persists. If so, then RF coupling would be entirely ruled out, and a simpler solution might be to plug the radio into a different circuit. Having read Mr. Silver's column for many years, I know he is a very smart guy and therefore might have already taken that step, but even smart guys occasionally forget something. What say ye?

p.s. I cannot provide a link to the story because only ARRL members have access to the electronic version, and otherwise you would have a hard copy to refer to.

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