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Author Topic: Coax routing and grounding  (Read 7766 times)
AD5HD
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« on: August 02, 2013, 08:24:50 PM »

Not really another grounding question as it is asking for opinions about how best to route coax and control cables from the ground bulkhead next to and bonded to the AC service entrance across the house to a ground window at the station.

One option is through the attic and another is to bury the lines around the house and drill through the wall near the station.  One is a heckuva lot easier but seems like it'd be more prone to lightning induced surges  Then again the ground lift propagating through the AC power system would be common between the station ground window and the AC power wiring, more or less, since the lines would follow the AC wiring distribution fanout, again more or less.

Running large metal conduit through the attic from the bulkhead but grounded only at the bulkhead and stopping just where the lines leave the attic through the closet ceiling seems like it'd be a good idea as an electrostatic shield but doesn't do anything to shield magnetic coupling from nearby lightning induced di/dt but then the Romex wiring isn't in a conduit so maybe it doesn't matter.  A lot less work and a lot cheaper without it anyway.

Another question is whether a #6 solid copper wire from the bulkhead to the station ground window is acceptable or if it should be copper strip.  Hard to see that it'd make any significant difference.  Doesn't seem like a good idea to connect the bulkhead to the ground window using only the coax braided shields anyway.

Details of the installation as follows for reference:

Antennas listed in decreasing distance from the ground bulkhead:

Rotatable, remotely tunable, 80m/40m shielded balanced receiving loops through DXE RFCC unun on ground mounted mast and connected with #6 solid copper wire to a nearby isolated 8ft ground rod with a Harger 302U clamp, F6 coax and control cable are buried to the ground bulkhead (antenna is built and tested, assembled, and awaiting installation)

HF2V on a DXE VA-BASE with an isolated DXE VFCC unun and a radial plate mounted to a ground mounted pipe connected with #6 solid copper wire to a nearby 8ft ground rod with a Harger 302U clamp and bonded to the ground bulkhead using Polyphaser clamps and 1.5" copper strap from Harger, BuryFlex coax is buried along with the receive loop F6 coax and control cable in a common trench to ground bulkhead (antenna except for top sections is assembled with DXE parts awaiting installation)

G5RV at 30-50' with ladder line transition to short RG8X pigtail into a DXE FCC050 feedline current choke grounded with #6 solid copper wire to a nearby 8ft ground rod bonded to the ground bulkhead using Polyphaser clamps and 1.5" copper strap from Harger, BuryFlex coax is buried to ground bulkhead joining a common trench with above lines after about 15' (antenna parts in house but not constructed)

Ground bulkhead: (built and assembled ready for installation)

Georgia Copper ground bar mounted on Harger HV standoffs in a fiberglass NEMA enclosure on side of the house next to the AC service entrance, earthed with three copper straps dropped below grade and clamped to a horizontal copper strap with Polyphaser clamps which is in turn bonded to two 8ft ground rods one on each side with Polyphaser clamps one of which is in turn bonded to the service entrance ground bus with #6 solid copper wire using a Harger 302U clamp

Lightning suppression:

Polyphaser RTC-1 for control cable, IS-50X-C0 for both BuryFlex coaxes, and one DXE F type suppressor carrying DC control current to loop antenna from station, all bonded with T&B CopperShield and stainless hardware

Grounding kits for each BuryFlex coax can be added inside the NEMA enclosure and terminated using Harger 216 bonding lugs bolted to ground bar, bonded with T*B CopperShield, and attached with stainless hardware

Cable run from bulkhead to station:

Coaxes and control line run up through soffit, through attic to other side of the house, down through a closet ceiling to the floor, over to the station, and terminate at the bottom right of  a 12”Hx24”Wx1/8” copper plate constructed as the station's ground window and mounted to the wall directly behind the station desk using Harger HV standoffs

Station ground window: (built, installed, and operational)

Each coax connects to a Diamond CX210 coax switch which is bonded to the copper plate with Harger zinc based joint compound and stainless hardware and with a low inductance 5W/47ohm load built into a PL259 on one output of each to terminate the lines when not in use per Polyphaser requirements

