[TowerTalk] [RFI] Back of desk grounding buss

Jim Lux jimlux at earthlink.net
Fri Mar 21 12:02:42 EDT 2014

On 3/21/14 6:41 AM, Charlie Gallo wrote:
> On 3/21/2014 n8de at thepoint.net wrote:
> ...snip...
>> Think of the 'ground bar' as a series-string of very low value
>> Now ... think of your equipment as being connected to the 'nodes'
>> between each of those resistors ... rig A at one end, then rig B
>> at the first 'node' ... rig C at the second 'node' ... etc.
> ...snip...
> Now thing of it a slightly different way..  The center of the bar has
> the cable to ground, and each section of bar is a LARGER conductor
> (aka lower impedance) than a wire the same length (greater cross
> sectional area - hence my question RE SIZE and material), and the
> wires are shorter.  The lumped impedance is therefore lower, as far
> as I can tell

Depends on whether you're talking about 50/60 Hz line or lightning or 
HF.  For line frequency, yes, the Z is lower with the big pipe/bar; for 
1 MHz and up (lightning) the inductance dominates, and there is very 
little change in inductance with conductor size, it's almost entirely 
determined by the length, which is roughly 1 uH/meter.   (going from a 
1x1 cm square to a 0.2 x 20 cm sheet changes the inductance something 
like 20-30%)

And in either round or wide flat, the impedance due to inductance is 
typically orders of magnitude bigger than the AC resistance

However.. by far and away, the most common death, injury, or damage 
scenario is a contact between power line and antenna or equipment. 
(either internal short inside box from power wire, or something outside) 
There's a lot more of those than there are lightning strikes or 
transients.   So, maybe we should be focusing more on 60 Hz bonding and 
whether it can take the fault current from a MV distribution line that 
contacts your antenna.

> Assume the grounding points on your rig, and your amp are 36 inches
> apart (as they are if you have an FT-1000MkV field and an Alpha 78
> next to one another with the amp on the right).  You can run say two
> 18" lengths of braid to a common point, for 36 inches (tight
> stretched, but all it 40 with droop etc), OR you could put in a buss
> bar that is 36" long, and 2x 4" jumpers to said buss.  Total length
> is about 4" longer, but the BUSS is more conductive than the braid
> Which gives you less poof?

either would work.  the key is that they are bonded together.  The 
differential voltage will be small.

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