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RFI
WN4AZY
#1 Posted : Friday, October 26, 2012 8:09:51 AM(UTC)
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This article is reprinted from the Volume 2 number 2 issue of LOGic Lines. © 1993 by Personal Database Applications, 1323 Center Drive, Auburn, GA 30011-3318 770-307-1511 http://www.hosenose.com. All rights reserved.

RFI in your shack
By Michael Klein, NV1L


LOGic users have long known the advantages of having a computer in the shack for full-time ham radio operation. And with the release of LOGic version 3, having your computer in the shack for everyday operations is essential for the serious DXer. The bad news is that computers can generate interference that causes problems for the HF band (and sometimes the VHF band as well). This can actually impede the operation of your station and your ability to log DX. The good news is that there is something you can do about it. Occasional computer users won't realize that there is a problem until they try to use the computer for that weekend contest. By then, it's too late.

What is RFI?
RFI is an acronym for Radio Frequency Interference. Computers generate RF, most often manifested as beeps, pops or a steady state whine. You may hear these anomalies across a wide range of frequencies in your HF receiver. The first thing we want to do in our RFI investigation is to clearly identify the problem, then pursue a systematic method of elimination. From experience I have found that starting with the 20 meter bands works well. Since most hams who own a beam antenna have one for the 20 meter band, this will certainly be an important band to clear up, not to mention the fact that every major DXpedition will be on 20 meters.

Boot up your computer. Unplug your antenna lead from your HF rig and sweep the entire band. Note anything unusual on a pad of paper. Next, turn off the computer and make note of any anomalies which are now gone. These are the frequencies which your computer is most likely interfering with (bear in mind that some household appliances like the refrigerator can cause RFI also). Now, reconnect your antenna lead, tune to one of the problem frequencies and turn on the computer. You should hear the machine boot, and there probably will be a correlation between the buzz activity and the RFI. This is good, because it absolutely identifies a problem for us to solve.

Chokes and Capacitors
The main weapons of the RFI investigator are Chokes and capacitors. RFI can best be illustrated as a "leak", where for some reason interfering signals are leaking into an area where they don't belong. There are two ways to stop a leak. You can plug up the hole, or you can drain off the pressure. Chokes will stop the RFI flow in a circuit, and capacitors will shunt (drain) it off to ground. For the most part, we are going to concentrate on using chokes for solving this RFI problem. Radio Shack sells a "snap" torriodial type of choke which is excellent for stopping computer RFI in the shack. Palomar Engineers also sells an RFI choke kit which contains an excellent cross section of chokes which will work for this application.

In this next operation, make sure you don't have any programs running which may be damaged by turning the machine off without closing the files. You may want to end any Windows sessions for this reason. Boot your machine, and turn your HF rig to one of the problem frequencies. Confirm that you are indeed hearing RFI in the receiver. Disconnect, one at a time, the following devices from your computer while it is running:

Modem: If you have an internal modern, disconnect the phone line and observe the results. Make a note of the effect, if any, on your pad. If you have an external modem, disconnect the serial cable at the computer end. Make a note of the results.

Printer: Cycle the power and note the results. Then unplug the printer cable at the computer end. Note the results.

Mouse: Unplug Mickey next. Note anything different.

Keyboard: First, place your hands over the keyboard and listen for any difference in the RFI level. If placing your hands on the keyboard increases the RFI, then you have a coupling problem. Unplug your keyboard and make a note of what happens.

Monitor: Now is the time to unplug your monitor from the computer. Again, note everything.

Anything else: Unplug any other outboard devices from your computer at this time, including any serial devices, like the rig controller. Keep accurate notes by disconnecting one device at a time and recording the results.

You now have an accurate record of each device's RFI contribution. Most, if not all of the RFI will probably now be gone. There are further measures that we must now take.

Solutions
With the CPU still running with nothing connected to it, take a piece of stranded wire, or wire braid (a shield braid from a piece of coax is perfect), and connect it between the computer case and your station ground (you do have a station ground, don't you?). Note any difference in the RFI level. In some cases, galvanic contact between the computer and station ground can result in an increase in RFI. If this is the case, you may want to try moving the computer's line cord to another AC circuit. In general, connecting the computer to a separate line circuit from the rig will help keep the line from introducing RFI up into the rig's power supply and into the audio. The computer's switching power supply is nothing more than a transmitter (oscillator). If connecting the computer to station ground causes more trouble, isolate it with a .01 uf disk capacitor in series. I like to use metal film capacitors for this type of application. Use at least a 200 volt cap. Place the capacitor at the ground end of the wire. Try changing the microprocessor speed using the "turbo" switch. The microprocessor's clock will transmit spikes according to its frequency. Occasionally, even after an aggressive RFI program has been initiated, residual RFI will impede station operation. Changing the clock speed will shift the RFI center frequencies somewhere else, and you maybe able to work a particular portion of the spectrum which otherwise would not be possible.

