[TowerTalk] Ground resistance

[Jim Brown]

On Thu, 8 Jun 2006 22:58:04 -0500, Gary Schafer wrote:

>> And remember that the nature of coax is to act as a common mode
>> choke at RF, so when you short the shield to ground, the center
>> conductor is pretty close to ground too. It isn't a PERFECT common
>> mode choke, so perhaps 1% of the lightning strike is present
>> between the center conductor and ground. So if the lightning is 50
>> kV, the center conductor might be 500 v above the shield. Enough
>> to blow something connected, but probably not to arc across the
>> switch.

>Unfortunately the center conductor is not at or near shield potential
>because of propagation qualities of the coax. Lightning energy will
>propagate on the coax like any other signal.

Huh?

>Also even if the center conductor is shorted to the shield at the antenna
>with a grounded type antenna feed the lightning energy will still find its
>way to the center conductor from the energy that is carried by the outer
>shield. 

I suggest you study Ott's discussion on the nature of the field inside a 
circular shield.

>This happens because of "transfer impedance". 

Huh? 

>At low frequencies, of
>which includes a large part of lightning, the skin effect that normally
>keeps energy on the outside of a cable from penetrating the shield falls
>apart and allows current on the inside of the shield. 

For any coax that your or I would use on a transmitting antenna, this 
happens BELOW a few kHz. Above that frequency, it's a common mode choke with 
a coupling coeffient nearly equal to unity. There is SOME energy in 
lightning below that transition frequency, but a tiny fraction of what is at 
much higher frequencies. Again, see Ott. 

>This is why it is as important to protect the center conductor as it is 
>the shield.

You misread my post. I simply observed that if no equipment is connected to 
an coaxially fed antenna, the ANTENNA would be pretty well grounded from a 
lightning protection point of view if the shield was grounded. The function 
of the arrestor like a Polyphaser is to protect radio EQUIPMENT connected to 
the coax by shorting the center conductor to the shield. 

Jim