Insulated Support Structures
Lightning Protection Information
Wooden Poles used in place of a steel tower inhibit lightning current flow to earth. More damaging energy will be on the coax cable to equipment antenna port.
Insulated Support Structures
Wood or fiberglass support structures are not a good idea. They are an insulator. The cabinet earth ground, earth ground system. The 6-inch copper strap earth ground lead should be routed on the opposite side of the
pole from the coax and conduits.
When large currents flow through any conductor, a strong magnetic field is developed around the conductor and can couple energy to other nearby conductors. A circular conductor will usually be surrounded with a cylindrical magnetic field varying in intensity as the current flow propagates down the conductor The circular
conductor’ s cylindrical field "pattern" is indicative of its magnetic field susceptibility boundaries.
A copper strap will also develop a magnetic field closely aligned with its physical shape. As current propagates down the strap, a magnetic field develops close in to the flat portion and extending out from the edges. The strap’ s field "pattern" is also indicative of its magnetic field susceptibility boundaries.
If downward circular conductors were arranged perpendicular to the flat side of the copper strap (opposite sides of wood pole), the magnetic field overlap would be reduced and mutual coupling would be minimized.
The strap will conduct most of the current to earth ground with little reverse EMF developed on the conduits.
• If an equipment cabinet is pad mounted, all the energy on the large surface area conduits and/or coaxial cables would be directed towards the cabinet (entering from the top or side) with resultant large currents through the cabinet to local earth ground. Below grade cabinet bottom entry with a low ground connection
on the coax would reduce through cabinet current flow (recommended). The conduits and/or coaxial cable shields should be grounded at the base of the support. The remaining energy would go to earth down the cabinet’ s internal earth ground conductor. The usual center pin/shield propagation differential voltage would
occur and could be equalized by an appropriate dc blocked center pin protector.
• If an equipment cabinet is pole mounted, current flow from the coaxial cables shields (to the top of the cabinet) and conduit (going downward from the cabinet) would pass through the cabinet, duplexer housings, and connector panel PCB ground plane. Duplexer internal ground connections could sustain
damage and PCB ground plane traces could be destroyed. If coaxial cables were brought down the outside of the cabinet, looped up, and entering through a bottom connector (preferred), the lowest inductance path would be through the bottom panel of the cabinet to downward ground conductors. The
additional coax and 180 degree turn would add series inductance. Current flow through equipment would be minimized and a bulkhead type center pin protector could be used as a bottom connector.