[TowerTalk] grounding (again)

Jeff DePolo jd0 at broadsci.com
Sun Apr 22 18:01:43 EDT 2018

> What I am planning to do is to add a 0.25" aluminum plate on 
> the inside (between the studs) in the shack.  The plate will 
> be about 12 inches wide (the KF7P plate is ~ 12 inches  wide)
> I want to tie the two plates together with some copper strap.  
> Any thoughts on the thickness of the strap?   

The thickness of the strap has very little effect on inductance.

> My thought is to use 1 inch wide strap, and use as many 
> straps as I can, across the tops of both plates.  Doing that, 
> I'll have to make "flag" folds (triangle) on several of the 
> straps to get them to line up with the 2 inch PVC pipe, but I 
> don't think that will be an issue.  

You didn't say how long the run length was between the exterior aluminum
plate and the interior plate so it's hard to gauge what effect using strap
versus wire would be.  But generally speaking, you have to go to a fairly
wide strap (like, 3" or more) to make a big difference in inductance as
compared to something like #2 wire.  I did the math on this a long time ago,
but as I recall, 4" strap has about half the inductance as #2 wire for a 12"
length.  1" strap doesn't have appreciably less inductance than #2 round

> Do I need Polyphasers on the plate in the shack?   

You always want the arrestors to be as close to the protected equipment as
possible.  The more distance you have been the common-point ground and any
endpoint, the more of a voltage differential there can be due to L and R.
Voltage differentials result in current flow, and when that current flow
occurs where it shouldn't, smoke can escape from semiconductors.  So, you're
better off having the arrestors on the interior panel rather than the
exterior panel.  How much of a potential (no pun intended) improvement there
would be by moving them depends on how much R and L there is between the
interior and exterior panel.

The other think to consider is that you will presumably have four runs of
coax between the panel inside the exterior box and the new interior panel.
If the shields of all of those cables are bonded to the respective panel at
both ends, adding additional bonding conductors between the two panels will
only incrementally lower the R and L between the two.  I'm not saying that
you should eliminate an additional bonding conductor, I'm just saying you
will already have a good bond between the two panels without doing anything
just by properly bonding the coax shields at both ends.

Assuming the conductor run distance involved isn't much more than the
thickness of the wall, I'd be inclined to just run a #2 between the interior
and exterior panels, ensure that the coax shields are bonded to both panels,
and call it a day.

					--- Jeff WN3A

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