A Polyphaser IS-PLDO-120US20A is bonded to the copper plate in the same way and to the left of the coax switches at the bottom of the copper plate to provide station AC power

The control cable terminates on a horizontally mounted single row barrier strip to the left of the AC suppressor and has MOVs bridged from each position to a load center neutral bus bar bonded similarly to the copper plate parallel and beneath it and several inches above the bottom edge of the plate

Individual equipment ground straps connect to the neutral bus bar using heavy tinned flat braid insulated with heatshrink and connect to each grounding terminal

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WX7G
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« Reply #1 on: August 02, 2013, 08:53:44 PM »

The coaxial cables enter at the AC ground end of the house and this is ideal. You do not want to ground them at the other end of the house. You have the ideal situation with one common ground entry for both AC power and coaxial cables and adding a ground at the other end is the worse thing that can be done.

With the single common ground there can be no lightning current through the house AC wiring. With grounds at both ends of the house lightning current will travel from the AC/coax ground through the house to the other ground and this is what one wants to avoid.

« Last Edit: August 02, 2013, 09:05:19 PM by WX7G » Logged
AD5HD
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« Reply #2 on: August 02, 2013, 09:00:59 PM »

If the coaxial cables enter at the AC ground end of the house do not ground them at the other end of the house. You have the ideal situation with one common ground entry for both AC power and coaxial cables and adding a ground at the other end is the worse thing that can be done.

A wire/strap back to the bulkhead is an option at the moment, didn't intend to install it as I'm not entirely sure what the common mode impedance through the Polyphaser AC suppressor looks like although I could measure it I guess.  Haven't thought to check if the earth of the power cord is tied to the case though, need to do that before moving forward, thanks for getting me to think about that.

BTW, nothing else enters or exits the station ground window except a VHF/UHF feedline but it goes through a coax switch the same way and a -C1 Polyphase mounted on the ground window.  I know, that's not a good thing but the antenna (AR270B) is in the attic, at least temporarily.  Gotta do something about that.
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WX7G
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« Reply #3 on: August 02, 2013, 09:09:13 PM »

The antenna in the attic is at a lower elevation than the other antennas and because of this it is shielded from a lightning strike. The attic antenna does not need a local lightning ground.

If I have a good mental picture of your system you have the makings of an ideal lightning protection system. However, adding the second ground turns it from an ideal system with no lighting current through the house to a system that conducts half the lightning current through the house.
« Last Edit: August 02, 2013, 09:16:06 PM by WX7G » Logged
AD5HD
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« Reply #4 on: August 02, 2013, 09:23:26 PM »

The antenna in the attic is at a lower elevation than the other antennas and because of this it is shielded from a lightning strike. The attic antenna does not need a local lightning ground.

If I have a good mental picture of your system you have the makings of an ideal lightning protection system. However, adding the second ground turns it from an ideal system with no lighting current through the house to a system that conducts half the lightning current through the house.


Well, yes, the antenna in the attic is lower, even the HF2V is lower than the trees that line the nature park on the other side of the alley but lightning does strange things and I guess I'm just paranoid, hihi.

When you say 'add the second ground' do you mean hard tying the ground window to the bulkhead?  They'll be tied by all the paralleled coax shields anyway but from what I gather NASA testing indicates they would make fine fuses should lightning current be directed along them, thus the thought to bridge the bulkhead to the window since the window technically isn't separately grounded.

I did notice W8JI has his station ground window tied back to the entrance bulkhead with copper strap to keep the potential difference to a minimum during a strike event rather than attempt to depend on the earth's ground lift which propagates at a different rate than through the electrical system.  Interesting problem and not easily simulated as far as I can tell.
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WX7G
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« Reply #5 on: August 03, 2013, 07:34:14 AM »

This can be simulated including the potential difference through the ground. It can be done using SPICE representing the earth ground as a grid of resistors. This can be used to determine the potential difference between the shack equipment and the earth directly under it.

The G5RV is the tallest antenna and it might shield the other antennas. This link describes how to calculate the protection zone.

http://www.hermi.si/en/how-far-from-a-lightning-protection-installation-are-we-still-safe

I would run the coaxial cables through the attic without a conduit. A conduit will reduce the displacement current from the cables but how much might the displacement current be?

