[TowerTalk] Shack to service entrance ground

[Jim Brown]

> At the risk of stating the obvious run all antenna circuits to enter near
> the service entrance and bond there. Continue the run inside the house to
> the “shack.”

For many reasons that depend on where everything is in any given home, 
the shape of the home, and what's around it (for example, concrete) that 
is NOT the obvious solution, and in many situations it's a poor one.

Here's one important reason. Lightning arrestors for coax work by
1) taking the shield to properly bonded ground at a suitable point, and 
2) shorting the center conductor to the shield, protecting the input 
circuit of the receiver and/or the transmitter/power amp output stage. 
#2 depends on that short being placed very close to the protected 
circuitry; if there's a lot of coax between the protector and that 
circuitry, a differential voltage will be induced on center conductor by 
current in the shield (from the strike), requiring a second protector at 
the shack. The strength of the induced voltage is proportional to the 
shield current and the resistance of the shield at the frequency(ies) of 
interest, and inversely proportional to the density/uniformity of the 
shield. A measure of this relationship is the "transfer impedance" of 
the shield, and is the ratio of the induced voltage to the shield current.

 > At a minimum you will almost certainly destroy the RS232/USB interface
 > on your rig and the RS232/USB port on the computer.

A VERY common cause of computer interfaces being fried are Pin One 
Problems in the victim equipment (that is, failure to bond cable shields 
to the shielding enclosure at the point of entry). This causes any 
shield current (RFI, or the strike) to wander around circuit common on 
the circuit until it finds the chassis, creating IR drop along they way. 
When it's only hum, buzz, or RF, we hear hum or buzz in the audio, or 
have RF feedback. When it's a strike, stuff fries.

Virtually all consumer products, everything from computers to home 
entertainment to TV sets, AND virtually all ham gear, are built with Pin 
One Problems. That's why ferrite common mode chokes and proper bonding 
inside the shack work to kill RFI!

73, Jim K9YC