On 6/13/2012 7:53 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...
Yah, but... For one of the authorities on lightning and protection go
to http://www.protectiongroup.com/PolyPhaser
>
> 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).
And it usually makes no difference *IF* proper grounding is used. IE all
components at any particular location end up at the same potentials.
Grounding coax to the top *and* bottom of the tower. Running a ground
from the tower to the "single point ground" where the cables enter the
house.
> 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...
Which is usually due to both the rise time DV/DT and magnitude of the
strike, but again with proper grounding rarely makes a difference.
> 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
Here we disagree as I'd call it very rare except for systems without a
single point ground. There is a large capacitance from the shield to
ground for buried cables which tends to drain off a lot of the voltage
and it slows the rise time. Where this is a problem is in dry climates
and soil. Here the soil is normally wet except for about one or two
months out of the year. My tower and antennas have taken at least 17
visually (witnessed)verified *direct* strikes in the last 10 years with
no damage.
> 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.
But the rise time or Dv/Dt of lighting is much lower in frequency than
that. OTOH there is no really typical lightning strike as they cover a
very wide range of rise times and power. The only thing predictable
about lightning is its unpredictability.
>
> 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.
This works only if all the circuits (including phone, coax, Telephone,
TV antenna, and what ever else) are the same length and follow the same
path after they enter the house or pass the ground. Quite often it's not
possible to do things this way as the antenna system, (towers and
antennas) are on the opposite side of the house from the electrical
system entrance. Very few homes are wired in a fashion to make this
possible. Most older homes have circuits connecting outlets in one room
to lights in another, daisy chain fashion. You may find circuits that
alternate lights and outlets running the periphery of the home and this
is not going all the way back to "knob and tube". My single point
ground is on the back of the house where the important stuff enters with
a direct, large cable taking a direct route to the house electrical
ground. There is no practical route for a peripheral ground around the
house to the service ground as it'd be well over 100 feet long going
either direction. Legally there is only one safety ground if they are
all tied together and this service entrance ground must be within a few
feet of the entrance. It's usually of such a nature that it only serves
as a safety ground. For lightning it serves as a tie point rather than a
sufficient ground for lightning. The single point ground for the
station is adequate for most lightning strikes. For the tower I run a
ground out radially from each leg for about 80 feet. Adequate ground
rod spacing is about twice the rod length (see above polyphaser link)
and the radial grounds are tied together for a total of over 600 feet of
bare #2 CadWelded(TM) to 32 or 33 8' ground rods There are two parallel
ground lines from the tower to the SPG with ground rods every 16 feet
(+/-) and polyphasers on each coax.
The ground system on my towers would be a very poor RF ground for a
vertical antenna. OTOH all equipment in each station is bonded
together. Also the soil is quite acetic which means no bolted ground
rod clamps. After a year in the ground you can usually pull the clamps
right off the rods. When we put in an underground electrical service
the ground rod clamps for the service entrance could be lifted right off
the rods as if some one had already removed the screws, yet the
electrician put back the same arrangement. The electrical inspector
told me that was code, but I could go ahead and cad weld them.
73
Roger (K8RI)
>
> You get 3 grounds, safety, lightning, and rf, when you ask questions of
> people who don't understand the above.
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|