[This kind of discussion often happens over at
TowerTalk@contesting.com. Search there for lots more info.]
Keep in mind there are 3 types of "grounds": RF, AC (power), and
lightning protection. (Actually, there's a fourth - low-level audio
grounding, that you need to worry about for microphone or soundcard
interfacing.)
The first (RF) is somewhat hopeless, as was pointed out, because a
real (broadband) RF ground has to be very short, say not more than
1/10 wavelength, to avoid reactive and resonant effects. That means
shorter than 1 meter for 10 meter operations, etc. That's not
practical in most installations. You don't really need a "true" RF
ground, if your antenna system is balanced properly - no common mode
RF on the feed lines, etc.
Full lightning protection requires serious measures. Some of us with
high towers or in strorm-prone areas do manage to take direct hits
routinely, but I expect most hams would have major damage from a
direct hit. The system has to be designed to divert tens of kiloamps
to ground while sending a surge of less than ~100 volts to your gear.
On the other hand, AC safety grounding is relatively straightforward,
if you know what you're doing. The main thing is to bond your grounds
together, so that fault currents (up to 100 A) find an easier path
than through your rig.
73 Martin AA6E
On 11/29/05, Richard Detweiler <rdetweil@hotmail.com> wrote:
> Hi James,
>
> I'm no expert, but this is what I understand about grounding. Maybee others
> can fill in some gaps.
>
> Should a lightning strike the AC wires, the AC ground is not enough to
> dissipate the current before it reaches the radio chasis. Even .001 ohms
> between the AC ground and the RF gound will create a huge voltage
> difference, and the station chassis will become the fuse.
>
> For instance, a cloud to ground strike has between 10 million and 1 billion
> volts. All that could hit the ground system but for discussion lets say it
> has at least volts 500,000 Volts by the time it gets into the ground system
> looking for a way to dissipate itself, the current at .001 ohms is an
> instantaneous 500 Million amps...
>
> That much current & voltage will likely fry something inside the radio. So
> the idea is to direct that stuff away from the station.
>
> Best is to use a large wire from the AC Main ground DIRECT to the RF
> Grounding system. Following the grounding guidelines really works.
>
> I've had strikes and the radios have survived so far. I lost two PC network
> cards, and a network router from the EMP induced into the cat 5 cable, so,
> I'm going wireless from now on.
>
> In reality, there is NO way to prevent the voltage on the ground system from
> going very high during a direct lighting hit or to really guarantee that it
> won't harm some equipment. But a well grounded station is much more likely
> to survive intact.
>
> The purpose for tying everything together with heavy gauge wire ( like #6 or
> #O ) is to prevent the voltage of any one area in the electrical/antenna
> system from rising too far above any other area of the system during the
> lighting strike. It's Ok if the voltage rises, As long as EVERYTHING else
> goes up with it at the same instant and it has a very low ohm path to
> dissipate itself. The more grounding rods, the faster the voltage will
> dissipate and the less the voltage will rise...
>
> Best wishes,
> Rich
> K5SF
>
>
>
>
>
>
>
> >From: "JAMES BRASSELL" <jimbrass@bellsouth.net>
> >Reply-To: Discussion of Ten-Tec Equipment <tentec@contesting.com>
> >To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
> >Subject: [TenTec] Station and AC Ground
> >Date: Tue, 29 Nov 2005 09:37:24 -0500
> >
> >Hey, All.
> >
> >Just a quick observation and question. I have read many posts on this site
> >about having the station and AC mains grounds tied together. For my
> >observation, when you have a separate station ground (and I do; a good one)
> >and all pieces of equipment are tied to that ground and the ground wire
> >from the AC plug is tied to the equipment chassis then you have effectively
> >tied the AC mains ground to the station ground. I have measured from the
> >AC mains ground to the station ground and it is zero ohms, with no voltage
> >(to the microvolt between them). I have looked in the equipment and the AC
> >ground is tied directly to the chassis, not through a board. My question
> >is, if the equipment is grounded and you have a good AC mains ground is
> >that not tying the mains and station grounds together? I could see where
> >one might have a problem if the ground in the equipment was achieved
> >through a circuit board and the equipment was not otherwise grounded. I
> >feed two verticals, GAP Titan and
> > Voyager, and run 1500 watts into them on a regular basis without any RF
> >problems. What say you?
> >
> >Jim, K4ZMV
> >_______________________________________________
> >TenTec mailing list
> >TenTec@contesting.com
> >http://lists.contesting.com/mailman/listinfo/tentec
>
>
> _______________________________________________
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>
--
martin.ewing@gmail.com
http://blog.aa6e.net
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