RFI should be eliminated from one system at a time. An extension of this is to eliminate the individual points of entry, or exit, for the RFI on each piece of equipment before proceeding to the next. It is very helpful to keep a simple written log of the changes and the effects as you proceed, since it is difficult to recall the results after making many changes.
Initially reduce the selected system to it's simplest form. For example if RFI is getting into a hi-fi system consisting of individual components such as tuner, amplifier, cd player and speakers. Disconnect all auxilliary components from the mains and audio leads into the amplifier, and if possible disconnect the speaker leads and use headphones. All leads should be disconnected at the point of entry/exit at the amplifier. You now have an amplifier in its simplest form.
Check for RFI (in the headphones). If it is present it must be entering the amplifier via the mains lead, headphone lead, or directly into the internal stages. Apply suppression cores in turn to the individual leads, checking the result after each lead is treated. If the RFI is not reduced by applying a suppression core do not leave the core on that lead and try the next one. RFI which gets into the internal stages is the most unlikely and is a much more difficult problem. Once an individual piece of equipment has been eliminated add other items one at a time until the whole system has been restored to normal. You will normally find that RFI is only entering at one or two points. Speaker leads (especially long ones) are often a problem. All suppression cores should be placed as near to the entry/exit points as possible for best effect.
Use of a plug-in mains filter is always a good precaution, as is a low pass filter at the rf output of the transmitter. Check that there is a good rf ground on the transmitter.
Computers are notorious generators of rfi in hf and or vhf receivers, and it is usually necessary to reduce the computer system and the receiving equipment to its simplest form when beginning the investigation. A plug-in mains filter is always a sensible precaution on both computer and receiver. This should not be the simple "surge suppressor" which is often used with computer equipment, but should incorporate a "low pass" rf filter as well. Most electronic/computer suppliers stock them.
The integrity of the shielding in the computer case is often very poor - nil if it is a plastic enclosure! If a metal cover is fitted it should have good metal-to-metal bonding to the main frame - it is common to have paint-to-paint contact only, apart from 2 or 3 attachment screws. Where possible clean paint away from the contact area and fit additional screws, say at 75-90mm spacing.
Disconnect all possible external leads from the computer and receiver, including the antenna, especially any interconnecting leads, which should be screened with grounding at the receiver end. At this point you will probably have only the keyboard and monitor attached to the computer to enable it to start.
Switch it on and note how much rfi is getting into the receiver without the antenna connected. I suggest tuning the receiver around the 2-4MHz region initially. If there is no discernable rfi then the most likely point of entry is via the antenna. In this case, connect the antenna, which should use coax as far as possible within the vicinity of the computer. The level of rfi will vary as the computer boots up. When the bootup has finished try switching off the monitor and note any change in rfi. This will indicate how much "extra" rfi is being generated by the monitor, which is traditionally not well screened. The monitor should be located as far as possible from the receiver.
Now begin applying suppression on each lead (one at a time) and noting the effect on rfi. The mains lead may require a few turns wound through a large core such as the FT-240-43/77, but the split beads are easier to fit to terminated cables such as video, keyboard, mouse etc. When you note a reduction of rfi, secure the suppressor on the lead and then continue testing the remaining leads in the same way. You may find that the beads work better at one end of a lead than the other or even one at each end may be necessary. Now reconnect leads to the computer one at a time and carry out the same proceedure as above, noting the effect each time.
Computer power supplies (switched mode) generate rfi over a very broad frequency range. The amount of rfi from them varies greatly, and if that is the main source of rfi in your system it could be worth getting a good "used" one from a computer recycler, as they are a cheap option. Fitting 0.01uF/250VAC capacitors from active and neutral to ground inside the power supply case may also be necessary if they are not already installed.
The suppression effect is additive, so that two similar beads are more effective than one. It is also additive in the suppression of different ranges of frequencies by using two beads of different material. There is no sharp cutoff between the different materials as far as suppression is concerned - it is a gradual change in effective suppression. More details of characteristics and physical size are found on our web site. Suppression cores take many forms, but the most common are the bead/sleeve which is threaded over a lead, toroid cores which allow several turns of cable to be threaded through the core, and split cores which are like a sleeve but are split into two which allows them to be placed around leads without removing connectors. The core material will determine the frequency range for best attenuation, but this is a very broad range and the effect of one material range overlaps another.
The split cores are usually a good starting point as they are easily fitted to existing leads. Two sizes are available, one takes cable upto 6.3 mm diameter and the other takes cable up to 12.7 mm diameter. Split cores use 31 or 43 ferrite material which covers RFI suppression from HF up to UHF. Small diameter cable will allow more than a single turn through the core with consequent increased attenuation. Attenuation is also increased by adding more than one core at the entry or exit point. Often, one core at each end is best. The split cores come with a nylon housing which holds the two halves together and clamps around the cable.
Ferrite materials which are suitable for suppression of RFI are:
I suggest starting with large size toroids in 43 and/or 77 material which can be fitted on the mains leads. In addition, get some of the 2X-43-*P2 and 2X-31-*S2 split beads depending on the cable sizes you have on your system. Once you have determined which types of cores best suit your installation you can always get more as required.