[TowerTalk] Rooftop vertical grounding
Michael Tope
W4EF@pacbell.net
Thu, 10 Feb 2000 18:32:39 -0800
Jeff,
One suggestion that I have heard here (on TowerTalk) in the past
is to ground the shield of your coax cable just outside the feedthru
entrance and then coil a few turns of the coax to form a choke
between the ground point and the feedthru panel. The choke together
with the ground connection forms a low pass filter that will help to
shunt high frequency lightning dishcharge energy to ground.
Running a separate ground conductor in parallel with your coax
will probably help as the shield of your coax will have less
current carrying capability than a big #1 ground conductor. As
far as disconnecting the station ground from the antenna ground
when not operating, I would tend to agree. If the equipment is
not physically connected to anything, the likelihood of lightning
damage would seem less than if there is some physical connection
to the antenna ground. Even the best of grounds will have some
finite impedance which implies that your tie point will "bounce"
above zero potential during discharge events. But I am not expert,
and lightning has been know to do some weird counterintuitive
things.
Good luck with your new antenna.
Mike, W4EF..................
----------
From: berferd[SMTP:jrp@dimensional.com]
Reply To: jrp@dimensional.com
Sent: Thursday, February 10, 2000 1:35 PM
To: towertalk@contesting.com
Subject: [TowerTalk] Rooftop vertical grounding
Hello everyone.
My name is Jeff and I'm a new ham. I received my general class
license approximately two weeks ago. Call sign is KC0HEP. I
live in Colorado. I'm not putting up a tower (yet), so if this
not the appropriate forum for my questions, please chastise
me and I'll be on my way.
My first antenna is going to be a vertical. The Butternut HF9V
to be exact. I'm planning on mounting it the roof of my house
with the counterpoise kit on a tripod. The antenna is about
26 feet in height.
I'm trying to figure out the best way to ground this thing in
case Mother Nature decides to zap it.
The antenna needs to be mounted in the middle of the roof because
there will be four rope guys going to the corners of the roof.
So the down conductor(s) will have to take a turn when it gets
to the edge of roof. I understand from reading messages in
the towertalk archive that the turn should be as small as
possible. Does anyone know how tight I can go? If I did get
a lightning strike and the bolt reaches the turn, I don't
know where it else it could go (other than down the coaxial
cable). All the fences around here are wood and there aren't
any above ground utilities. So wouldn't it continue down
the conductor to ground in spite of the turn?
I'm thinking about two down conductors of #1/0 gauge going down
adjacent corners of the house into separate ground rods tied
together. Is this enough? Can I get away with only one down
conductor? It is my understanding that the down conductor
surface area should be at least equal to or larger than the
total surface of the coax. I'd like to use insulated conductors
if possible. Any reason why I should use bare conductors?
Since my shack will be in the basement, I'm looking at
installing an entrance panel at the top of the wall of the
shack (have to see how much room I have between the top of
the foundation and the floor above to see if I can squeeze
one in there). So the outside of the entrance panel would
be tied into earth ground and the inside would be tied
into the equipment bus bar. I have a electrician coming
over to see about moving my service ground (which currently
resides under a concrete patio) somewhere else so I can
tie into it.
The part that scares the hell out of me is tying my shack
bus bar into the antenna ground.
Let's say I've done everything by the book with respect
to lightning protection. What if the energy from the
strike isn't dissipated by ground quickly enough? What's
to stop the energy from finding it's way to my equipment?
I'd like to have a good RF ground for sure, but I'm worried
about protecting my equipment. I'd like to set something
up where I could not only disconnect the antenna feed
from the shack, but also disconnect the bus bar from
ground when not operating. I'm thinking that this way,
if a lightning strike does occur (when I'm not operating)
the equipment is isolated. Of course *all* connections
would be severed (power, etc.)
Is this a good idea? Comments? Flames?
Thanks for reading this far.
Jeff
KC0HEP
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