[TowerTalk] Replacing station ground
jim at audiosystemsgroup.com
Wed May 30 16:02:36 EDT 2018
On 5/30/2018 11:04 AM, Bob Shohet, KQ2M wrote:
> I am planning to completely revamp my station placement and station ground and have been thinking about and researching different ways of doing this for several years. I have also studied what was done at many stations at which I have operated, but that only confuses the issue more as each house and terrain is different and every station owner has their own way of doing things. :-) I have also spend considerable time reading the Polyphaser material and W8JI and K9YC’s websites. So I have several questions and would appreciate your help.
> First, here is what I am planning to do:
> Install a metal Hoffman box at each tower and then ground the coaxial cables, rotator cables and control cables at the box (the control cables are already grounded to the tower) with a #2 Copper wire grounded connecting to multiple short ground rods (because of the ledge right underneath) and the tower.
> Then I will run all the cables to a larger metal Hoffman box grounded just outside the shack and a few feet from the utility ground, telco and cable grounds. Then, in a trench ~ 1’ deep around two sides of the house to form a perimeter ground, I plan to connect a series of 8’ copper clad ground rods culminating with a connection to the Hoffman box outside the shack and to the utility, telco and cable grounds, so that I will have a single-point-ground connected to a perimeter ground in the trench. I will then fill in the trench when done.
> 1) How far away should the trench be from the house foundation? 6”? A foot? more? I know that there should be some separation but do not know how far is best. I live in Western CT near the NY border where we get a lot of ice and snow and it has gotten down to –20 F; so frost heaves are always an issue.
A good engineering guideline is that all rods should get good exposure
> 2) I assume that the best and safest way to connect all the grounds would be to use #2 copper wire running between the Hoffman box and the utility/telco and cable grounds. Since these ground wires are all connected ABOVE ground, is there any risk to removing the wires and replacing them with a single #2 Copper wire below ground? If there is risk with removing the utility ground temporarily to do this, then should I simply dig down and then connect #2 copper wire to the same utility ground rod below ground – thus avoiding the need to remove the utility ground above ground to connect all the grounds? Is there a problem that could be created by having an oxidized copper clad ground rod of the utility ground with connection above ground and then simply sanding a spot lower on the same rod below ground in the trench and then connecting the Hoffman box to that below ground?
Any danger from disconnecting and repairing/replacing connections to the
power service ground rod can be eliminated by doing it last, after other
rods have been bonded to each other and to other house grounds. I hope
that you plan to use CADwelds. I prefer the ones that KF7P sells. Note
that they must be matched to the number and size of wire for each
> 3) I need to get about 6 control cables, 6 rotor cables and 8 coaxial cables into the shack. How would you suggest that I do so without allowing critters and insects in?
> I see two potential options:
> A) There is a small window to the shack about 12” high and 24” wide – if I replaced the window with an aluminum plate (which would be grounded to the single point ground), I will use standoff coaxial connectors to get the coaxes inside, but I don’t think that I have enough room to mount everything on the plate, so how do I get the control cables and rotator cables inside without making another hole and allowing the critters in?
> B) If I don’t replace the window with a panel, then how do I get all the cables inside without allowing critters and insects inside other than making a big hole in the house above the top of foundation and then stuffing it with steel wool and other material? This would allow cold air and humidity to get into the basement as well and provides a potential way in for all kinds of insects and mice. Not a desired outcome.
> 4) Due to the extremely violent wx that I get at my qth I still like the idea of disconnecting everything when not in use. With this in mind, does it make sense to have another aluminum panel inside the shack mounted on wooden beams near the window (and grounded to the spg with #2 wire) to which I attach all the coaxial, control and rotator cables inside the shack and then simply disconnect them and move them away when not in use? These outside and inside cables would also be disconnected from all the equipment and outlets and ground inside the shack would be disconnected from the single point ground outside.
I've used two different solutions to get cables into the shack. The
first was to mount feedthrough Polyphaser arrestors on a copper panel
mounted in a cutout in the wall. Neighbor K6XX built the panel in his
shop and helped me mount it and mount the arrestors. The second was a
couple of Hoffman boxes mounted to the wall, one with terminal strips
for control cables with provisions for arrestors on each conductor, and
the other with the arrestors that Array Solutions sells. Each box has
one or more feedthrough PVC conduits to get through the wall into the shack.
I strongly prefer second arrestor solution for two reasons. First, the
Polyphasers do occasionally go short circuit, either from a lightning
hit or operator error (TX into the wrong antenna). In addition to cost
(they cannot be repaired by replacing the GDT element), they are a major
PITA to replace. Second, I much prefer the circuit used in the Array
Solutions arrestors, because it is far more likely to discharge any
gradual charge buildup before it reaches a point that triggers the GDT.
KF7P makes some nice custom Hoffman-style boxes for the arrestors he
sells, but neighbor W6GJB designed a very nice (and simple) mounting
method to increase the number of arrestors that can fit in the box, and
make it easy to replace a blown GDT. With all hinged cover Hoffman
boxes, cables enter the bottom of the box through strain relief fittings
sold for use in power system box knockouts, and cables have tape or
heatshrink to build up their diameter to minimize the opening for
insects. Boxes are purchased with the optional steel panel, arrestors
are mounted to the panel, and the panel is bonded to the ground system.
My shack utilizes both the Polyphaser panel and this newer Hoffman box,
and both are bonded at the same point.
I've photographed the box, and at some point I'll write it up and put it
on my website.
As to #4, W8JI has observed that this should not be necessary if
everything is properly bonded both inside and outside the shack.
73, Jim K9YC
> I would appreciate any thoughts, suggestions or warnings about what I propose. Thank you for your help!
> Bob KQ2M
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