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.
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
