"Rob Atkinson, K5UJ" wrote:
> This sounds like you can't have a separate safety ground and rf ground.
> This is a new one to me. It sounds like you have to either clip the 3rd
> prong on your power supply cord plug and use your rf ground or hook your rf
> ground to your safety ground wiring somewhere and use the ac safety for all
> your grounding. Not getting this.
Hi Rob,
You are about right on not having a separate safety ground and a separate rf
ground. But don't clip off that ground pin. I am looking at it more as a
lightning ground but as you will see it also can be your rf ground..
By "single point grounding" it is best done with a bulkhead panel where the
antenna cables and power cables come into the shack. You should have your
antenna cables grounded to the panel and the power outlets mounted on the same
panel with the power safety ground tied to the panel. From the bulkhead panel
run copper strap or a couple of parallel copper straps to your ground system.
Hopefully your ground system is close to the bulkhead panel. Mount your coax
lightning protectors to the panel as well as power surge protectors for the
power lines. If you have a phone line that is to be connected to any equipment
in the shack mount a surge protector for it on the same panel.
Now you can connect your equipment ground straps to this same panel. This way
you will have no ground loops through the equipment. If you do get a lightning
strike it may come in on an antenna line or it may come in on the power line.
Because you have surge suppressers on all lines, if one gets hit all lines will
get elevated together in voltage. This voltage could go up to several thousand
volts. But because all lines go up together and come back down together there
will be no difference of potential between any of them. Your equipment will also
get elevated to the same voltage but again because there is no path through the
equipment there is much less of a chance of any damage.
Now if you did not have the power line protectors and the power line ground
attached to your antenna lines and they were both attached to a radio, and a
strike came in on the power line it would go through the radio and back out the
antenna connection to the antenna system ground. This would leave the radio in
the loop where lightning current could flow through it.
That is the basic idea of single point grounding. Don't run another ground lead
from the radio to an additional ground rod. With a lightning strike the two
separate grounds will not get elevated the same amount and you will have current
through the equipment.
The polyphaser site has some excellent articles on this.
The key to ground leads is short and large surface area. A good lightning ground
is also a good rf ground. Although a good rf ground is not necessarily a good
lightning ground. (elevated vertical antenna ground example)
When putting in ground rods it is best to run them out in a spoke shape like the
spokes of a wheel. Start at the nearest point to the shack, but not right up
against the house. The soil right next to the house usually stays very dry and
the ground conductivity is very poor there. Get it out from under the eves where
it gets some water. Then from there run radials out like you would for a
vertical antenna ground. Along each radial place another ground rod separated
from the first by the sum of the lengths of the two. For two 8 foot rods
separate them by 16 feet. It does no good to tie another wire around the outside
radius of your ground system, like the rim of a wheel. Or to put cross tie wires
between radials. You are better off making another radial with that wire. As you
get out that far the impedance gets too high to do much good.
A decent lightning ground needs more than a couple of ground rods. If you get a
strike of any significance a couple of rods will not handle it. Also short wide
copper strap needs to be run to the ground system. 6" wide if preferred. More
than one run in parallel of smaller strap works too. Put them edge to edge
spaced a little apart to reduce the mutual inductance.
Cad welding the connections to the rods is preferred. If not good clamps need to
be used.
On tower connections never connect copper directly to the galvanized tower. Use
a bronze clamp to isolate it or a stainless clamp with an added piece so that
the copper never touches the tower.
Solid copper strap or solid heavy wire should be used outside. Don't use
stranded wire or braided wire. These will corrode between strands and they are
impossible to clean again when the connection needs to be renewed.
A good ground is part resistive and capacitive coupling to the earth. This is
why the radials work well in addition to ground rods. An excellent lightning
ground would be a buried radial system for a 160 meter vertical. After all most
of the energy in a lightning strike is concentrated from DC to 1mhz.
Remember that all lightning strikes are not mega strikes. There are many minor
strikes where damage can be prevented with not so perfect a grounding system.
This got longer than I intended. Guess I should stop now.
73
Gary K4FMX
|