On Mon, 26 Sep 2011 22:08:23 -0700, "Sal" wrote:

"Jeff Liebermann" wrote in message
.. .
Well, if you're going to go through all that trouble to install a
proper ground, you can do as well using Unistrut instead of a 2x4.
http://www.unistrut.us

Unistrut is wonderful. About 20 years ago, I used to engineer installations
in a big government lab in San Diego and it was the customer's preferred
method for anything structural that didn't involve concrete. :-)

I learned you could do anything with Unistrut. It's the Erector Set / Lego
Blocks for grown-up kids.

"Sal"

Agreed. However, you have to be careful using Unistrut for outriggers
on towers. Unistrut doesn't do well when twisted (in torsion) by a
top heavy antenna. The only way that seems to work well on a tower
are two 1 1/2" channel Unistrut outriggers, with a connecting pipe for
mounting the antenna, dish, or whatever. A single outrigger usually
results in a the antenna twisting around in the wind. (1 1/4" will
work with light antennas).

Something like this:
http://i668.photobucket.com/albums/vv44/ahwwwjeah/My%20Tower/DSCF6120.jpg
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Thanks for all the info

I am going to ground out the satellite dish to the tower.

Now it is grounded to my new plazma and I would feel more comfortable with
it grounded to the tower. Don't want nothing happening to that new plazma.
Now of course if it ever got hit by lightning I would imagine the receiver
would get fried, maybe not the tv, the coax goes into the receiver, then
into the yamaha sourround sound then into the tv. A jolt has to stop
somewhere along that line.





"Jeff Liebermann" wrote in message
...
On Mon, 26 Sep 2011 22:08:23 -0700, "Sal" wrote:

"Jeff Liebermann" wrote in message
. ..
Well, if you're going to go through all that trouble to install a
proper ground, you can do as well using Unistrut instead of a 2x4.
http://www.unistrut.us

Unistrut is wonderful. About 20 years ago, I used to engineer
installations
in a big government lab in San Diego and it was the customer's preferred
method for anything structural that didn't involve concrete. :-)

I learned you could do anything with Unistrut. It's the Erector Set /
Lego
Blocks for grown-up kids.

"Sal"

Agreed. However, you have to be careful using Unistrut for outriggers
on towers. Unistrut doesn't do well when twisted (in torsion) by a
top heavy antenna. The only way that seems to work well on a tower
are two 1 1/2" channel Unistrut outriggers, with a connecting pipe for
mounting the antenna, dish, or whatever. A single outrigger usually
results in a the antenna twisting around in the wind. (1 1/4" will
work with light antennas).

Something like this:
http://i668.photobucket.com/albums/vv44/ahwwwjeah/My%20Tower/DSCF6120.jpg
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

" Tuuk" wrote in message
...
Thanks for all the info

I am going to ground out the satellite dish to the tower.

Now it is grounded to my new plazma and I would feel more comfortable with
it grounded to the tower. Don't want nothing happening to that new plazma.
Now of course if it ever got hit by lightning I would imagine the receiver
would get fried, maybe not the tv, the coax goes into the receiver, then
into the yamaha sourround sound then into the tv. A jolt has to stop
somewhere along that line.

I know what an opto-isolator is. (Provides a signal path with a short-range
modulated light beam -- no copper connection) I don't know if any have been
designed/sold for lightning protection.

Anybody?

"Sal"
(KD6VKW)

On 9/26/2011 3:45 PM, Jeff Liebermann wrote:
On Mon, 26 Sep 2011 12:04:21 -0700, Jim
wrote:

On 9/25/2011 7:06 PM, Jeff Liebermann wrote:
The end and
highest antennas were also grounded with a undersized #4 copper wire
running down the pole to a ground rod at the base. We didn't get any
lightning hits because there was an all metal forestry lookout at the
highest point on Santiago which took all the hits.

#4 is plenty big enough for lightning grounding. The current is high,
but the duration is short. You see larger lightning conductors for
mechanical reasons (e.g. where it might get damaged, or where it has to
move repeatedly).

One of the neighboring towers took a direct hit. The #6 AWG he was
using literally exploded, spraying melted copper everywhere and
blowing pieces of concrete block all over the building.

http://en.wikipedia.org/wiki/American_wire_gauge#Table_of_AWG_wire_sizes
Fusing current (32msec) for #4 is 34,000 Amps. The average lightning
hit is about 20,000A but can go up to 200,000A.