A first-order calculation with the lightning ground resistance of 12.5 ohms (four ground rods), cable-to-ground capacitance of 2 pF/inch, 50' cable run, no ground at the end far end of the cable, and a 30,000 amp lightning strike having a current risetime of 30,000 A/us, and zero potential change along the ground under the cable run.  

dV/dt is (30,000 A/us)(12.5 ohms)/1us = 375 kV/us

The displacement current from the cable I = Cdv/dt = (100 pF)(375 kV/us) = 37.5 amps

The displacement current from any one house AC circuit branch will be about the same.

Now let's see what the current along the cable is if one ground rod is connected to the cable at the end of the run. With four rods at one end and one ground rod at the other end, 1/5 of the 30,000 amps or 6000 amps will travel along the cables and the AC wiring to the radio equipment.

37.5 amps is not a problem but 6000 amps may be.
« Last Edit: August 03, 2013, 08:24:16 AM by WX7G » Logged
AD5HD
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« Reply #6 on: August 03, 2013, 10:04:27 AM »

This can be simulated including the potential difference through the ground. It can be done using SPICE representing the earth ground as a grid of resistors. This can be used to determine the potential difference between the shack equipment and the earth directly under it.

The G5RV is the tallest antenna and it might shield the other antennas. This link describes how to calculate the protection zone.

http://www.hermi.si/en/how-far-from-a-lightning-protection-installation-are-we-still-safe

Excellent!  Many thanks.  An extremely useful site, never saw it before.

BTW, there's a line of trees at least 80-100' tall (as far as I can tell with a laser rangefinder) along the other side of the alley behind the house and lining the boundary of a large nature park.  But I've assumed the equipotential surface over the local area can be easily disturbed enough to expose both the HF2V and the G5RV.  Better safe than sorry, I think.

I would run the coaxial cables through the attic without a conduit. A conduit will reduce the displacement current from the cables but how much might the displacement current be?

And that's where I left it, without conduit.  Figuring a nearby strike generates primarily a low impedance field within the induction field the loop area would matter more, assuming the station ground had a relatively low impedance return to ground through any of the possible paths through the coax shields to the AC wiring in relatively close proximity in the attic or from the ground window plate on the wall, albeit spaced away with HV standoffs, to the AC wiring within the wall behind it which was unavoidable.

A first-order calculation with the lightning ground resistance of 12.5 ohms (four ground rods), cable-to-ground capacitance of 2 pF/inch, 50' cable run, no ground at the end far end of the cable, and a 30,000 amp lightning strike having a current risetime of 30,000 A/us, and zero potential change along the ground under the cable run.  

dV/dt is (30,000 A/us)(12.5 ohms)/1us = 375 kV/us

The displacement current from the cable I = Cdv/dt = (100 pF)(375 kV/us) = 37.5 amps

The displacement current from any one house AC circuit branch will be about the same.

The /1us in he first equation is a typo, no worries.  30A/us seems a fair number, no problem there.  And the capacitance between coax and ground also sounds fair although I've never thought to calculate it, thanks for that.

So the numbers yield a reasonable amount of current through the shield braids and distributed fairly evenly over the length as displacement current yielding about 750mA/ft for the total coax bundle leaving the greatest current through the braid at any one point being highest at the ground bulkhead end which makes sense.

And as such it would seem relatively pointless to bridge the ground window and the ground bulkhead unless there's a sneak path to ground at the window that I'm missing.

Extraordinarily helpful, many thanks.  Nice to know my original design is justified, that is no conduit, no wire between bulkhead and ground window, cables through attic, and no ground connection at the station.

Now let's see what the current along the cable is if one ground rod is connected to the cable at the end of the run. With four rods at one end and one ground rod at the other end, 1/5 of the 30,000 amps or 6000 amps will travel along the cables and the AC wiring to the radio equipment.

37.5 amps is not a problem but 6000 amps may be.

And the coax shield braid wouldn't handle it anyway, burning open in the process.  I really hate ground loops...

