[TowerTalk] Grounding

Michael McCarthy, W1NR w1nr@eecorp.com
Thu, 03 Feb 2000 18:03:15 -0500

Lee Buller wrote:
> Some people are telling me I should tie all the grounds together...the
> ground rod under the tower, in the shack and the AC mains together.
> HMMMMMM.  Is that practical?  If I am running a 12 volt rig from a
> battery...what do I need to ground?  I need an RF ground...right?  If the
> rig is running from a 13.8 volt power supply (like an astron) would the P/S
> be grounded through the AC Main if wired properly?  Still the 12 volt rig
> would not be grounded except maybe through the negative lead of the power
> cord...but some of those are floating as well.

   Both the NEC and the ARRL Handbook state that all grounds must be bonded.
Both for safety and for keeping potentials at the same level.  During a
lightning strike, there will be a rise in the potential of one of the
grounds.  If they are not bonded together, there will be a difference
created and the possibility of an arc over from one ground to another.
Murphy's law states that the arc over will most likely be through your
radio equipment!

> I know there is an issue of ground differential....but for the life of me I
> cannot figure out why to tie all the grounds together.  Some people
> advocate that and I am just at a loss to understand why.  I just don't want
> any AC stuff  attached to my RF ground.  Why would I want to have all that
> ground wire going through to Romex back to the entrace panel and then to
> ground.  Sounds like it would be no ground at all.  What is the difference
> between an RF ground and an AC ground?

   Your RF ground needs to be low impedance and as short as possible.  I use
3" wide copper strap around the operating bench to a copper feedthrough panel
that has the lightning arrestors on it.  This goes to a ground rod outside,
again using 3" copper strap.  That ground rod is connected to the tower
ground with #4 bare solid wire buried in the ground.  There is also a run
of #4 from the copper panel to the main electrical ground at the water
meter.  This keeps ALL grounds "bonded" to the electrical ground.

GL and 73 de Mike, W1NR
**************** Give Every Man His Dew **************
Michael A. McCarthy, President
Everest Engineering Corporation, Consultants
4 Barnes Circle, Marlborough, MA 01752
(508) 460-6737, FAX (508) 460-7951

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