[TowerTalk] Tower grounds
jimlux at earthlink.net
Wed Jun 13 05:51:15 PDT 2012
On 6/13/12 4:53 AM, David Robbins wrote:
>> 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,
This is why the emphasis on "bond everything together" at the entrance
to the building in the NEC. It may seem quite dorky, but that little
grounding block for cable TV or satellite installation actually does
something useful: it makes sure the shield of the coax is at the same
voltage as all the rest of the stuff near by.
> 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.
Well.. the three grounds are interconnected, but the connection may not
be very effective in one direction..
COnsider a RF ground at 30 MHz and the AC safety ground (greenwire).
The NEC required bonding of the two makes sure that the 30 MHz ground
isn't "hot" relative to someone standing on the soil or touching
equipment hooked up to greenwire ground. But that bonding conductor
(which can be many wavelengths long) doesn't do much for the RF
properties. In fact, one could legally bond the two with a big RF
choke. you've met the NEC safety requirement (e.g. if a power line
falls on your antenna, it blows a fuse or the antenna, rather than
killing the person touching the radio)
Lightning is more about "bonding" than "grounding".. making sure
everything goes up and down together so there's no "leaping the gap"
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