Peter,
A few questions:
1. How close is your antenna system (or any part of it) to the offending
PC?
2. Does the 746 Pro go quiet with the antenna removed? If should, but if
not, then there may be multiple coupling paths between the PC and rig.
3. Is the PC a stock commercial item with FCCand CE mark labels, or have
you (or someone) done "custom mods"?
In any event, remember FCC Part 15 does not require ZERO emissions, but
emissions limited to a given level and measured at 3m from the unit.
Add-ons and custom mods will often increase the emission levels to well
above the legal limit. Your data shows some of this effect. BTW: When
you opened up the case of the PC, does the case have internal metal
shields? If so, are these intact and making contact as designed? Keep in
mind that reducing emissions is only one way to attack the problem; the
other way is to increase the separation distance between the PC and your
antennas.
Keep us posted, and best wishes on the project.
73, Dale
WA9ENA
> [Original Message]
> From: Peter Laws <plaws@plaws.net>
> To: <rfi@contesting.com>
> Date: 1/2/2010 11:41:28
> Subject: [RFI] Motherboard RFI
>
> Took a little time before the RTTY Roundup to try to track down some
> noise on my IC746Pro. PC was definitely the cause noise is there even
> if I stop the system at the GRUB screen so likely not disk-related
> since they were pretty inactive (but not a sure thing).
>
> Pulled the cables off one by one. Got to the motherboard's COM1 and
> all noise vanished ... well, at least where I was listening on 17 m
> (around 18130.0). OK, a couple snap-on chokes got rid of some but not
> all. Decided to try it on a different serial port (I have a Siig
> 4-port that had two free on the octopus). No noise. Cool!
>
> Not so much. As I started to plug in other things to the motherboard
> (not the PCI cards), I got the same noise there. OK, took off the
> USB-PS/2 adapter from the mouse and ran right into USB. A little
> noise there that a choke didn't really make go away - I'll try moving
> that to the USB hub, but that hub still plugs into the MoBo.
>
> Keyboard is also a problem and it's NOT USB. Put a few snap-ons on
> the cable and the AT-PS/2 adapter (yes, you read right). and again,
> couldn't get rid of it all.
>
> Opened up the case with the system running and poked around. I noted
> an odd thing. With COM1 -- the noisiest -- connected, I could reduce
> the noise by pushing on the top of some of the caps. OK, odd. Am I
> adding capacitance? I pushed on other things, potentially conductive
> and not ... and got the same.
>
> So what could be "loose" that would generate (I assume) common mode
> noise? It's only stuff plugged into the MoBo - nothing plugged into
> the sound, video, or PCI serial card.
>
> Any ideas on a real fix? The noise is only in specific places in the
> spectrum, so I'll play in RTTY RU today/tomorrow, but now that I know
> that it's actually fixable, I'd like to!
>
> Peter
>
> --
> Peter Laws | N5UWY | plaws plaws net | Travel by Train!
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