> -----Original Message-----
> From: towertalk-bounces@contesting.com [mailto:towertalk-
> bounces@contesting.com] On Behalf Of Jerry Keller
> Sent: Thursday, January 04, 2007 10:26 AM
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] Antenna/Tower Grounding (Lightning Protection)
>
> In his well-stated list of grounding advice, Gene AD3F includes: "3.
> You
> ought to ground each coax cable shield to the tower at the top and at the
> bottom of the tower."
>
> I've seen this advice before, but have never understood the reason for it,
> or what the adverse consequences might be if everything else Gene
> recommends
> is done, and this is not (as is the case with my system). Can anyone
> explain this for me, please?
>
> 73, Jerry K3BZ
The idea is to try and keep all voltages at the same level so as not to have
anything arcing as in between the cable and the tower.
Further to that, grounding the coax at the very bottom of the tower greatly
reduces the voltage on the cable that goes to the shack.
If you were to bring the cable off the tower a few feet above ground rather
than at the bottom of the tower the voltage difference between a few feet
above ground and ground level may be several thousand volts difference.
The tower has significant inductance and with a high current strike
lightning provides there is a very large voltage difference between the top
of the tower and the bottom of the tower because of this impedance. The
tower looks like a large resistor divider during a lightning strike. The
further up from ground you go the higher the voltage level you will find.
There was a time when it was standard procedure for all 2 way etc. radio
installations to bring the coax away from the tower to the building at about
10 feet above the ground in a cable tray or on a messenger cable to the
building. This was very bad for lightning protection. Now the recommended
way is to bring the cables all the way to the bottom of the tower before
coming off. There are still a lot of installations that bring the cables off
at some height above ground by necessity. But if you have many cables and or
very large cables and a good ground system of heavy conductors at the
building then you can get away with doing so because there is a lower
inductance path to earth via the many/large conductors.
73
Gary K4FMX
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|