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Re: [RFI] Noisey PSU (Laptop) on HF Decodeing

To: "RFI@contesting.com" <RFI@contesting.com>
Subject: Re: [RFI] Noisey PSU (Laptop) on HF Decodeing
From: "Jim Brown" <jim@audiosystemsgroup.com>
Date: Tue, 29 May 2007 17:57:23 -0700
List-post: <mailto:rfi@contesting.com>
On Tue, 29 May 2007 20:36:17 -0400, Tom Rauch wrote:

>Not only must the PS make noise, the noise must somehow get 
>into the receiving system.

I own five IBM T-series laptops (T20, T21, T22, T41), and none of 
the power supplies have RFI issues. In Chicago, I used them within 
a few yards of receiving antennas. Here in CA, I've got a lot more 
separation to most of my antennas, but also a much lower noise 
level. I DO, however, hear VHF noise (2M SSB) from mouse movements 
that I blame on the video system, and is greater if the laptop is 
plugged into power. 

Noise sources need an antenna to radiate it. Internal wiring will 
radiate if the box is poorly shielded and current loops are large 
enough. A good designer will minimize those loop areas, using 
circuit layout and multilayer boards to create a ground plane 
(stripline or microstrip). 

External wiring will radiate if the designer was careless about 
allowing coupling to it. That external wiring can usually be 
choked effectively with ferrites. You're usually out of luck with 
the internal antennas. 

One thing I'm learning is that some notebooks will run on a fairly 
wide range of DC voltages. My IBM laptops all work fine on the 
13.8 VDC supply that runs my radios -- I just need to be careful 
not to overcharge the internal batteries, and the IBM software 
allows me to monitor battery voltage and current. My wife's Dell 
also seems to run OK on 13.8VDC, but doesn't provide the 
monitoring software. So, if I had a notebook with a noisy P/S, I'd 
spend some time monitoring power supply voltage and current, then 
work up an alternative DC supply to replace it. I've done that 
with these laptops to create a monster UPS that can hold them up 
during a long power outage.  


Jim Brown K9YC

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