[Ham-Computers] Re Ground Loop Potential

[Duane Fischer]

Phil, 	
	
My computer is disconnected with the wall outlet when I am on the air. None
of the other four computers are affected, nor is anything else in my house.
Unless a computer tower sitting with no wire plugged into the wall outlet
can be damaged by RFI, that can not be my cause Phil.	
	
DBF

----------
From: Philip <ndb_fch-344 at sbcglobal.net>
To: Computers Ham <ham-computers at mailman.qth.net>
Subject: Re: [Ham-Computers] Re Ground Loop Potential
Date: Tuesday, February 06, 2007 11:52 AM

Good morning Duane,

Yes, I'm aware that your computer isn't in the actual ham shack (room), but

in a separate room.  Unfortunately, that doesn't necessarily make any 
difference, or protection from power-line carried RFI.  The powerlines all 
return to the same point, as well as acting as antennas.  Powerlines can 
make pretty good conductors and distributors of RF, especially when 
everything is on the same side of the "Pole Pig" (Transformer).  Think 
"Carrier current" distribution.

Let me cite the following examples that I'm directly familiar with.

When I was stationed at MT Horiatis Greece in 70/71 I got permission to set

up and operate the inactive MARS station in my barracks room.  The rig was 
Collins "S" line with a 30S1 amplifier (their big floor standing puppy). 
The equipment all needed some work to "put right", site maintenance 
installed a 220 Volt outlet in my room for the 30S1 (the entire site ran on

60 Hz US power standards) and a HyGain 14AVQ was roof mounted with suitable

radials (I paid for that antenna myself).

It took about a month to get the station fully operational.  It was first 
tested by checking into the MARS Nets with the basic set-up, sans 30S1. 
That was successful and the following evening I got brave and fired up the 
30S1 amplifier (the only amplifier's I had previous experience with were
the 
30L1's at some of our remote sites).  Carefully following the Collins
Manual 
I got it tuned up on the MARS Frequency and was just checking into the net 
when I got some "Knocks on the door".  It seems that my nice pleasant but 
garbled voice was coming out of the speakers of most of the stereo systems 
in the barracks, even the ones that were turned OFF!

I found it hard to believe that it was coming out of the stereos that were 
turned off and told the owners so. But they insisted it was occurring.  So
I 
QSY'd off the net freq. slightly, sat a gentleman at the mike and told him 
to "test count" (what a clown he turned out to be).  Anyway, sure enough
the 
audio was coming out of the speakers of turned off stereos, all the way
down 
to the far end of the hall (opposite end of the barracks from me)!

I didn't have any ferrite chokes, but I spent the next day hand-winding
some 
RF chokes and with "borrowed" capacitors from the parts bins of our tropo 
site I was able to "silence" most of the racket.

Now you ask, how did it get into the (passive) speakers of turned off 
stereos.  Only thing I can figure out is that the speaker leads acted as 
"antennas" and the RF was rectified in the "unpowered" audio output 
transistors.

Now to a more recent experience that I have here.  You're aware of my local

conditions, Mobile home with no real estate room for antennas etc.,
grounded 
sheetmetal roof.  Most of the antennas are above the roof and shielded from

inside the house by the metal.

The computer is directly grounded to the station ground bus, using good RF 
practice (short braid etc.).  I only run 100 Watts, but even so it's not 
unusual for my beautiful "Melodious" voice to come out of my premium Yamaha

(active) computer speakers.  Not always, it depends on the band in use. I 
suspect that the RF is picked up directly on the shielded lead going to the

subwoofer block that contains the 35 Watt amplifiers.  When QRM'd, my voice

comes out the "satellites".  This even though there is moderate separation 
from the antenna, a sheetmetal "shield"  and only 100 Watts.  It appears 
that it's only affecting the speakers, probably being rectified in the
first 
audio stages, and doesn't appear to affect the computer itself in anyway. 
I've never bothered to try to "clean it up".

Final note:
Now, pump that power up to 1500 Watts (or even a thousand) like you do 
during nets and it's very conceivable that you could get enough RF pumped 
into the speaker lead of your MoBo to damage the sound chips, even with the

computer turned off!  Especially if you consider that in my experience in 
Greece there was enough RF rectified to be fairly loud in PASSIVE stereo 
speakers that aren't known to be efficient.  Certainly enough RF could be 
present on your computer speaker lead to damage an "iddy biddy" audio IC! 
You might consider using some of the battery powered active speakers that 
are available for computers and Ipods, that would at least eliminate the 
possibility of RF coming into the speaker from the power-line and then back

into your computer.   A few turns of the speaker lead through a ferrite 
toroid, right at the computer might also take care of the problem.

Phil, KO6BB
OTR Quote:
"Never Steal A Butcher's Wife"


> Phil,
>
> I do 'not' have a computer in the Ham Shack.
>
> There is never a computer 'on', not even in standby mode, when I am on
the
> air. I do a total power shutdown, always have.
>
> Duane W8DBF

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