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Re: [TowerTalk] Tower grounds

Subject: Re: [TowerTalk] Tower grounds
From: David Robbins <>
Date: Wed, 13 Jun 2012 06:53:02 -0500 (CDT)
List-post: <">>

>You are ahead to recognize there are two grounds: one for lightening
>protection, the other for radio frequencies.

no, there is either NO ground, or ONE ground, or 3 grounds, depending on who 
you talk to and about what type of phenomena... 

There is NO ground when you talk about lightning or rf and your components are 
more than a few meters apart (or less if you are talking vhf/uhf/microwave). If 
your tower is not right next to the shack you can develop very large voltages 
between the ground connection at the tower and the ground connection at the 
shack... for an average lightning stroke to the tower that could be hundreds of 
thousands of volts for periods of several microseconds... more than enough for 
all sorts of nasty things to happen.  One of the nasties is called a 
back-flashover where the voltage on the 'ground' conductor or tower or coax 
shield gets so high that it flashes over the insulation to the power carrying 
or signal conductor... this is actually one of the most common causes of 
failure in equipment connected to coax coming in to the house, or even to power 
outlets, the ground voltage is raised by even a nearby lightning stroke and to 
equalize the voltage it flashes over back to the power conductor(often 
 through your power supply) or coax center conductor (through your radio 
receiver input). if you have a 10m vertical mounted on the ground the voltage 
along the ground system goes from a min to a max in a short distance along the 
ground, try to measure voltages at 2m frequencies with test leads a half meter 
long and the voltage at the scope or meter ground can be much different thant 
the voltage on the equipment you are measuring.

Legally there can be only ONE ground. Most building codes require that all 
equipment be grounded to the same building power entrance ground for safety 
purposes. Fortunately doing this also helps with preventing voltage differences 
between equipment and you in the shack due to rf currents on coax shields or 
other cables. Note, it does not prevent the currents, nor will it 'drain' them 
away to 'ground'... it just keeps all the equipment in the shack at the same 
potential so you don't get bit when you touch two different things.

You get 3 grounds, safety, lightning, and rf, when you ask questions of people 
who don't understand the above. 

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