Acceptable Lightning Ground?
Dave wrote:
"I would like to propose a grounding arrangement for my (hypothetical)
antenna and get some feedback on it."
Assuming the tower is not base insulated, start by grounding each leg of
the tower with a no. 6 AWG or larger cable directly to its own ground
rod.
Best lightning protection comes from folded driven element antennas
grounded directly at the tower top or other mounting point and
connecting the antenna with coax which is grounded again at the tower
base. If a folded driven element is not used, a short-circuited
quarter-wave stub can be connected directly across the antenna
drivepoint to supply the grounding to the tower that a folded element
provides.
Standard practice requires a no. 6 AWG or larger cable directly
connecting the base of the tower to the ground connection used by the
electrical service to the site.
For enhanced protection, large RF chokes as used in tower lighting
circuits in broadcast stations, are placed in each power wire, including
the neutral, between the electrical service and the radio connected to
the antenna. A-C capacitors are connected between each end of the
high-current chokes and a common connection directly to the station
ground. If there are 3 wires you need 6 caoacitors. Across each
capacitor you need a metal oxide varistor appropriate for the line volts
and the joules you may be called upon to dissipate when lightning
strikes. We also shunted the capavcitors with conventional thyrite
arrestors to back-up the MOV`s. This pi-filter with arresters worked
like gang busters. It is not excessive but did prove necessary for
solid-state equipment protection.
Best regards, Richard Harrison, KB5WZI
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