[Towertalk] Grounding mast to tower

[Pete Goudreau]

K7LXC@aol.com wrote:

> In a message dated 4/1/02 7:14:41 PM Pacific Standard Time,
> goudpj@mac.com writes:

>> How does one go about guaranteeing a good RF ground between a
>> rotating mast and the tower proper? I'm assuming that a thrust
>> bearing isn't adequate, even a Rohn TB-3, but I'm sure I could be
>> quite wrong.

> 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.

>> Additionally, a lightning hit to an attenna or the mast would just
>> arc across or flash ionize the air and form the conductive path to
>> the tower in that manner, does this jibe with others' experience in
>> these matters?

> Boy, you lost me on that one. If the antenna or tower is hit, then
> your tower ground should be useful. In addition it'd be helpful to
> have the cables from the tower protected before they go into your
> building. Then they'd probably be okay.

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.

> I don't know what a flash ionization is.

Flashover between two objects, through the air, which is ionized by 
the field.

> 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.

Thanks,
Pete, AD5HD