On Sun, 15 Feb 2004 19:33:27 -0800, Frank Gilliland
wrote:

In ,
wrote:

snip
You guys are all over this topic and it's really blowing my hair
back.

Can you guy's condense it to your opinion of the best/cheapest
way to ground lets say..an omni on a roof-top or tower? Best
being a relative term.

I'm sure the advice will save at least one person from getting
blasted, and make people aware that you can't just put a hunk
of metal in the air without considering lightning.

Thx.


Ok......

First, sink a ground rod at the point where the coax will enter the house. If
you have a basement next to the ground rod, sink a couple more rods so they are
separated by ten feet or more and in the path of the coax. Some people like to
prep the ground by soaking with sal****er, but in my experience that makes the
rod corrode before its time, then the salt leeches away into the ground water,
leaving a very poor ground in a very short time. I use galvanized pipe as
opposed to copper rods because they have a larger diameter (more rod-to-ground
contact = lower resistance), and they last longer. You can also thread the pipe
for a super-neat installation of a waterproof box......

Next is the coax installation. Run the coax down the mast or side of the house
to the ground rod. Don't run the coax near any wiring inside the house. You can
detect house wiring with a cheap metal detector. Avoid sharp turns as much as
possible. At the ground rod it helps to have a waterproof box because you need
to cut the coax, ground the shield, and shunt the center conductor to ground
with a choke (as per the diagram I made yesterday).

About the choke: This serves two purposes. First, it shunts static electricity
from the antenna to ground. Second, it provides a path for lightning if it
should strike. Since the lightning will arc across every loop in the choke, the
inductance value isn't critical just as long as it is high enough to block your
RF (about 1 mH or larger for HF and above), and the wire size is large enough to
handle some current (#14 or larger should be fine). One of those heavy-duty hash
chokes for ignition noise is ok, or you can wind your own -- use a large iron
bolt for a core and wind about 50 turns of #10 or #12 house wire. If you aren't
running much power, you can slit the insulation along the length of the coil so
it will arc at a lower voltage.

Then run the coax up into the house along side the grounding strap. Tie them
together with cable-ties if you want. Again, it's important that you run these
so they don't come close to any house wiring, and they should enter at a
location that is clear of easily combustible stuff. Terminate the ground strap
with a really big alligator clip, jumper cable clamp, or whatever you have that
makes a really good connection and can be easily disconnected. The ground clamp
from an arc welder is almost ideal.

Above the point where the coax and grounding strap enter the house, make a hook
or post where you can hang both when not being used. Remember that if lightning
hits, it's very possible that these will jump straight out from the wall and
dance around like a water hose, so make sure they are held securely in place.

Inside the house, make yourself a grounding bus bar of copper or aluminum. Put
this on the back of your bench and use it to ground all your equipment. To this
you clamp on your grounding strap. When not using your bench, unhook the
grounding strap along with the coax and stow it away. Also, to protect the coax,
use an appropriate socket and short it out before you stow it.

When should you unhook your antenna? There is a thing called the 30-30 rule: If
you hear thunder less than 30 seconds after you see the lightning, unplug. Don't
plug in again until 30 minutes after the last thunder. That's pretty safe. You
can even improve on that by building a lightning detector, many of which will
indicate lightning even before you can hear the thunder.


How's that?

Oh, I almost forgot..... coax length!!!!! The low impedance of the ground will
be reflected at the radio when the length of the coax & ground strap, from the
ground rod to the radio, are 1/2 wavelength. This means 1/2 wavelength -without-
consideration of velocity factor because we want a low impedance -ground-. And
this means -- you guessed it -- 18 feet of coax!



Very nice Frank, The only thing I have different on my setup, is a
ground window where my coax and rotor cables enter the house. That is
a metal plate, a scrap aluminum plate I found at work (12" x 12").
Its mounted over the entrance to the crawl space of my house
(no basements down here) I mounted coax and rotor feed throughs to it.
When are storm is approaching I disconnect my cables outside so that
none of cables enter the house.