Make sure that all of the computer's case screws are tight, and that all of the blank panels are installed. There should be no unnecessary holes in the case. If you have an older "flip-top"style case, you should install bonding wires between the top and bottom portions of the case. The hinges are not sufficient bonding for RFI prevention.

Going back to our note pad, lets take a look at what we can do in each case of RFI.

Internal modem: Try wrapping about twenty turns of the phone line around a ferrite rod. You can find a perfect ferrite rod inside of an old balun. Tape the turns with electrical tape. Keep the rod as close to the modem as possible. I once had severe RFI problems and almost all of it was caused by radiation down the wire going to the RJ-11 jack. If you don't have a rod, use in series several of the snap-together chokes from Radio Shack. This should clear up this problem. Another tactic to use in extreme cases of internal me-

External Modem: About the only thing you can do is either disconnect the modem cables when not in use, or install chokes on the serial cable and phone line. The serial cable may be fairly thick; you won't be able to wrap turns. Just install several chokes in series along the cable at the computer end, and tape them in place. Treat the line as the internal modem above.

Printer: After moderns, printers are known to cause the most problems with RFI. Note whether or not having the printer's power on or off makes a difference. I've seen cases where just turning the printer on made it much worse, or better! This may make the difference between having to disconnect your printer every time you want to operate or not. If you can live with the printer, fine. If not, treat the cable with chokes like the external modem.

Mouse: You can wrap turns of the cable around a rod or chokes. Nothing special here. Fortunately, mouse cables are usually long.

Keyboard: Desolder the DIN connector from your keyboard connector (noting the wire pinout and colors!). Slide ferrite top torroidal chokes onto the cable with the connector removed. The RFI kit that I mentioned earlier contains torroids which will fit perfectly over most keyboard cables. Reconnect the DIN connector, and tape the chokes in place right at the connector. Don't be conservative here. Use six to eight of the chokes. You don't want to have to do this one again! Use the snap chokes as an alternate.

Monitor: Most monitors already have a choke in the data cable. You may have noticed a bulge in the cable - this is the choke. Try adding more in series if you want to, but chances are they won't do any more than the factory-installed Choke. If you have a real RFI problem with the monitor,' ~6U'li have to shield it. Most people will find that the monitor is "clean", and doesn't contribute much if anything, to the overall RFI problem. Here's how to shield your monitor:

Purchase some kind of shielding paint, like the kind sold by the Master Bond company. Another alternative is copper foil, but we'll cover the paint method first. Unplug your monitor from the AC mains, and disconnect it from the computer. Open the monitor cabinet and clean the inside with alcohol. Take a long piece of #22 AWG stranded wire, and strip about ten inches of it bare. Glue the tip of the stripped end somewhere near the top of the inside of the inside of the monitor case. Paint tae entire inside surface of the monitor case with the conductive shielding paint, covering the bare wire in the process. As the paint dries, it will become sticky and allow you to push the wire down into the paint. Apply as many coats as necessary in order to completely cover the surface of the monitor cabinet uniformly. Be sure to cover the outside lip of the cabinet, so contact is made when the case is reconnected. An alternative to painting the lip is to glue copper foil along the joints of both cabinet sections and paint over the foil inside the cabinet to assure contact. In my experience I have found the paint to be extremely conductive, and have measured zero ohms between any point in the cabinet and the end of the wire. Reassemble the cabinet and connect the drain wire to your station ground, or computer chassis case. Be sure to bring the drain wire out through a vent hole!

If you decide to use copper foil instead of paint, use strips instead of large sheets. It will be easier to install that way. Clean the inside of the cabinet with alcohol first to remove any residues which may impede adhesion. Apply contact cement to both surfaces (some copper foil is sticky-backed and ready to apply) and completely cover the inside surface of the monitor cabinet. Overlap the sections, and make ventilation holes afterwards by pushing a sharp object through the molded holes in the cabinet from the outside. Next, solder the junctions together as best as you can. Naturally, some of the contact cement will have spread out through the overlapping strips of foil, but you will be able to solder most of the joints together. It is not necessary to solder the entire length of each joint, just enough to insure a good shielding effect. Take a piece of stranded wire and solder it to your shield and reassemble the cabinet. Make sure that you have contact at the joint between cabinet parts.

You will now notice a big difference in the RFI generated from your monitor if you have an older one with no inline choke. This shield will make the difference between your being able to use the computer in the shack, or not.

Using these RFI fighting techniques will allow you to become a more effective operator by using your computer in the shack. Your enjoyment of the hobby will increase as you decrease the amount of RFI emanating from your computer.
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