[TowerTalk] [Bulk] Terminate braid at top of tower...or not?

[Grant Saviers]

I wonder about the widely shared concern about crank up section 
electrical continuity.  Some crank up towers have hard stops and the 
elasticity of the hoist cable can hold them tight against the stop. Then 
the hoist cable itself is an electrical path.  The runners *must* 
contact on at least one leg, there is no way a section is meta-stable 
free standing inside another.  Perhaps the contact has some low 
resistance as Zn on Zn but the runners to legs have significant area to 
make a capacitor for rf.   Plus the section overlap makes for at least 
two contact points between sections. Lightning will not notice these 
"gaps".  Perhaps in a gusty wind a tower may shift enough to have 
variable contact paths between sections and that could cause noise 
issues, but then why isn't this noise heard by crank up owners on their 
yagis as the tower shifts its Z?

Then to address your logic, I agree that a tower must be a lower 
inductance path than coax given the tower effective diameter.  So 
bonding periodically helps to insure equal (and lower) potentials to the 
shield.     This bonding would be difficult on a crank up.   I gave up 
on fixed standoffs that promised cable "loops on the tower, not the 
ground".  All of my standoffs are pass through loops and I manually 
flake the cable bundle figure eight style on the ground.

So far, I haven't seen any data to back up the continuity concern.

Grant KZ1W


On 11/1/2014 9:44 AM, N3AE wrote:
> I've always wondered about terminating coax shields on the top of a crank-up tower. On one hand, I think that by doing that, you're encouraging lightning currents to go down the coax shield as the resistance between the tower sections of a crank-up is likely to be larger than the coax shield (assuming the shield is also bonded at the bottom of the tower). But perhaps the inductance of the coax is larger than that of the tower so most of the lightning current would still traverse the tower? On the other hand, by bonding the coax at the bottom and not at the top, you may have flash-over issues at the top because of the difference in potential of the tower and the coax shield at the top. Maybe I'm making this more complex than it is, but my gut tells me it is a bit more complex than it may first seem.
>
> All the commercial towers bond coax shields at the top, bottom and every 70-100 ft or so, but they are not crank-ups with unknown continuity between the sections.
>
> N3AE
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