[TowerTalk] Antenna/Tower Grounding (Lightning Protection)

[Jim Brown]

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.  

73,

Jim Brown K9YC