In fact every line coming into the ground bulkhead has 30, 32, or 40 ferrite toroids, depending on type, on them as they do coming into the ground window.  All coax, control, and power lines routed over the ground window and making connections with any equipment in the station are likewise suppressed for RF CM currents, including the power cord of the DC supply plugged into the Polyphaser suppressor.

All lines and coax between equipment including the lines for the footswitch, paddles, and key are likewise suppressed.  There are a total of 340 F-50A-77, 70 SB-2401-43, 90 SB-2401-73, 128 FT-82-77, and 100 FT-82-42 along with a dozen Radio Shack UU cut core ferrite AC line suppressors, and at least 16 TDK clamp-on suppressors of two kinds within the station build.  All interconnects are made with RG400 or F6 to reduce transfer admittance and crosstalk wherever possible.
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WX7G
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« Reply #7 on: August 03, 2013, 10:50:28 AM »

I'll simulate the potential difference between the station and the ground underneath it.

Ferrites are not be needed for lightning protection and the ferrites on the cables routed to the bulkhead will be completely saturated during a strike. I do military EMC and HEMP design and one goal is to tie every box together via a low impedance path. Ferrites between boxes do just the opposite and is something we would never do.  
« Last Edit: August 03, 2013, 10:56:19 AM by WX7G » Logged
WX7G
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« Reply #8 on: August 03, 2013, 12:13:25 PM »

I ran a simulation with each 1 meter square having a resistance of 40 ohms.

With the shack 10 meters from the ground system and connected by the AC wiring and coaxial cables the shack reaches 375 kV relative to the ground underneath it. Adding another four rod grounds at the shack greatly reduces the potential relative to the ground underneath it. However, midway along the house the potential difference between the AC wiring and the ground underneath is 150 kV.

To limit the potential difference between the house wiring and the ground underneath it it appears that the house needs to have a series of connected ground rods or a buried bare wire encircling it. I can model this later this weekend.
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AD5HD
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« Reply #9 on: August 03, 2013, 12:32:45 PM »

I ran a simulation with each 1 meter square having a resistance of 40 ohms.

With the shack 10 meters from the ground system and connected by the AC wiring and coaxial cables the shack reaches 375 kV relative to the ground underneath it. Adding another four rod grounds at the shack greatly reduces the potential relative to the ground underneath it. However, midway along the house the potential difference between the AC wiring and the ground underneath is 150 kV.

To limit the potential difference between the house wiring and the ground underneath it it appears that the house needs to have a series of connected ground rods or a buried bare wire encircling it. I can model this later this weekend.

The ferrites are there to provide higher CM impedance to RF so coupling to the antennas, especially the vertical, doesn't add noise to the receive antenna ground or find its way back to the station.  They don't have anything to do with lightning suppression, I was just mentioning them in the context of avoiding ground loops which come in many flavors.  You're absolutely correct otherwise though of course, no argument there.  A perimeter ground is not feasible but I certainly wish it were.

Edit: Forgot to mention that the potential difference between the wiring or the station ground window and the earth beneath has to include the dielectric properties of the soil so the differential propagation of ground lift can be seen in a simulation.  And the difference isn't nearly as important as the difference between equipment tied to the ground window (thus the ground window existence), the ground lift through the wiring/coax is a common mode event and either the shack sits on a very large slab of polypropylene and no one enters or leaves during a storm or one is smart enough to stay away from operating during a storm to avoid experiencing the common mode lift difference relative to the slab/ground.
« Last Edit: August 03, 2013, 12:40:44 PM by AD5HD » Logged
WX7G
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« Reply #10 on: August 04, 2013, 12:51:53 PM »

Although it contains some errors this paper on lightning and amateur radio stations is good.

http://www.bwcelectronics.com/articles/WP30A190.pdf

The author advocates single-point grounds, which is what you have if you don't connect the "ground window."

A single point ground will reduce lightning current through the house AC wiring and coaxial cable to near zero. But there is then the "ground lift" issue, as you call it. Ground lift in the vicinity of the radio room can be reduced by connecting the radio equipment to the ground window. And it can be reduced even more by placing ground radials under the radio room. The downside to the ground window is that 1/5 of the lightning current takes a path through the house to the ground window.

