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Subject: Re: [RFI] PC keyboard RF immunity?
Date: Thu, 24 Dec 1998 09:30:38 EST
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Here is a copy of an article I wrote for the "Frankford Radio Club"
newsletter. You may find it helpfull in resolving your computer RFI problem.
73, es Merry Christmas
John - W2YR
QUIETING THAT NOISY COMPUTER By: John Gagen, W2YR
Having had some of my DX searching capabilities impeded by my noisy computer,
I began to investigate the cause and the cure for all of the "Birdies" being
generated by my computer and being picked up on my HF receiver. I was also
receiving excessive RFI on the computer when operating on 160, 80 and somewhat
on 40 meters
Having tried to solve this problem with the common split ferrite (snap-on
type) chokes sold at flea markets, radio shack, etc, and found that they are
NO GOOD for lower freq. HF attenuation. If you look at the impedance curve of
freq. vis impedance, you will find that these number 43 type materials are
good for VHF attenuation. However, they offer very little attenuation at the
lower HF frequencies. Therefore, now knowing
the problem, I went to the electronics catalogs and selected a ferrite
material that had a high impedance for the 160 and 80 meter amateur radio
bands. The result was number 75 and number 77 material from Amidon Inc. I
bought several sizes of these ferrite cores from Amidon. The only problem is
that "SPLIT" type ferrite cores in a snap on configuration don't seem to be
available from any manufacturer. I assume this is due to the fact that the
impedance properties would most likely be severely degraded by splitting this
higher density type of material. Results: I choked all my computer VO cables
with these units and I no longer had RFI problems interfering with my
computer. This is also true concerning emanations that were coming from the
computer. I can now take a portable AM broadcast radio and place it on top of
my computer case and not have it plagued with interference. However, I still
hear the din of other neighborhood computers whining in the background. The
following is the source and part numbers for the toroid chokes I used:
P.O. Box 25867
Santa Ana, CA 92799
Telephone: (714) 850-4660
Part numbers are:
small ferrite bead, number 75 material, FEB-75B-101, $4.50 PK/12
larger core unit, number 77 material, FB-77-1024, $2.00 EA
very large core, number 77 material, FT-240-77, $9.00 ea, (great for cables)
For those of you who would like to go the ultimate route and use super high
quality RFI proof I/O cables, the Black Box Corp. will make you special cables
for your particular VO needs. They provide a very nice ordering form (chart)
that will help you with whatever specific cable connector assembly you need. I
spoke with Black Box customer service on the telephone and they said they
would be happy to provide potential customers their "Custom Cable Form" for
Remember to specify the following when ordering your cables:
*Cable type -foil and copper braid shielding.
*Special instructions- EML/RFI.
* Keep cable length away from 1/4 wave multiples at amateur freqs. (calculate
* Do not attach shield to connector hood on the peripheral end of the cable.
(cut drain wire)
Black Box Corp.
1000 Park drive
Lawrence, PA 15055-1018
Telephone: 800-321-0746 (24 hours, 7 days a week)
In regards to problems with close neighbors complaining about your radio
interfering with their computers. I advise all hams to take the "HIGH ROAD" on
this issue. Tell them to take their problem up with the computer manufacturer.
DO NOT, under any circumstances, work on a neighbors computer even if he is a
close friend. The reason for this is that at a later date if and when any
problem occurs (especially a data crash) you will be blamed. FCC rules on
computer RFI are simple and clear cut. Computers are part 15 devices and as
such they may not interfere with any licenced radio service and must accept
As I said previously, most interference problems occur due to pick up on the
I/O cables. The worst of these is the keyboard cable and the mouse cable.
Choke these cables with the proper toroid chokes and your most likely
disappear. In severe cases, problems will MFJ electronics sells an RFI proof
computer keyboard (model MFJ-551) for $39.95 if you need one you can order one
directly from MFJ at:
MFJ Enterprises, Inc.
Some new stunts computer manufacturers are now doing is to use unshielded
cables for I/O lines and eliminate the RFI power line filter, on the AC input
of the computers switching power supply. I have seen some computer power
supplies, missing this filter, that generate noise all over the spectrum. The
only remedy then is to install an AC line filter inside the switching power
supply. This should only be done by a qualified service technician and not by
the average computer owner. If you need one of these AC line filters, they are
available for $5.41 ea.(part # 562-857-03/47) From:
Telephone (800) 346-6873
I have also noticed that some peripheral devices are generators of RFI/EMI
themselves and in almost all instances, I have coincidentally found unshielded
plastic cases. A remedy for this problem is to spray the inside of the plastic
case with a special shielding paint. I had to do this on my monitor to
suppress emanations. Beware that this spray is nasty stuff. If you do use it,
make sure you spray in a well ventilated area and wear a face mask (The stuff
stinks). Pro Distributors sells this product called "EMI-RFI Shield" catalog
number 10-4807 for $29.95 for a 12 oz Spray can. Their address is as
5135 69th St.
Lubbock, TX 79424
Remember that EMI/RFI tempest situations are specific and each installation
will have to be dealt with on an individual basis. The above tips I have given
should help guide you in "Quieting That Noisy Computer."
Meanwhile, good news comes in waves, recently computer manufacturers have
petitioned the FCC to relax the emanation standards presently imposed upon
them! Does this possibly mean we are all in for another round of fights with
the neighbors over RFI while big business makes a greater profit.
Administrative requests: rfi-REQUEST@contesting.com