On Mon, 2007-04-23 at 13:18 -0400, John K Gotwals wrote:
> RF wise, my situation is not very desirable: My shack is on the second
> floor, one end of a 110 feet dipole fed with open-wire line is 15 feet from
> my computer, the feedline droops some but does parallel the dipole on its
> run to the shack, and I sometimes operate at the kW level on 160 thru 10
> meters.
>
> When I would operate high-power, my computer would beep and the keyboard
> would be disconnected. In addition, I would also hear a hum in the computer
> connected Bose speaker system. The computer keyboard, optical mouse, and the
> Navigator digital mode interface are all USB connected to my computer. My
> system is now clean, and it took very little in the way of filtering to
> clean it up, but it took a fair amount of time to experiment with various
> configurations until I found one that works.
>
> I recommend a four page article titled "RFI Page" at
> http://www.aa5au.com/gettingstarted/rtty_rfi
> In this article, the author states that he has not had good results with the
> clip-on types of inductors, and that has been my experience also. Ferrite
> toroids and rods seem to work better, and the rods are easier to use with
> cables which already have attached connectors. I purchased 4 inch long
> ferrite rods (part # R-050400-33) from CWS ByteMark
> http://www.cwsbytemark.com/ and they did the job for me. Although the
> optical mouse, Navigator interface, and the keyboard all had small circular
> inductors around the cable, the keyboard was being disconnected. I wrapped a
> portion of the keyboard cord, next to the computer, around a ferrite rod,
> and taped the wire in place. I did the same thing for the stereo cord
> running from my soundcard output to my Bose speaker system, but at the Bose
> end of the cord. Now my computer no longer beeps when I operate at the kW
> level, and the hum is absent from the Bose speakers.
>
> One final comment - Although type 33 material is not normally used at HF,
> other more suitable ferrite material does not appear to be available in the
> form of rods.
>
> John, N9JG
For RF filtering, ferrite with a much higher permeability (and loss at
HF) is preferred to what one would use for a transformer core at the
same frequency. Essentially the higher the permeability the better so
long as the DC or LF current in the wire doesn't saturate the core and
kill off that HF loss.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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