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diletante Mar-31-00 02:26 PM
Two of my computers share a table with my amateur radio station. RFI is always a problem at some level and I have had to work on both the radio/antenna system and the computers. Part of the solution has been to avoid using both at the same time, but these days I am using one of the computers to operate the radio and the kids are always on the other machine, so it is time to clean house.

The biggest annoyance is the speaker systems. The RF gets detected in the amplified speaker system and makes a racket when you key the transmitter. Of the 2 machines, the one that is not connected to the radio is the worst by far. The computer can be turned off, and the sound system will still do its thing. There is no evidence that the RF is getting into the computer via the cables.

I fixed a keyboard problem with one of the Radio Shack ferrite cores on the cable (about 6 turns through the cable), but I tried these cores on the speaker wires with no useful effect. I plan to get some type 43 torroids or beads (the big kind, like the ones that ship on monitor cables and some keyboard cables) and try those. Small capacitors could also be added at the terminals, but it would be better to install them inside, which I don't want to do. I wonder if anyone else has experience with these problems? (How about you Larry? Are you a ham?)


1. RE: RFI (Ham)
lbyard Mar-31-00 06:03 PM
In response to message 0
Rob, One cannot change the laws of physics. You must shield, choke, replace, or move your speakers/wires. There are speakers that are designed not to interfere with monitors. I would presume that if they are shielded well enough to prevent the transmission of a magnetic field to a monitor, they are shielded well enough to protect them from external RF. Of course, you could build Faraday shields around your equipment... I would try moving the speakers and using shielded wire. Hang them from the wall somewhere. Or you could move yourself. That is, get two desks, set one of them in front you and the other behind, and buy a swivel chair. Don’t underestimate the ability of one of those RF impulses to interfere with a speaker cable and to inject enough noise into an unpowered speaker to hear it. The way I check on that kind of problem and track-it-down is with a homemade probe connected my spectrum analyzer. I would be quite concerned if I were you about this problem. I wouldn’t be so worried about noisy speakers as the possibility of your equipment interfering with emergency receivers in hospitals, mobile equipment on trucks, etc. Your rig must be emitting all kinds of harmonics. I have seen those sort of problems and they can be rather difficult to fix. I would inspect my coax connectors and the standing wave on the transmission line. Regrettably, I am not a ham. When I was very young I got bored learning the Morse code and never got my license. So, you will have to take this advice from a lowly Electrical Engineer. Larry

2. RE: RFI (Ham)
diletante Mar-31-00 07:20 PM
In response to message 1
The RFI thing isn't as dangerous as it sounds. I am only running 1 to 25 Watts and my antennas are matched (SWR very low). I am meticulous about this and have had the radios on spectrum analyzers (radiating trash RF is sort of like refusing to bathe -- tough on friends and neighbors). It looks like the particular speaker system is very susceptible. It is a SoundWorks system with a subwoofer and amplifier. It is further from the radio gear than the other sound system, which only makes a slight whispering noise when I transmit data.

It is fairly common to run across a device that won't reject radiated or out-of-band RF. (I could tell you stories.) The FCC doesn't require much in the way of RF immunity. The premise is that it would be too expensive to require all electronic items to be robust in radiated fields. Instead, manufacturers are required to meet less stringent standards and then required to repair equipment that malfunctions on a case-by-case basis. Most consumers have no idea of their rights in such cases. The normal reaction to RF interference is to blame the radiator. At the same time, look how hungry everyone is for wireless electronics.

Well, I was guessing that you might have some first hand experience with RFI and computers. The ferrite cores are expensive and it would be nice know what was the best thing to buy. I will try to get back to this post with some info so others might benefit.

By the way, I am very impressed by how you take on all comers here. Also, this is the weekend to sneak the new motherboard and all the rest into the old case. It will be another adventure for me. I don't do enough of this to ever be good at it. Have a fine weekend.


3. RE: RFI (Ham)
Ra Byn Dec-12-00 06:54 PM
In response to message 2
See if you can locate a book called

"Audio & Video Interference Cures" by Larry Kahaner published by Hayden Book Company. Publishing date is 1979.

I checked the copy I have out at the local library.

The cover says. How to eliminate and prevent intereference caused by...transmitters,CB radios,home appliances,atmospheric noise sources neon lights, hair dryers and other offenders.

I hope you find it & it helps

ra byn james

4. RE: RFI (Ham)
diletante Dec-12-00 09:35 PM
In response to message 3
Thanks for the help. Since writing this post I have nearly eliminated my problems. The cure was to double up on the ferrite cores and wind the coils carefully. I used 2 stacked cores (the rectangular type sold by Radio Shack) on each of the leads enterring the amplifier/sub-woofer box of my speaker systems. Each lead was wound about 10 to 15 turns through the stacked cores, and the ends of the coil were kept separated -- this was important. Information on how to wind the coils (RF chokes) was included with the cores.

The ARRL also puts out a book on curing RFI, but I don't have it. I should probably make a trip to the library.


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