One might want to check a different source for fusing current. The
standard test lightning stroke has a 2 microsecond rise time (10% to
90%) and a 50 microsecond fall time (to 50%) typically represented by a
double exponential (1-exp(-t/a))*exp(-t/b), so when you used the
Onderdonk equation, you want to use a 50 microsecond pulse

In general, fusing goes as a function of the "action", I^2*time.

The equation from Onderdonk is usually used for this kind of thing (as
shown in the table you cited). Onderdonk's equation assumes a
rectangular current pulse, but that works for the most part and for
lightning, it would be a conservative estimate.

For a 50 microsecond pulse, the fusing current is 22 amps/cmil (vs .87
A/cmil for the 32 ms pulse)

AWG4 is 42kcmil, roughly, so the fusing current is well over 8000kA

Another approach to analysis might be to consider the energy dissipated
in that wire. It's a bit tricky, because as the wire heats up, the
resistance increases. (by a factor of 4 or 5 at the melting point, as I
recall)

But let's consider a meter of that wire which is 5.189mm in diameter.
it has a resistance of 0.82 milliohm (at 25C). Let's round up to 5
milliohms.

Let's say our lightning stroke has an RMS current of 50kA (peak current
would be higher), so we have (50E3)^2*5E-3 = 12.5 MW. But that lasts
for only 50E-6 seconds, so we deposit 12.5E6*50E-6 = 625 Joules into the
meter of wire.

That wire has 21 sq mm cross section so the one meter length is about
21E-6 cubic meters or 21 cc. Copper has density about 7 g/cc, so we've
got around 150 grams of copper there. At a specific heat of 0.385 J/g
every 58 Joules will raise the temperature 1 degree. Since we're
dumping in about 600 joules, that lightning stroke will raise the
temperature around 10 degrees.

In practice, a typical lightning strike has several strokes, so you
probably dump 4-6 times that amount of energy into the wire. But still,
you're talking maybe a 50-60 C temperature rise, which is LONG way from
the 1000 degree rise you need to melt the copper.


Your observation of total destruction of a copper conductor was probably
from some other mechanism. The electromagnetic forces are the best
candidate. I used to do some quarter shrinking, and a few thousand amps
in sub millisecond pulse would destroy a AWG10 coil from the magnetic
forces.

An unsupported turn of AWG 4 or 6 wire would probably be destroyed by a
20kA pulse with a rise time of 2 microseconds. The di/dt is 400E9 A/sec,
so the flux is pretty spectacular.

The other source of disaster in high power pulse discharges is if there
is a loose connection or a gap in the conductor. The several tens of
volt cathode drop in an arc at 20kA gets the peak power up pretty high,
and unlike in the example above where the 12MW is distributed over a
meter of length, you get that megawatt dissipated in a few cm, so the
energy density is much higher.


http://www.lightningsafety.com/nlsi_info/media.html
Other references offer averages from 5,000 to 50,000A. I would call
#4 marginal but probably adequate for California, which doesn't get
much lightning.

On 9/27/2011 1:10 PM, Sal wrote:
" wrote in message
...
Thanks for all the info

I am going to ground out the satellite dish to the tower.

Now it is grounded to my new plazma and I would feel more comfortable with
it grounded to the tower. Don't want nothing happening to that new plazma.
Now of course if it ever got hit by lightning I would imagine the receiver
would get fried, maybe not the tv, the coax goes into the receiver, then
into the yamaha sourround sound then into the tv. A jolt has to stop
somewhere along that line.

I know what an opto-isolator is. (Provides a signal path with a short-range
modulated light beam -- no copper connection) I don't know if any have been
designed/sold for lightning protection.



Typical optos can stand off 5kV or so. Speed is a problem There are off
the shelf opto isolator widgets for things like RS232 and phone lines
that are fairly inexpensive. Get to RF, though, and the price goes way up.

If you need more, you go to a fiber optic link of some sort.

On Tue, 27 Sep 2011 13:10:30 -0700, Sal wrote:

" Tuuk" wrote in message
...
Thanks for all the info

I am going to ground out the satellite dish to the tower.

Now it is grounded to my new plazma and I would feel more comfortable
with it grounded to the tower. Don't want nothing happening to that new
plazma. Now of course if it ever got hit by lightning I would imagine
the receiver would get fried, maybe not the tv, the coax goes into the
receiver, then into the yamaha sourround sound then into the tv. A jolt
has to stop somewhere along that line.

