When I transmit I hear myself loud and clear on the speakers ...

Q Matt, WA5DE, asks, “When I transmit I hear myself loud and clear on the speakers that are mounted within my computer monitor! Can you help?”

A Ouch! Having the speakers in the monitor really makes it tough. In general, components such as monitors don’t pick up much RF—wires do. In the order of likelihood, your primary the suspects are:

• Audio/monitor cable
• Monitor ac line cord
• Computer ac line cord
• Other wires connected to the computer, such as printer, modem, etc.

These wires usually pick up RF in the common mode, meaning that all wires in a bundle pick up the same signal, acting as if they were a single wire. The cure is—no surprise here!—a “commonmode choke.”

Common-mode chokes are usually made from ferrite material, often a donut-like torodial core. You don’t mention the frequency involved, but on HF, you usually need at least a few turns of wire through the toroid to be effective. I usually suggest that hams use #43 ferrite for the upper HF range, #75, #73, #77 or -J material for the lower HF range, with #43 being a good all around material that works well on HF and VHF. I recommend an F(T)-140 size core for smaller wires and an F(T)-240 size for larger wires. That monitor cable, though, might be a bit big for even an F(T)-240 size. (140 = 1.4 inch outer diameter, 240 = 2.4 inch.)

Some hams attempt to use one of those “split bead” ferrites. Unfortunately, you don’t get enough inductance from one or two of them to do much for HF. About 4 or 5 in series might be effective on VHF and up.

What I would first do is simplify the system. Disconnect all peripherals (printer, modem etc) and see if the problem is still there. (It probably will be.) Then, try FT-240-43 ferrite cores on all the ac line cords. I would also buy a RadioShack 15-1111 ac line filter and try it on the line cords. We are approaching it backward, with the least likely steps first, but it will be hard to do an effective job on your thick monitor cable, so we are exhausting every other possibility.

One troubleshooting step is to determine the power threshold that causes the RFI before and after each step. If you do something that makes the RFI appear at 50 W rather than 20 W of transmitter output, you know you did something that did have an effect. You can then try a better version of the same fix (more turns on the ferrite, etc). If it makes an additional improvement, you are on the right road. If it does not make an additional improvement, you have RFI caused by several different problems, each one of which may have a different power threshold.

If all else fails, try moving or re-orienting your antenna, or placing a number of split-bead ferrites on your monitor cable.

The ARRL Web page has some RFI packages that will help, with company addresses and the like. In addition, The ARRL RFI Book has an excellent chapter on computer interference.

From QST August 1999

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