use direct series link coupling from the antenna series variometers to the
Bushings. Feed-through bushings couple the helix house outputs to
the antenna down-leads. The bushings are usually gas-filled to reduce the
size of the insulator for a given flashover rating. The gas feed line to the
bushing shall be 3/8 inch (9.4 mm) or greater in diameter. Details will be
set forth in the BESEP.
Shielding. To reduce losses into surrounding materials, the
interior of the room containing the tuning coils is shielded. The entire
interior, including floor, trenches, walls, and ceiling, shall be lined with
copper or its equivalent in aluminum. Doors shall also be covered. Seams and
joints shall be continuously brazed or soldered for copper and welded for
aluminum. The shielding shall be bonded to the building ground system.
Because of the high RF induction heating capability of the signal, no magnetic
material should be used inside the shield. Any ferrous material used shall be
shielded, and the shield shall be grounded. Shielding thickness shall be
provided in the BESEP.
Bonding. For 50-kilowatt helix houses, reinforcing steel shall be
welded at joints between longitudinal and transverse rods, along lines 10 feet
(3.05 m) apart both ways, forming buses that are carried and grounded to the
buried ground system surrounding the helix house. When transmitters are more
than 50 kilowatts, reinforcing steel in walls and ceilings shall be welded at
every joint where transverse rods pass longitudinal rods.
Safety. Entrance to the helix house shall be restricted during
system operation by an interlocked, non-magnetic metallic barrier bonded to
the ground system.
Cabling. Transmission lines may be run above ground or
underground. Direct burial is preferred, where practical, to protect cables
and to permit maintenance and/or security vehicular traffic on the antenna
Lighting. Incandescent light fixtures shall be installed on the
bulkhead, 8 feet (2.44 m) above the floor.