Kurt,
See my inserted comments.
73, Dale
WA9ENA
-----Original Message-----
>From: KD7JYK DM09 <kd7jyk@earthlink.net>
>Sent: Dec 20, 2013 8:31 PM
>To: rfi@contesting.com
>Subject: Re: [RFI] Daisy-chain grounding [was DigiKeyer II]
>
>"connections on each device can create impedance "bumps" increasing overall
>impedance all along the chain. Even if only two wires are connected to the
>exact"
>
>I have a 25' tower with antennas near the top. On the very top, the highest
>point, is an air terminal (lightning rod). This is connected to a thick
>wire, braided #2 I think, then straight line to a 3/4", 10' long copper clad
>rod into the ground at the base of the tower.
- - - - - - - - - - - - -
OK, but I have 3 ground rods around the concrete foundation for each of my 3
towers. The big tower (for VHF and up) is 77 ft high and uses 2m/70cm dual
banders as the lightning rod. That tower uses the same big grounding rods as
you - 10 ft by 3/4" - real monsters. The 3 rods at each tower are tied into a
ring using #2 AWG stranded bare wire, and a jumper runs from each tower leg to
that grounding ring using #4 bare tinned wire.
If your #2 is stranded wire, is it the "coarse" stranding of electrical wire or
the fine braid as used in welding cable? If the wire is insulated, your are
probably OK either way, but see my comments in an earlier posting about not
using braid exposed to weather (which could apply to welding cable). Two of my
3 towers happen to be aluminum, so I did not run a separate grounding wire from
the top to the ground rods, and also did not do so on my one steel tower. I
let the towers be the lightning current conductor. The big tower has sustained
2 direct hits, one estimated at 60,000 amps, without a problem to itself (the
antennas are another story!).
- - - - - - - - - - - - -
>
>Co-ax runs down the tower, clamped about every foot, then bends upward into
>a junction box about five feet from the ground.
>
>Coming out the side of this box is 2" conduit run, about 30' underground
>with the co-ax to feed the equipment in the house. Along with this, from
>the junction box only is a #6 braided wire, parallel with the co-ax, until
>it passes the ground for the subpanel, under the house, there, the #6 ends.
>This ties the co-ax on the tower, the tower and the household ground system
>together. The co-ax continues another distance, feeds one radio downstairs,
>then another upstairs. The ground wire does not follow the co-ax the entire
>way, but the radios are connected to the household ground.
- - - - - - - - - - - - - -
OK so far, but where are the lightning protectors on the coax? They should be
mounted right near that ground rod by the house. Since you are tying the tower
ground to the house ground there (correct idea), that is where your protectors
should be located.
A comment about your #6 wire parallel to the coax: Is it run inside that
conduit or separate from the conduit? It should be separate and about 6 inches
to a foot below ground. That will allow it to intercept ground currents that
are part of any gradient field that may develop should lightning miss the tower
and strike ground the area. (Yes, that happens.) Also, that wire should be
bare and in contact with the ground all of the way between tower and house.
- - - - - - - - - - - - - -
>
>Should I bring the ground wire the whole distance with the co-ax and connect
>it to the radios as well?
- - - - - - - - - - - - - - -
Well, you do need to connect the radios to that common point ground in some
manner. Yeah, you could continue the run of #6.
- - - - - - - - - - - - - - -
>
>Another idea along this "impedance bump", what about winding the second
>ground wire into a choke at the junction box to force a strike down the
>direct run from the air terminal to the 10' ground rod?
- - - - - - - - - - - - - - -
Bad idea. Do NOT add any extra L to the tower grounding scheme. You want as
much of the strike current as possible to go to that ground. That's the main
reason I let the tower structures themselves serve as the strike current
conductors. I do have all coax lines (about a dozen of them on the big tower)
bonded to the tower structure at the top and bottom ends of the coax run so
that the coax shields are at the same potential as the tower itself. That
reduces arcing thru the cable jackets between cable shield and tower. Yes,
that also forms parallel conductors to the tower legs, but I suspect the ,most
of the current does flow in them due to being "fat" conductors (as much as 3"
diameter) and having a direct, low-Z path to earth ground.
- - - - - - - - - - - - - - -
>
>Kurt
>
- - - - - - - - - - - - - -
Hope this helps. I can truthfully say that with all of the grounding and
bonding, plus use of protection on EVERY I/O line for the shack or house, I
have had zero problems inside from direct lightning strike action.
73, Dale
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