I know what an opto-isolator is. (Provides a signal path with a
short-range modulated light beam -- no copper connection) I don't know
if any have been designed/sold for lightning protection.

Anybody?

"Sal"
(KD6VKW)

These fine folks make a heavy duty 75 Ohm protector:

http://www.alphadeltacom.com/

On Tue, 27 Sep 2011 07:53:06 -0400, " Tuuk" wrote:

Thanks for all the info

Y'er welcome. Now, if you want a usable answer to your question, it
might be helpful if you describe the tower, what manner of DBS dish,
and some of the distances involved.

I am going to ground out the satellite dish to the tower.

If your unspecified type of tower is properly grounded, the tower will
protect the dish from a direct hit. Still, it's a good idea to ground
everything to reduce the effects of induced currents from a nearby
hit.

Grounding the dish is probably a good idea. Grounding the LNB at the
dish is a bad idea. Depending on model of LNB, many of them have no
DC connection between the LNB case and the dish ground. This is not
for lightning protection but to prevent ground loops. In general, you
want to do it the way the installation manual suggests and the NEC
electrical code demands, which is a grounded barrel connector
somewhere close to the utilities ground.
http://www.dbsinstall.com/whatis/Whatisgood-5.asp
Such a ground is NOT to protect against lightning, but to protect
against getting electrocuted if the satellite receiver magically loses
its protective AC ground and leaks some 117VAC onto the coax.

This discussion has some good comments on DBS dish grounding.
http://www.dbstalk.com/showthread.php?t=16300

Now it is grounded to my new plazma and I would feel more comfortable with
it grounded to the tower. Don't want nothing happening to that new plazma.
Now of course if it ever got hit by lightning I would imagine the receiver
would get fried, maybe not the tv, the coax goes into the receiver, then
into the yamaha sourround sound then into the tv. A jolt has to stop
somewhere along that line.

The general idea is to give the current a better path to ground than
through your expensive electronics.

Incidentally, I just found this on using wood for mounting:
http://www.dbsinstall.com/diy/GroundPostInstallation.asp

What's inside a Polyphaser cellular lightning protector:
http://802.11junk.com/jeffl/pics/lightning/index.html
Note the 4 ceramic spark gaps in series. You get 4 hits, and then it
shorts out.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On Sep 25, 7:46*pm, Jeffrey Angus wrote:
On 9/25/2011 6:29 PM, Ralph Mowery wrote:

"Jeffrey *wrote in message
Several commercial tower sites I used to service used wooden
cross supports between telephone poles.

The thing with lumber is that it can hold moisture and cause the tower to
rust where the wood is.
Also if it is treated wood, the chemicals may cause the same thing only
faster.

Each cross brace has about a dozen 2" diameter galvanized stubs to
attach the antennas to. They are bolted directly to the cross braces.

All the wood was treated with Creosote just like the poles themselves.
The site was at least 40 years old the last time I was up there in 2005.

The stubs didn't look any worse than the usual stubs bolted to metal
tower assemblies.

Jeff

--
"Everything from Crackers to Coffins"

Same here Jeff, creosote poles and cross arms 30 years old with hot
dipped galvanized hardware. No sign of deterioration.

Jimmie

On Tue, 27 Sep 2011 20:32:28 -0700, JIMMIE wrote:

On Sep 25, 7:46Â*pm, Jeffrey Angus wrote:
On 9/25/2011 6:29 PM, Ralph Mowery wrote:

"Jeffrey Â*wrote in message
Several commercial tower sites I used to service used wooden cross
supports between telephone poles.

The thing with lumber is that it can hold moisture and cause the
tower to rust where the wood is.
Also if it is treated wood, the chemicals may cause the same thing
only faster.

Each cross brace has about a dozen 2" diameter galvanized stubs to
attach the antennas to. They are bolted directly to the cross braces.

All the wood was treated with Creosote just like the poles themselves.
The site was at least 40 years old the last time I was up there in
2005.

The stubs didn't look any worse than the usual stubs bolted to metal
tower assemblies.


Same here Jeff, creosote poles and cross arms 30 years old with hot
dipped galvanized hardware. No sign of deterioration.

Jimmie

Ask John Franklin in Santa Barbara how his telephone poles are doing on
Gibraltar these days...