[TowerTalk] station grounding question
gm3sek at ifwtech.co.uk
Tue Nov 29 03:40:42 EST 2005
>> If you want the best isolation of equipment, a separate ground lead from
>> each piece of equipment or rack of equipment would be run to the single
>> point ground plate. That would eliminate currents being fed from one piece
>> of equipment to another.
>Only if each lead is the same length and the same impedance/reactance.
>The rapid rise time for the current and the wide range of frequencies means
>only a slight difference in "electrical" length could cause large voltage
Good point. As our stations grow larger and more complex, the 'star' grounding method becomes
less and less realistic. You wind up with connections of very different lengths running back
to the ground plate, and the whole star design is undermined by cross-connections through
signal cables. That is exactly the situation we're trying to avoid.
The same is true of the busbar layout, because most busbars don't have a low enough inductance
to prevent surge potentials developing along their length.
At some stage, we have to step across to a method that is aiming to create one large
groundplane. Unlike a star, the aim of a groundplane is to short-circuit all potential
differences anywhere across its surface. That then allows you to add interconnections freely.
Again this will be far from perfect, but it is practical to stand all the rigs on a really
wide metal sheet, running the whole length of the table. Each piece of equipment is then
bonded to the sheet with the shortest possible length of wide strap. In order to work anything
like a groundplane, the sheet needs to be REALLY wide... I'm thinking of at least 12-18in,
almost the full front-back depth of the rigs. If it still looks anything like a busbar, it
isn't wide enough yet :-)
Likewise the sheet should be bonded back to the single-point entry with a wide strap, ideally
acting as a conduit for all the incoming cables.
As I said, it isn't perfect, but I think it would be better than a star layout that has been
fatally undermined by cross-connections and unequal lengths of ground leads.
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