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Accomplished DX-er
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Registered: 02-2009
Location: Northern Ireland
Posts: 403
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CPU QRM !


I wonder if anybody has any ideas how I can cut down the level of noise picked up on my transceiver from my desktop computer switch mode power supply.

Whenever my PC is on, 80M and any frequency below for that matter is more or less unusable because of the noise that the computer PSU emits, this only happens when I am using coax to feed my antenna, which I am now since removing the ladder line fed doublet antenna. I thought coax was supposed to shield the cable from picking up noise, it's strange that the twin feed picked up less, almost no noise, I can't go back to the twin feed as the antenna system is different now and needs to be used with coax.

I have tried routing the radio equipment mains power cables to a far away socket, I have ferrite barrels on all the cables in the radio room, computer equipment included, but still massive QRM while the PC is on. I have even removed the Earth cables from the mains plugs to see if that would cure it (NOT RECOMMENDED!)

Any ideas would be appreciated, by the way the switch mode PSU in the PC was an upgrade from the original which I no longer have, I wonder is there any other way to power the PC without having to use this nuisance switch mode stuff and without having to buy a laptop!.

Last edited by mikefox whiskey, 11/Aug/2009, 10:44 am


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Patrick
15/Mar/2009, 6:54 pm Link to this post Send Email to mikefox whiskey   Send PM to mikefox whiskey
 
Hot Fusion Profile
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Registered: 03-2008
Location: South West England
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Re: CPU QRM !


Is it actually the PSU?
Computers are full of square wave digital signals at all sorts of troublesome frequencies as I'm sure you know.

What sort of antenna id=s it? Is this the Windom you spoke of?

It may be that the co-ax is picking up signals due to the presence of RF on the screen, in which case try a choke balun on the co-ax - do a search for 'ugly balun' if you aren't already familiar with this, or wind the co-ax a few times through a ferrite ring. The 'ugly balun' is the better method, but the ferrite ring will at least show whether the problem can be cured using this method. If it does work, you could then go ahead and wind the co-ax balun for real.

Other than that, no suggestions.

There is a company who makes linear PSUs for computers, but it was a while back - maybe serch for that if all else fails?

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Andy - 26 FB107
15/Mar/2009, 9:32 pm Link to this post Send Email to Hot Fusion   Send PM to Hot Fusion
 
mikefox whiskey Profile
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Accomplished DX-er
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posticon Re: CPU QRM !


Hi Andy, I'm guessing it's the power supply, when the PC is in standby mode there is QRM in fact even when the PC is shut down noise is still picked up by the transceiver on certain frequency's, mainly below 5MHz, at worst it's +30 on the S meter!, it only disappears when I turn off the power supply at the back of the tower (or pull the mains plug out).

As regards my antenna, it is the windom I talked of and it does have an isolator on the coax (I'm quite pleased with this antenna though that's another story).

Anyway thanks for the suggestions and I will have a look at the linear power supply's you mentioned. Cheers

Last edited by mikefox whiskey, 18/Mar/2009, 10:46 pm


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Patrick
18/Mar/2009, 9:43 pm Link to this post Send Email to mikefox whiskey   Send PM to mikefox whiskey
 
John193 Profile
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Registered: 11-2008
Location: South Glos
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Re: CPU QRM !


Try running the mains lead tthrough an RF choke Patrick that cures 99% of EMI from PC's. Thats another reason i hate pc's. About 10uh should kill it. Regards John193

I think I mentioned QRN sensaticity when you mentioned you were hanging a windom, The balanced windom doesnt suffer the same noise fortunately.
26/Mar/2009, 12:04 pm Link to this post Send Email to John193   Send PM to John193 MSN
 


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