TT:
So I conclude that lightning energy impinging on a buried coax cable
will at most have a surface/shield current flowing on it, which can be
shunted to ground by a properly applied ground connection at the SPG panel
outside my house/shack. My planned system configuration exactly. Any
energy on the center conductor (which got there by striking one of my
antennas at the tower top) ought to be taken care of by the arrestor, at the
bottom of the tower - again, my configuration.
73 de
Gene Smar AD3F
-----Original Message-----
From: Tom Rauch <w8ji@akorn.net>
To: towertalk@contesting.com <towertalk@contesting.com>; teamw
<teamw@quixnet.net>
To: <towertalk@contesting.com>
Date: Monday, July 23, 2001 11:50 AM
Subject: Re: [TowerTalk] Routing Cables - Tower to shack
>
>> I get the impression that in some readers minds buried cable is immune to
>> the effects of lightning. However, the earth is not a magnetic shield. A
>> cable a foot or two underground can have a surge induced in it of a
>> thousand volts or so from a lightning strike a quarter mile away, or
more.
>> (Twice, one when the strikes magnetic filed expands outwards and again
>> when the field collapses). Those surges can be on the shield of your
>> buried coax cable and on the center conductor.
>
>The main advantage of the earth is the cable is tightly coupled to
>earth around the cable, and is very unlikely to have much potential
>difference between it and earth from distant strikes. Put simply,
>even if the shielding is less than perfect it all moves together in the
>same direction at the same rate. Buried cables are mostly immune
>unless very dense conducted charges along the earth's surface
>from a close-by strike create problems.
>
>The *induced* charges that are most problematic are induced when
>the cable or metallic structure is vertical or suspended above
>ground.
>
>I think what mostly is missed is lightning is NOT dc, it is a varying
>waveform. The energy peak is at 10kHz, with considerable energy
>extending higher in frequency.
>
>The key to lightning protection is proper grounding, and avoiding
>ground loops through equipment. Virtually all of the problem deals
>with connections to shields, there is very little differential mode
>current or voltage induced inside a shielded cable. The exception
>would be a case where you have a very large antenna element
>connected to the center conductor.
>
>If you depend on a lightning arrestor you are deluding yourself. The
>vast majority of the problem is common mode, not differential
>mode, voltage between conductors of incoming lines or cables! I'd
>recommend reading the NAB (National Association of
>Broadcasters) Handbook. They aren't trying to sell anything but
>protection. In the section on antennas and towers, they cover site
>lightning protection, and list other references.
>
>
>73, Tom W8JI
>W8JI@contesting.com
>
>List Sponsor: Are you thinking about installing a tower this summer? Call
us
>for information on our fabulous Trylon Titan self-supporting towers - up to
>96-feet for less than $2000! at 888-833-3104 <A
HREF="http://www.ChampionRadio.com";>
>www.ChampionRadio.com</A>
>
>-----
>FAQ on WWW: http://www.contesting.com/FAQ/towertalk
>Submissions: towertalk@contesting.com
>Administrative requests: towertalk-REQUEST@contesting.com
>Problems: owner-towertalk@contesting.com
>
List Sponsor: Are you thinking about installing a tower this summer? Call us
for information on our fabulous Trylon Titan self-supporting towers - up to
96-feet for less than $2000! at 888-833-3104 <A
HREF="http://www.ChampionRadio.com";>
www.ChampionRadio.com</A>
-----
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
|