Of course another way to reduce ground lift is to reduce the ground system resistance. You have four rods that might have a resistance of 50 ohms each. According to the author ground rod resistance drop by a factor of 2.5X during a lightning strike (pages 14-17). If so, your ground resistance might be 5 ohms. Hit with a 25 kA strike (page 9) the ground lift is 125 kV.

Perhaps the way to deal with ground lift and the lightning current through the house is to reduce the single-point ground resistance. Eight additional 8' rods spaced 16' apart could give a ground resistance of 1.7 ohms. The resulting the 25 kA ground lift is a safer 43 kV. At 8 kA/us the dv/dt with the 1.7 ohm ground is 16 kV/us. A person touching the radio equipment with this dv/dt experiences (100 pF body C) a current of just 1.4 amps which is equivalent to a 2 kV ESD event. This will be felt but should not be painful nor harmful.
« Last Edit: August 04, 2013, 12:54:02 PM by WX7G » Logged
K1CJS
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« Reply #11 on: August 04, 2013, 03:49:29 PM »

All this discussion about the exact results and measurements of a lightning strike is really doing no good for anyone.  Bottom line is unless you spend many hundreds of dollars and go with a professionally installed system, you're going to have some damage if you suffer a direct strike.  PERIOD.  Even if you do get a professional system, odds are worse than even that your antennas will still be damaged.  If you have no damage, you didn't get directly hit--or you got very, VERY lucky.

What most of us protect against is the transient strike potential, and for that, single point grounds and the claims that that is the only way to offer positive protection is just so much eyewash.  The ground halos that many of the professionally installed systems use have many MANY ground rods located around the circumference of the circle.  Are those advocates of a single point ground going to tell the professionals that they're wrong?  Single point ground systems may look good on paper, but in actual use have their own drawbacks.  Every system does. 
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AD5HD
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« Reply #12 on: August 04, 2013, 04:46:53 PM »

Although it contains some errors this paper on lightning and amateur radio stations is good.

http://www.bwcelectronics.com/articles/WP30A190.pdf

The author advocates single-point grounds, which is what you have if you don't connect the "ground window."

A single point ground will reduce lightning current through the house AC wiring and coaxial cable to near zero. But there is then the "ground lift" issue, as you call it. Ground lift in the vicinity of the radio room can be reduced by connecting the radio equipment to the ground window. And it can be reduced even more by placing ground radials under the radio room. The downside to the ground window is that 1/5 of the lightning current takes a path through the house to the ground window.

Of course another way to reduce ground lift is to reduce the ground system resistance. You have four rods that might have a resistance of 50 ohms each. According to the author ground rod resistance drop by a factor of 2.5X during a lightning strike (pages 14-17). If so, your ground resistance might be 5 ohms. Hit with a 25 kA strike (page 9) the ground lift is 125 kV.

Perhaps the way to deal with ground lift and the lightning current through the house is to reduce the single-point ground resistance. Eight additional 8' rods spaced 16' apart could give a ground resistance of 1.7 ohms. The resulting the 25 kA ground lift is a safer 43 kV. At 8 kA/us the dv/dt with the 1.7 ohm ground is 16 kV/us. A person touching the radio equipment with this dv/dt experiences (100 pF body C) a current of just 1.4 amps which is equivalent to a 2 kV ESD event. This will be felt but should not be painful nor harmful.


I reviewed the Polyphaser data on the AC suppressor and it does tie the ground station plate to AC power ground where it's plugged in.  This isn't a local earth of course, just the ground connected from the service entrance where the bulkhead is bonded anyway.  So the idea to keep the instantaneous potential difference between the coax and control lines as they terminate on the ground window is to connect the ground bulkhead and the ground window with a low inductance strap.  The point is to get close to equal ground lift propagation times, thus making the interconnection required.  I knew there was a reason I put a ground clamp on the window back when, duh.

This is still a single point ground system since the ground window isn't earthed locally with a ground rod at the station.

