[TowerTalk] Tower grounds
k1ttt at verizon.net
Wed Jun 13 04:53:02 PDT 2012
>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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