Jim--
In the answer you posted Z(below) you said:
>
> All equipment in the shack should be bonded together by very short, beefy
> copper. This combination of bonds is what goes to the earth electrode.
>
This is NOT what the grounding instructions say in my Yaesu FT-950 manual.
Traditionally, this list adheres to : "Do what the manufacturer says". I am
getting several interpretation of "SPG" here, and am confused. In the past,
I have read your posts with close attention and admiration; I need a
clarification.
Another question: You mention "Short, Fat" as desired form for connections
here, but in many other places, "Short, Flat" (as: Copper strap) seems to be
preferred, as having low inductance and lots of area for "skin" effect.
Thanks-- Bill--W4BSG
--------------------------------------------------
From: "Jim Brown" <jim@audiosystemsgroup.com>
Sent: Wednesday, August 04, 2010 12:03 PM
To: "Tower Talk List" <towertalk@contesting.com>
Subject: Re: [TowerTalk] A different question (I think) on SPG
> On Wed, 04 Aug 2010 12:19:12 -0400, n8de@thepoint.net wrote:
>
>>You are confused by the term SPG. It does NOT mean that EVERYTHING is
>>tied to ONE POINT!
>
> Unfortunately, you are too. :)
>
>>The ground you have outside your shack is the SPG for your
>>equipment/shack, provided all 'boxes' in the shack are tied to it and
>>not simply to each other.
>
> WRONG! DEAD WRONG!
>
> All equipment in the shack should be bonded together by very short, beefy
> copper. This combination of bonds is what goes to the earth electrode.
>
> Further, ALL earth electrodes must be bonded together by short, fat
> conductors. This, in fact, is Gary's problem, it is a common problem, and
> it
> is the same problem I had in my house in Chicago. The fact that it is hard
> doesn't mean that we should not try to solve it. If we don't, lightning
> may
> do it for us. :)
>
>>The common practice, many years ago, was the 'daisy-chain' the ground
>>from box to box to box, etc ... and then ONE wire went to ground.
>>This is a problem waiting to happen.
>
> Again, I strongly disagree. There is nothing wrong with short fat
> conductors
> bonding equipment together and bonding the combination of those bonds to
> the
> earth electrode system. AND all of that MUST be bonded to the power system
> ground.
>
>>Tie EACH of the 'boxes' in the shack to the SPG outside with SEPARATE
>>ground wires. Thus, the SPG you have serves its purpose.
>
> Running a half dozen wires from your operating desk down to a ground rod
> with no connection between them in the shack is a recipe for hum, buzz,
> and
> RFI!
>
>>The grounds at each tower are great for dissipating the initial 'blow'
>>if struck, but NOTHING will keep the 'strike current' from travelling
>>from tower to shack.
>
> I don't agree. Remember, lightning is NOT A DC EVENT, it is a TRANSIENT
> event, which means it is an RF EVENT. The towers are far enough away from
> the house that the inductance of the connections between them provides
> SOME
> insolation between them.
>
> The most important thing here is to ground each tower AND ALL THE WIRING
> as
> effectively as possible. That means bonding coax to the tower at top and
> bottom. It also means bypassing CONTROL LINES to the tower at top and
> bottom. Coaxial ferrite chokes on the lines between the tower and the
> shack
> can also help keep the strike (at least until they explode). :)
>
>>The idea is to MINIMIZE the potential damage.
>
> And to minimize the LIKELIHOOD of damage.
>
> Lightning current will usually find the lowest impedance path to ground
> relative to the point(s) where the charge is building. It it's building by
> your tower, you want to give it the lowest possible impedance path to
> earth
> at the base of the tower, and you want the impedance back toward the house
> to be high. It is the ratio of those two impedances that protects your
> house. The impedance back toward the house is the inductance of the coax
> and
> the control lines.
>
> Getting back to Gary's situation. If it's not possible to bond from one
> side
> of the U to the other in a straight line, I would build a perimeter ring
> of
> ground rods around the house at spacings equal to their length, bond them
> all together, and then bring bonds to them from the power service
> entrance,
> CATV, telco, and shack. I would NOT bond the towers to the house ground. I
> would also bond the power system green wire to that ring at multiple
> points.
>
> What I've described is almost exactly what was done at an AT&T Long Lines
> site on a mountain top where I've had a ham station. There are multiple
> ground rods around the perimeter of a 60 x 100 ft building, all tied into
> building steel close to each rods, and the power system service is bonded
> to
> that at the point of entry. At that site, the tower is right next to the
> building, it's 36 x 36 ft at the base, there are rods near each leg, they
> are bonded together, to the tower, and to the building perimeter.
>
> 73, Jim Brown K9YC
>
>
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