Topband: Chassis Bonding

[Jim Brown]

On 1/28/2014 7:28 PM, Tom W8JI wrote:
> Jim,
>
> You are giving terrible advice.

I believe that it is you that are stuck in old, misguided concepts that 
are simply wrong.

>
> 1.) Equipment is not always 6ft length from something else

It is in nearly all ham stations. Except perhaps some larve ones like 
yours.
>
> 2.) #10, ignoring connection imperfections, has about .01 ohms per 10 
> ft. That's up to 0.2volts with 20 amps of power supply loop current.

Not a problem unless you have bonded DC- to the power supply chassis.

>
> Also, almost every Ham radio manufacturer in the world isolates the 
> low level audio shields inside radios except at one point because they 
> are well aware of issues with the large currents in 12V solid state 
> high power systems.

Yep. It's called "The Pin One Problem," and is a well documented cause 
of hum, buzz, and RFI.

>
>>> 20A DC, and, as we discussed several years ago, 20A DC modulated if 
>>> it's
>> SSB, but NOT 20A of hum or buzz.
>>
>
> You better take some time and put a scope on a power supply lead with 
> a SSB transmitter. While the supply outputs DC, the power supply 
> loading by the radio is a time-varying resistance that varies between 
> 5-10 ohms at zero audio crossings to about .5 ohms (for a 100W radio) 
> at audio peak. If you couple into the PS line with an audio amp, it 
> sounds almost exactly like a SSB signal with no BFO except a bit bassier.

We had this discussion four years ago, and I wrote an applications note 
on it, crediting you with alerting me to it.  It is here. 
k9yc.com\PowerSupplyBondingAndAudioDistortion.pdf

>
>>> If the bond lead has .01 ohms resistance, there is the potential for 
>>> over .2 volts hum or noise.
>>
>> The source of the hum and buzz is LEAKAGE current, not power supply 
>> current.  Leakage current rarely exceeds 10-20 mA unless some piece 
>> of gear has developed a fault.
>>
>
> You are considering power line AC to the safety ground in commercial 
> equipment. 

No, I am talking about the SOURCE of the hum and buzz that we have 
dumbly blamed on "ground loops," and because we blamed them on "loops," 
we did all the wrong things to solve the problem.

> Ham stations have grounds, and those grounds cannot be bonded 
> perfectly to the mains, nor will many people bond them at all. 

WRONG!  We MUST bond all grounds in a facility for lightning safety, and 
for safety of personnel.  And when done completely and properly, it 
SOLVES problems.

> This is just the real world we live in. The power mains try to use the 
> Ham gear grounds as a ground. If I read the neutral to ground bond at 
> my house, I typically have 1 amp or more showing from the mains ground 
> (the primary return is always bonded to the transformer secondary 
> center tap at the pole) trying to ground to my tower grounds and radials.

You are getting confused by the multiple uses of the word "ground." I 
suggest that you study my tutorial on Power and Grounding for Audio and 
Video Systems (on my website) which clarifies these issues. And there is 
NOTHING special about RF that makes our realities any different.

> The voltage driving that current isn't high, it is caused by the 
> neutral voltage  drop back to the substation, but it is a real voltage 
> and current that exists.

No, it is caused by triplen harmonics combining on the neutral (and 
ground) of 3-phase systems to add rather than cancel. That's the source 
of power line "buzz" -- 180, 360, 540, 720 Hz.  While we don't have 
3-phase power in our homes, the power on the street is nearly always 
3-phase, and often a very nasty variation of 3-phase called "high leg 
delta."  It's 240V delta, with all three phases fed to light industrial 
customers, and one of the legs center-tapped to feed single-phase users. 
Those 3-phase users get no neutral and no ground, so their harmonic 
current finds its path to ground through OUR neutral.  THAT'S the source 
of our "buzz."  And yes, it's not unusual to see an amp or more of that 
stuff on our neutral. High leg delta is all over mixed residential/light 
business/industrial neighborhoods of nearly all cities, and it's what 
I've got in the Santa Cruz Mountains.

>
> Besides that, we have power supply negative lead currents that are 
> also NOT DC, because the leads have a time-varying load resistance.

Which is why bonding DC- at the power supply is a very bad idea. And you 
and I have talked about that too, and we disagree.

>
> It's an imperfect world, and a simple $1-$2 isolation transformer 
> fixes it all so far as audio line ingress is concerned.
>
> Telling people to not use transformers and to bond things instead, 
> will eventually cause nothing but long term grief. I can assure you no 
> manufacturer in their right mind would listen to that advice. ANY good 
> interface by almost any manufacturer has shield isolation, with a 
> chassis bond only at one point. If they don't do that, the help lines 
> get busy with complaints.

Actually, exactly the opposite happened in the pro audio world more than 
15 years ago when Neil Muncy alerted us to The Pin One Problem, and 
manufacturers corrected them. Instead of "audio ground," and "mic 
ground," the shield goes straight to the shielding enclosure. Rane was 
one of the first to do this in a big way, and they reported after a few 
years that their support line costs had dropped by a factor of 10!

I suggest that you download and study AES48.  Or you can read the same 
material in tutorials on my website.

73, Jim K9YC