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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