> -----Original Message-----
> From: towertalk-bounces@contesting.com [mailto:towertalk-
> bounces@contesting.com] On Behalf Of Dudley Chapman
> Sent: Wednesday, November 23, 2005 11:47 AM
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] TowerTalk Digest, Vol 35, Issue 85
>
> >
> >Message: 4
> >Date: Wed, 23 Nov 2005 09:44:56 -0500
> >From: "Mark ." <n1lo@hotmail.com>
> >Subject: Re: [TowerTalk] Radials, lightning ground, RF ground
> >To: towertalk@contesting.com
> >Message-ID: <BAY102-F834211FAF2FAA9DCC130693550@phx.gbl>
> >Content-Type: text/plain; format=flowed
> >
> >Hello Dudley,
> >
> >Let me try to help clear the confusion. to the best of my understanding.
> >There are three separate entities to consider:
> >
> (...)
> >
> >Now, you see, that insulated or elevated wires in an RF counterpoise or
> >radial system, cannot sink any current to earth, and cannot act as an RF
> >ground or lightning ground.
> >
> >AN RF counterpoise/ground screen having light gage bare, buried wires or
> >mesh, can be both an RF counterpoise AND and RF ground, but would be
> burned
> >up by lightning strike current.
> >
> >A heavy duty RF counterpoise made from #6 bare, buried copper radial
> wires,
> >such as already described here, used on commercial broadcast
> installations
> >(very costly), can fulfill all 3 functions: RF counterpoise, RF ground,
> and
> >lightning ground.
> >
> >Hope this helps.
> >
> >--...MARK_N1LO...--
> >
>
> Mark,
> Actually, I was aware of those distinctions in the roles of grounds and
> counterpoises. However, your short treatise was extremely well written
> and
> concise. We all should print it out and paste it into our notebooks.
>
> I tried to articulate those differences to the OM who made the original
> request, but failed to lay it out as succinctly as you did. The confusion
> I
> expressed to Gary's response was regarding his view that the low current
> rf
> radial system would handle most of the lightning surge. I can see that
> the
> high frequency components couple to the ground capacitively, but I am not
> sure if the light gauge wire would handle the massive current surge.
>
> So my question to Gary is this: Suppose I install the light gauge
> insulated wire radials and the few safety and ground connection ground
> rods
> around the base of the tower/vertical. In my view, I still need to
> install
> heavy gauge ground radials bonded to ground rods along their length in
> order
> to handle a lightning surge. Are you implying that the second set of
> lightning radials is redundant?
>
> Dudley - WA1X
Hi Dudley,
I have been away for a few days so didn't get to respond.
Not actually seeing your installation I hesitate to make definitive comments
about your installation.
If you add up the surface area of several small ground leads / radials I
think that you will find that they will outnumber the surface area that you
would have with just a few large diameter ground leads that are normally
used in grounding connections.
One of the reasons for large diameter ground leads to ground rods is to
lower the inductance of the lead. Having a large current carrying capacity
does little good if the impedance of the lead is high. The full surge never
makes it to earth via the ground lead.
Multiple paths / wires in the ground system allow the energy to divide among
them so the current is divided among the leads.
The same thing happens when you have several antenna cables coming from the
tower. They all share the current and put less of a burden on any single
line.
This is the same reason that a rotator cable of multiple conductors is
usually protected with MOV's. The surge energy is shared by all the
conductors in the cable so no single MOV has to handle the whole amount of
energy on the cable.
No mater what is struck on a tower, by the time the energy gets to the
bottom, all conductors are sharing the current.
Remember that lightning is part RF energy and must be treated the same as
any other RF signal when you are trying to get it from one point to another.
This calls for short low impedance ground leads.
Someone posted the letter from Mike Holt on grounding specs for towers. One
of the comments about exothermic welding of the connections said it was to
provide the lowest inductance path for high frequency lightning surges.
It actually has little to do with inductance but more to do with maintaining
a low resistance path through the connection. A fraction of an ohm of
resistance with high currents can allow very high voltage rises at the
connection.
So even though some people have the right methods they are sometimes for the
wrong reasons.
To answer your question directly about installing separate lightning grounds
along with your radials; the more the better!
If you are going to use insulated radials lying on top of the ground then I
would install several ground rods as appropriate for the lightning ground.
But as long as you are going to the trouble of putting in radials I would
use bare wire and bury then a few inches in the earth and make dual use of
them. Depending on soil type some ground rods connected to some of the
radials may also be in order.
Remember that a good lightning ground is also a good RF ground.
73
Gary K4FMX
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