[TowerTalk] CATV & Phone grounds

[Jim Brown]

On Tue, 11 Jul 2006 12:13:49 -0700, Bill Turner wrote:

>As I understand it, lightning
>really does flow in one direction, making it DC but having a
>square-wave nature. Is that where the HF component comes from?
>Lightning doesn't really change directions, does it?

First, think of the current flow in the output stage of any 
electronic amplifier. The instantaneous current flow has a DC bias 
-- that is, it is always flowing in the same direction -- but it 
has strong RF components, namely the signals they are amplifying.  
The initial charge that sets off the lightning strike is the DC 
bias, but the fact that it comes in the form of a fast risetime 
pulse can cause oscillation in the resonant circuit that is the 
discharge path. 

As others have noted, the HF components comes from the very fast 
rise time of the current discharge. Remember that from the 
mathematical analysis that Fourier taught us more than a century 
ago, any transient or non-sinusoidal waveform will resolve into an 
infinite number of sine waves of increasing frequency (or if it is 
an impulse, a series of sine waves of decaying strength). If the 
single is repetitive, the higher frequency components will be 
harmonically related to the repetition rate. The resulting 
waveform can still come out with a DC bias, and the waveforms 
don't necessarily all need to begin at the same instant (thus they 
can resolve into any DC value, based on the physics of what else 
is happening in the circuit). 

Hope this helps.

Jim Brown K9YC