albertson.chris@gmail.com said:

What about putting a skylight high on the roof and putting the antenna up in
it?

What's magic about inside vs outside the roof/skylight envelope?

I have a large pine tree out front.  It's roughly 3x the height of my (one
story) house.  What are the chances of any lightning hitting my house rather
than the tree?  What if I put an antenna on the top of my house so the tree
is only 2x the height of my antenna?

Of course, that depends on how far the tree is from my house.  Not far.  Call
it 45 degrees from the back of my house to the top of the tree.  An antenna
on the top of my house would probably be below that sight line.

Is there a good book or URL on lightning vs antennas?  Again, I'm interested
in both the technical issues as well as the local zoning/legal issues.

--
These are my opinions.  I hate spam.

On Mon, Jun 11, 2012 at 10:31 PM, Hal Murray hmurray@megapathdsl.net wrote:

If the skylight is transparent to radio frequency then it might work.
But many times low emisivity glass (tat means the modern energy
savings type) has metallic coatings that attenuate radio signals.

Also from inside the skylight can you see the horizon all around for
360 dregs.  for that to happen the skylight must be taller than the
peak of the roof.  That would be an unusual skylight

A direct strick is not required.  What if the lightening hit a power
pole down the block?  there would still be a huge electric field and
the potential between the antenna and ground would be huge and there
would be current.  If the lightening hit the antenna there is nothing
you can do, grounding will not help.  What he prepared for is the much
more likely chance of a nearby strike.

Here is "The Book"
http://members.rennlist.org/warren/LightningProtectionAndGrounding.pdf

 What if I put an antenna on the top of my house so the tree

The local building code almost always reads "do what NEC says".  In
other words few cities will invest the time and $$$ to write their own
code and will defer to the Nation Electric Code.  Technically the most
sites papers are all for universities in Florida.

--

Chris Albertson
Redondo Beach, California

On 6/11/12 10:31 PM, Hal Murray wrote:

http://www.amazon.com/The-Art-Science-Lightning-Protection/dp/052187811X

Martin Uman and his collaborator Rakov have probably forgotten more
about lightning than everyone on this list collectively knows about it.
This one is a bit pricey still, but is the definitive tome.

A Dover Press version of Uman's "The Lightning Discharge" is <$20, and
well worth the investment if you're interested in lightning.

Ronald Standler's book "Protection of Electronic Circuits from
Overvoltages" is a great source on overvoltage protection in general.
$20 in paperback. Lots of useful information on how to design/purchase
transient suppression for all kinds of signals.  And surprising
information on how certain kinds of techniques can actually make things
worse.

One wants to be careful about texts published by manufacturers of
protection equipment.  Yes, they typically have valid information, but
it is coming from a source which wants you to "buy more stuff", so
they tend to be a bit more conservative (more protection = better, even
if the physics doesn't support it).  That said, much of the high quality
peer reviewed research on things like lightning rods (aka "air
terminals") does come from companies making such things.

Also, there are several pubs out there widely distributed aimed at
applications like FAA Control Towers or high reliability 24/7 land
mobile radio.  The recommendations in those books may prove to be
somewhat of overkill for a couple reasons:  most amateurs don't need
that level of protection; the suggestions aren't always supported by the
physics, but are there because "they don't hurt", triggering the "nobody
got fired for buying IBM mainframes" phenomenon... if it's small
differential cost, why not do it, because if we don't do it, and
something goes wrong, we'll be blamed.

Legality wise

Nat Elec Code (aka NFPA70) has bonding and grounding requirements.  Art
250 on grounding, Arts in the 800s on antennas.  Expensive ($80-100) if
you buy it, but since it forms the basis for California's Title 24
(State Electrical Code), a scanned version is online at
https://public.resource.org/.  The antenna grounding stuff doesn't
change very much, so an older code found at a used book store might also
work.

NFPA780 is the lightning protection code.

IEEE 1100 - The Emerald Book - is very useful on grounding, transient
protection, etc. issues in general. Pretty expensive.. find it in a library.
http://standards.ieee.org/findstds/standard/1100-2005.html

Hi gang
Just to upset the apple cart a bit, high trees do not necessarily
protect a large area from lightning.
In my past life as a range officer at a large shooting facility, we were
hit by lightning directly in front of the firing line during a storm.
The tree line was about 20 to 30 ft behind us and at least 50+ feet higher.
There was even evidence of arcing between 2 pieces of conduit stuck in
the ground about 5 ft apart. first and last time I saw a fire ball.
For induced charges from nearby strikes nothing beats a good ground and
lightning protector system.
If a direct hit, I hope your insurance is paid up.
My gps antennas are about 25 above ground just above my roof line. The
mast is grounded to 2 ground rods and the coax fed through gas tube
lightning protects. No problems even with hits several hundred yards away.

Ewing (Rix) Seacord
K2AVP/4/499
eseacord@verizon.net

845-628-0892 Home
914-262-9186 Cell
914-233-3886 Skype Notebook

On 6/12/2012 1:31 AM, Hal Murray wrote: