Ac Service Protection - Harris DX 25U Technical Manual

In medium and short wave installations
the antenna ground plane is very important
⁄
1
as it is of the radiating element. RF
2
current leaving the antenna must return via
the ground path (ground wave). For this
reason the "antenna coupling unit" must be
close to the base of the tower and securely
connected to the ground plane.
Figure A-6 shows the basic elements of a
properly designed antenna system.
• Good ground plane.
• Ball gap on tower.
• Series inductor in tower feeder.
• Antenna coupling unit connected to an-
tenna ground.
• The π
circuit is equivalent to the normal
Tee used by Harris.
• Underground coax.
• Guy wire length broken by insulators
and grounded at the bottom end.
The transmitter building must be given
extra protection to insure reliable equip-
ment operation. A low impedance safety
ground system must be installed using 3
inch wide copper strap hard soldered at all
joints and connected to multiple ground
rods located at the perimeter of the building.
The ground rods should be wet to make
good connection to the earth water table. All
equipment cabinets within the building
must be connected to the ground straps for
safety reasons.

AC Service Protection

a.4.
See Figure A-7. All incoming ac lines
should have a choke connected in series to
limit the high frequency surges on the lines
followed by a surge protector. The surge
protector must be connected to the building
ground system by short direct connections.
A surge protector is a solid state device
that is a high impedance until the voltage
across it reaches its rated clamping voltage
at which time the impedance suddenly de-
creases. The protector will then conduct
hundreds to thousands of amperes to
ground. All protectors are rated for maxi-
mum voltage and maximum surge energy.
If the surge energy exceeds rating of the
device it will normally short and for this
reason must be fused so it will disconnect
itself from the line being protected. When
this happens all protection is lost so some
warning system must be used to tell the
operators that a new protector should be
installed.
Speed is essential to protect equipment
from current surges with rates of rise ex-
ceeding 10,000 amps per microsecond and
pulses that last no longer than 100 micro-
seconds. Very short, low inductance ground
Figure a-3. Regulators in Delta and 4/Wire WYE Systems
straps are required to pass surges of this
type.
The surge protectors must be selected for
the line to ground voltage and the maximum
energy to be diverted. Bigger is always bet-
WARNING: Disconnect primary power prior to servicing.
Figure a-2. Lightning Incidents to Tall Structures
ter in this case. There are several manufac-
turers of surge protectors:
• Lightning Elimination Assoc., Inc.
• Current Technology
• Control Concept
a-2