I have a problem when I transmit on HF...

Q Arnold, AA3HO, asks, “I have a problem when I transmit on HF. When my wife is on the Web my transmissions apparently garble the incoming and outgoing data, preventing her from reaching her desired sites. The computer does not lock up, and the dial-up connection isn’t lost. What tips can you give me to locate and fix the problem?”


A There is a whole chapter in the ARRL RFI Book (http://www.arrl.org/catalog/6834/) on computers—I can’t reproduce it all for you here. It covers interference both ways—to and from computers.

Here is a plan of action. The book points out that many of the same fixes for RFI to computers are those for RFI from computers. These may be found in an article on the TIS Web site: http://www.arrl.org/tis/. Click on RFI/EMI in the menu, then choose “Computer Interference.” Try those fixes.

Also, from personal experience, make sure everything is grounded. All of your ham equipment should be grounded together. Ground your computer; a strap made from discarded coax cable shield running from a screw on the computer’s metal case to ground works nicely. A good ground point for the computer is the little screw on the cover plate of your wall power socket. Test this by shutting off the circuit breaker to that plug and use an ohmmeter between the screw and the ground wire hole. At the very least, make sure your PC is attached to your station ground.

Incidentally, the problem may not be the computer at all and may be your external modem, if that’s what you use. Use the same techniques on it if possible.
 
Finally, make sure you have no RF coming back into the shack from the antenna. If, per chance, you are using an end fed random or long wire antenna, you may have to try another type. For balanced antennas, such as dipoles, consider adding a 1:1 balun at the feed point. 

From QST September 2000 

I’m teaching an Amateur Extra course and the term “common mode” has come up in our discussions of interference ...

Q Jerry Seligman, W7BUN, asks, “I’m teaching an Amateur Extra course and the term “common mode” has come up in our discussions of interference. Unfortunately, the definitions in the License Manual and in The ARRL Handbook fail to explain it in easily understandable language. Can you provide a brief explanation?”

A The best definition of “common mode” appears in the new ARRL RFI Book. Basically, common mode is a situation where a given signal appears equally on more than one conductor in a cable, with the same voltage and the same current flowing in the same direction relative to a particular device (see Figure 2). The return path of the signal is typically outside of the affected cable.

A good example is RFI to a telephone via the line cord. The 4 (or 6 or 8, depending upon the phone system) conductors act as a multiconductor, end-fed antenna.



Figure 2—In (A) there are differential-mode signals conducted between two wires of a pair. This signal is independent from earth ground. In contrast, a common-mode signal (B) is in phase on all wires that form the conductor (this includes coaxial cable). All wires act as if they are one wire. The ground forms the return path.

From QST July 1999