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Re: [RFI] RF Quiet Laptop for Mobile Console?

To: "RFI List" <rfi@contesting.com>
Subject: Re: [RFI] RF Quiet Laptop for Mobile Console?
From: "Jim Brown" <jim@audiosystemsgroup.com>
Date: Mon, 22 May 2006 07:51:40 -0700
List-post: <mailto:rfi@contesting.com>
On Thu, 18 May 2006 13:12:59 -0500, doc wrote:

>Can anyone recommend a laptop for this context?

>1.  Sitting on top of a metal box that will house
>     HF & VHF-UHF gear as well as a scanner.
>     (must not radiate tons of RF noise)

Most ham gear is shielded, and radiates little RF trash. It's the 
antennas that have the potential for problems. 

>3.  Runs natively from 12vdc.

My IBM T22 and T41 work quite nicely from a Targus 12v universal 
power supply that I bought several years ago. The T41 lacks a 
serial port, so I put one of these in it. It works fine. We've 
used both on two Field Days as our CW logging computers. 


At my old QTH, I had several HF antennas within 10 ft of the 
laptop and never heard RFI on HF. I DID, however, hear trash on 2 
meters CW or SSB when I moved the mouse or panned the screen when 
the laptop was powered from AC, and I was never able to suppress 

Expect to do some serious work on your serial interconnection with 
any computer that you use, because virtually all have pin 1 
problems, and an antenna as close as in a mobile rig is likely to 
excite them. A pin 1 problem is when the cable shield (or signal 
return of an unbalanced circuit) goes to the circuit board rather 
than to the chassis. To fix it, use the shell of the DB9 as your 
shield connection and signal rather than pin 5. 

It also helps a lot to use twisted pair for the serial cable. I 
recommend CAT5 (or CAT6/7). Shielding helps a bit above about 20 
MHz, but makes no difference below that. Use one pair for each 
circuit in the RS-232 connection you are using. For a typical 
serial connection where only pins 2 and 3 are used for control 
with pin 5 as the return, use orange for pin 2, blue for pin 3, 
orange/white and blue/white for return (the shell of the DB9). If 
your rig(s) require other circuits, use the remaining pairs to 
connect them. 

The above precautions will virtually eliminate all RFI from your 
ham gear to the serial connection. With a "standard" serial cable, 
my Elecraft K2 would lock up the computer at about 12 watts 
loading a long wire running a few feet away. With this wiring, I 
could crank my Ten Tec Titan up to full power (1.5 kW) into the 
same antenna. 

I've never heard RFI FROM a serial connection to ham gear, but 
Ethernet puts out lots of RF trash throughout the HF spectrum, and 
should be avoided if possible. If you must use it, the RFI app 
note on my website will tell you how to suppress some of it. 
Follow the link to Publications. 


Jim Brown K9YC

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