And the radial field at the HF2V as well as the burying of the coax and control lines helps to dissipate nearby strike energy before it gets to the bulkhead.  Adding the ground kits to the coax lines at the bulkhead also helps to strip off any remaining transient current, depending on parasitic inductance and resistance between the shield tap point and the grounding system attached to the bulkhead.  Adding some ground rods at the periphery of the radial field, a total of five, would provide more dissipation and dispersal of charge away from the bulkhead.  And this is actually on my original drawings, bonded together with copper strap buried a foot to two deep.  But what a serious pita and beneath the radial field to boot.  Whee.

Do I expect antennas to survive a direct strike?  Heck no.  I'd like to survive nearby strikes and I certainly have no intention of operating during storms or during periods where lightning is possible.  If the antennas are damaged or destroyed, big deal, they can be easily rebuilt or repaired.  Radio equipment?  Well, not so much even though I built it.

But if the ground window and house ground wiring at the ground window experience near the same ground lift the only current flowing is a common mode current from the ground window and ground wiring to local earth through the slab as there's only displacement current flowing.  Just don't get in the way by operating while referenced to the slab when the ground window and all the equipment attached to it spike up some tens to hundreds of kilovolts above earth.

Bottom line is I don't want the station earthed locally, I want it to follow the house wiring common mode potential so no current flows between connections of equipment tied to the ground window, that's all.  Any displacement current that flows from anything at that potential is of no consequence unless I'm dumb enough to interpose my bulk resistance between that potential and the slab.  Not a good idea.  Although I did buy a 2'x3'x1/4" sheet of polypropylene back when to set the station on to avoid any possible conductive leakage path that could arc but never installed it as the risk is non-existent.  That and the dielectric material increases the capacitance and thus the displacement current.
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AD5HD
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« Reply #13 on: August 04, 2013, 05:00:51 PM »

All this discussion about the exact results and measurements of a lightning strike is really doing no good for anyone.  Bottom line is unless you spend many hundreds of dollars and go with a professionally installed system, you're going to have some damage if you suffer a direct strike.  PERIOD.  Even if you do get a professional system, odds are worse than even that your antennas will still be damaged.  If you have no damage, you didn't get directly hit--or you got very, VERY lucky.

What most of us protect against is the transient strike potential, and for that, single point grounds and the claims that that is the only way to offer positive protection is just so much eyewash.  The ground halos that many of the professionally installed systems use have many MANY ground rods located around the circumference of the circle.  Are those advocates of a single point ground going to tell the professionals that they're wrong?  Single point ground systems may look good on paper, but in actual use have their own drawbacks.  Every system does. 

More like thousands, price out Polyphaser PEEP and interior/exterior ground plates, etc., etc.  Yikes.  And it all really doesn't work all that well without using hardline/heliax either.  A commercial quality perimeter ground is possible but add another ton o' bucks.  And a service entrance series protector, and on and on and on till your house and station look like a broadcast station.  So it ends up more expensive than the station.  And, yep, the antennas and suppressors will eventually be damaged and need replacement.  And halo grounds make it all the more expensive and complicated, no argument there.  But then I don't need to guarantee operation through multiple direct strikes all the while making it possible to be working on the station components during a strike like a commercial broadcast station can.

The only point of the single point ground system for home use is to allow it to be bonded to the service entrance ground, regardless of how 'good' it may be, so that extraneous currents can't be induced to flow between the antenna system and the house AC system.  Ain't trivial and certainly not meant to guarantee no damage in the event of a direct strike but it should guarantee minimal damage.  This assumes of course a nearby strike doesn't induce a severe transient on the power lines coming into the house, another problem altogether.  I haven't addressed that yet although I did request it from our utility company but no surprise, no reply.
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W2RWJ
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« Reply #14 on: August 04, 2013, 06:46:12 PM »

A commercial quality perimeter ground is possible but add another ton o' bucks.  And a service entrance series protector, and on and on and on till your house and station look like a broadcast station.  

Exactly - see  http://www.eham.net/ehamforum/smf/index.php/topic,90898.0.html for some of what goes into a full-on commercial grade system.

The patch panel will be tied to the trapeze with #3/0 wire, and a ring ground circling the building, with driven ground rods every 20 feet around the ring.

73 Martin Flynn
W2RWJ
« Last Edit: August 04, 2013, 06:51:58 PM by W2RWJ » Logged
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