> -----Original Message-----
> From: firstname.lastname@example.org [mailto:towertalk-
> email@example.com] On Behalf Of Jim Brown
> Sent: Thursday, January 04, 2007 1:15 PM
> To: Tower Talk List
> Subject: Re: [TowerTalk] Antenna/Tower Grounding (Lightning Protection)
> On Thu, 4 Jan 2007 11:05:29 -0500, Gary Schafer wrote:
> >However as Jim noted 2 microseconds is a more typical lightning
> >rise time.
> Think AM Broadcast -- according to IEEE data, on average, the
> energy in lightning has a VERY broad peak around 1 MHz (so broad
> that there's lots at 2-4x that frequency and 0.2X that frequency).
> It is a VERY big mistake to think of lightning as DC. Yes, there's a
> DC component, but with LOTS of RF power. Think MF (300 kHz-3 MHz),
> think LOW IMPEDANCE to earth at 2 MHz. The impedance to earth of a
> ground system will be the parallel combination of all your
> connections to earth (ground rods, building steel, tower footing,
> cold water, radials), PLUS the inductance of the wire connecting
> them. The capacitance between a big radial field and the earth can
> be a significant component of lowering that impedance.
> Jim Brown K9YC
Jim is correct. A well designed lightning ground will also make an excellent
RF ground for a vertical antenna. If it will not serve as an RF ground then
it is not an adequate lightning ground either.
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