K7LXC@aol.com wrote:
> In a message dated 4/2/02 6:42:22 AM Pacific Standard Time,
> goudpj@mac.com writes:
>>> The mast is metal, the TB and its holding bolts are metal, it's
>>> attached to the metal tower - seems like that's enough for me.
>> Well, the only contact between the mast and the tower is through
the
>> point contacts across the balls, or rollers, in the bearing, which
>> doesn't seem like a low impedance path.
> There are over a dozen ball bearings in the Rohn thrust bearings so
I
> suspect that it's a pretty good contact. Even if there are some ohms
> of resistance, there is virtually no gap for the strike to arc
across
> so you still wind up with a decent path.
OK, thanks for that, had no idea if these bearings are ball, roller,
tapered roller, or bronze bushing. The problem with anything other
than a bronze bushing is that the point contact of the balls, or the
line contacts of the rollers, is just that, a tiny contact area.
This creates a large contact resistance and an incredibly large
current density at that point.
My guess is that the overall dc resistance is low enough to keep the
mast at the tower's potential so it can't build up a charge, but is
too high to handle any surge currents.
> Look at it like this - where else is the strike going to go?
Putting a
> strap from the mast to the tower won't work because of the inductive
> resistance (?) so you're stuck with that path anyway.
Well, that's my question actually, where do surge currents flow
during a strike to an antenna or mast?
Adding jumpers of some kind has been recommended, but that strap
would only be good for static grounding, and not very helpful for
surge currents. Braid or stranded wire won't hold up to strike
currents and solid wire or strap would eventually fail no matter how
carefully applied and the inductive nature of the loop around the
rotator would naturally, as you say, impede fast risetime surge
currents anyway.
This is one of the points I was trying to find an answer to. That
is, if the paths to the tower are all relatively high impedance, a
strike to an antenna element, boom, or mast isn't as straightforward
a thing to manage as one to the tower. And this isn't covered in the
Polyphaser book or any other source I've researched so far. Just an
engineering problem is all.
>> Sure, that's the plan, but if the tower is grounded and the mast
>> isn't well grounded to the tower, the surge current from a strike
to
>> an antenna or the mast has to jump the connection point to get to
>> tower ground so a transient high potential can exist.
> Believe me - those bearing contact points (which wouldn't suffer any
> real damage due to a strike anyway) would be the least of my
worries.
They're the least of my worries as well. My worry is in earthing
everything at the top of the tower so as to inhibit charge buildup
that could trigger a strike. If that's something that's possible,
then the probability of a strike is then reduced closer to the
region's random strike probability. I'd feel a lot better if this
was something that could be reduced to an engineering problem with a
cost effective solution.
>From what you've said, it appears that the bearing itself provides
adequate earthing of the mast for charge dissipation. The other half
of my original question was in wondering how effectively capacitance
of the antennae and mast is coupled to the top of the tower for use
in shunt loading it as a vertical. It seems that it's not a problem
either although it strikes me that it could be quite variable.
> The main point of a ground system is to keep harmful transients out
of
> the building so I think you're getting too hung up on something at
the
> extreme OTHER end of the scenario.
No, I guess I just didn't state my givens clearly. The entire
arrangement is to be installed by the book, according to Polyphaser
and appropriate codes. The coax feedlines will be grounded to the
tower at the top and the bottom using LMR-400 grounding kits on
BuryFlex. The tower will be well grounded and attached to an
earthing field of strap connected ground rods. All feedlines and
control lines will be buried and earthed at an external master ground
bar carrying Poylphaser dc blocked suppressors, etc., etc.
I wasn't worried about the mast end of the system in terms of damage,
just trying to find out if there was a way to optimize earthing of
the rotating parts so as to avoid charge buildup that could trigger a
strike. Just engineering it to death.
>>> Everything except the insulated elements are connected to the
tower
>>> and to the tower ground so they should be okay. You get arcing
when
>>> you have potential differences between conductors - that's why
>>> everything in the ground system is tied together so they all rise
>>> and fall at the same rate. Then you don't have any arcing.
>> Understood. But if the mast isn't well grounded to the tower, there
>> will be arcing. I'm assuming that even the minimal contact to the
>> tower via the thrust bearing and rotator bearing is adequate to
keep
>> the mast grounded to the tower, just that it isn't a low impedance
>> path for surge currents and thus some arcing could conceivable
occur.
> It's well grounded - okay? Yes - lightning is pretty unpredictable
and
> yes - some arcing could conceivably occur but I'd suggest spending
> your time, energy, and money on the more important *other* end of
the
> system.
Well, yes, lightning can be pretty unpredictable but there are ways
to mitigate its likelihood as well as its damage. And at the
potentials and charge densities associated with a strike, there isn't
so much arcing as there is vaporized metal fed plasma that
essentially electrically short everything within reach, and it will
reach whatever it takes. Nasty stuff. As long as its made as
unlikely as possible and the damage constrained to places well away
from the house and the equipment, I'll be happy.
Thanks very much,
Pete, AD5HD
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