on 12/12/02 7:18 AM, Jason Hissong at jhisson1@columbus.rr.com wrote:
>>> You want to keep the voltage potential between the top of your tower and
> the
>> earth to be a minimal as possible. Sometimes, it doesn't matter how good
>> your grounding system is and you will still take a strike. But you lessen
>> your chances which is half the battle.
>
> What about the antenna itself? The tower will be grounded like crazy, the
> antenna is only "half-grounded" with the shield being grounded. The rotor
> cable will of course be grounded as well.
>
> Thank goodness it is winter as I can do this in smaller chunks with little
> or no chance of lightning for the next couple of months.
It depends on your antenna itself. Many antennas are DC grounded. Some are
not. DC Ground and RF Ground are two very different things, by the way.
You ensure your antenna is properly grounded/protected by adding a surge
arrestor in your coax line. You can buy these from ICE or Polyphaser. The
nice things about the ICE and most of the Polyphaser units is that there is
a DC connection between the center conductor of the coax and ground. If you
pop a box open (I have the ICE unit), you will see a large torroid connected
to ground. This serves to connect the antenna to ground and also it serves
to bleed off any static charge that may tend to build up on the antenna due
to wind, snow, and charges during a thunderstorm. It will also help your
receive noise level as this static is now drained.
In parallel to the torroid is a gas discharge tube. This tube is what will
shunt the bulk of an RF strike to ground. Once the voltage on the center
conductor rises above a certain amount, the gas in the tube ionizes and
flashes over creating a very low impedance path to ground.
Finally, there is a DC blocking cap on the center pin in series with the RF
path. Why is that there? Well, if lightning hits your antenna, energy will
propagate on two paths through the coax. One will be on the outer shield of
the coax. The other will be on the inner conductor. Remember that
lightning is not DC but actually low frequency RF. So the energy propagates
on the surface of the conductors. Since the energy on the center conductor
must travel through dielectric, its speed of propagation is slower than the
energy traveling on the outer surface of the shield. Therefore, the strike
energy on the inner conductor will reach the suppressor AFTER the strike
energy on the outer shield. This will create a DC potential across the
coax. W/o that blocking cap, that DC could be seen by your radio equipment,
thus causing a problem! There is some level of debate on how much effect it
has in the real world. Polyphaser's site has some pretty convincing
reading.
Put the suppressor on the outside of the house at the point where you will
bring the coax inside. It needs to be connected to ground by a very low
inductive strap or wire bundle or something like that. Additionally, it
helps to run your coaxes through a grounded metal box when entering your
house. The reason is that the EMF from a strike can induce a voltage on
your coax on the other side of your suppressor. That metal box acts as what
is called a Faraday shield to help prevent such occurances.
Another great source of lightning protection info is on Bill Hidder's (N3RR)
website: www.erols.com/n3rr
Look for the lightning protection hyperlink.
There's a lot to understand and deal with when it comes to preventing and
dealing with strikes. Most hams have no clue. I didn't until I started
reading about it and asking detailed questions from guys like Bill.
73,
Jon
NA9D
-------------------------------------
Jon Ogden
NA9D (ex: KE9NA)
Citizen of the People's Democratic Republik of Illinois
Life Member: ARRL, NRA
Member: AMSAT, DXCC
http://www.qsl.net/na9d
"A life lived in fear is a life half lived."
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