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Re: [CQ-Contest] FT-1000D HUM in RX audio

To: "Bob AD5VJ" <ad5vj@ad5vj.com>, <CQ-CONTEST@contesting.com>
Subject: Re: [CQ-Contest] FT-1000D HUM in RX audio
From: "Tom W8JI" <w8ji@w8ji.com>
Reply-to: Tom W8JI <w8ji@w8ji.com>
Date: Wed, 18 Apr 2012 21:15:37 -0400
List-post: <cq-contest@contesting.com">mailto:cq-contest@contesting.com>
Hi Bob,

General tech info for contestors, I just hope it doesn't destroy the remote 
station and potential cheating complaining that is so interesting and 
exciting. :-)

> I redid the entire station grounding scheme after taking all equipment off
> of the bench, got behind the table and installed a grounding buss made 
> from
> an unused shelf support bracket. I installed a ground from each piece of
> gear to this home brew grounding bar and took the bar to ground about 8 
> feet
> from the shack to a grounding rod.
> The hum still persisted. I then realized I had not included the computer 
> in
> the grounding scheme. After connecting the computer to the grounding bar 
> the
> hum immediately went away.

Years ago, when we did broadcast studios, we all knew better than to have an 
UNbalanced audio line that was grounded to chassis at both ends, in separate 
pieces of gear. If you look at radios, even radio manufacturers figured that 
out. They almost always float the low level audio line from chassis except 
at one point along the path.

The problem is the common point to chassis should be at the cabinet entrance 
in RF environments, or at least be bypassed heavily to chassis for RF at the 
entrance. Otherwise, RF can get in the case on the mic lines.

Even if you look at microphones, the shield generally floats from any 
potential chassis path to avoid ground loops.

The proper way to share unbalanced audio lines, if the shield cannot be 
floated from ground safely at one end, is to use isolation transformers at 
one end of the path, or somewhere along the path. This breaks the shield 
path for low frequency currents.

The hum can be fixed sometimes by low impedance bonding, forcing the chassis 
to chassis path through the binding cable to be many times less resistance 
than the shield path, but that does not address the real problem. The real 
problem is the chassis path on each end of the shield, which will 
superimpose low frequency noise and hum laterally along the cable.

Consider if you have a rig with an external supply. You might have 20-30 
peak amperes with modulation superimposed on the ground lead between the PS 
and the rig. If the PS has a ground path back to the other gear, your 
undecoded voice can modulate the audio line, making it sound like RF in the 
audio, even though it is really a low frequency ground loop and not RF at 

It sounds to me like someone building equipment, or perhaps an error in the 
wiring, has created some ground loops through a neglectful design. I'd 
isolate grounds on cables, rather than depending on a low resistance bonding 

73 Tom 

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