RCA BTA-250M Instructions Manual page 11

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Atmospheric Static Accumulations
In certain localities atmospheric conditions build
up high static potentials on the transmitting towers,
making it imperative to provide a drain path to
ground for these accumulations.
If no direct path is
provided, the charge builds up potential until flash-
over occurs either across the tower base arc gap or
across one of the capacitors in the coupling system.
Where tower lighting chokes are used and one
side of the ac supply line is grounded, the lighting
choke will act as a satisfactory discharge path.
When neither side of the ac line is grounded, or
when a toroidal tower lighting transformer is utilized,
a drain path must be provided.
Such a path
already exists in the BTA-250M transmitter through
L105, L 106, and L 107. Should it be necessary to
break this path to ground, by the use of a series
capacitor, for example, it may be necessary to pro-
vide an alternate path. To check for the exist-
ence of a suitable dc path to ground, turn off
the transmitter and tower lighting circuit. Connect
an ohmmeter between the tower and ground. Any
resistance up to approximately 250,000 ohms will
provide a satisfactory return circuit. When no dis-
charge path is indicated, one may be supplied by
the installation of an r-f choke or a 100,000 to
200,000-ohm Clobar resistor. Connect either the
choke or the resistor from the antenna feed line to
ground. The line terminating unit will generdlly
serve to house the component used.
If antenna current meter fluctuations result from
the addition of the choke or resistor, it will be
necessary to add the blocking capacitor discussed
under the preceding heading "Antenna Current
Readings."
Electrical Storms
Areas subject to lightning storms require a direct
electrical path from the tower to ground, to avoid
capacitor and antenna current meter burn-out by
lightning striking the tower. This requirement is
generally met by arc gaps installed across the base
insulators. These arc gaps should be carefully ad-
justed during bad weather-which could be either
dusty or humid conditions. Spacing of the arc gaps
should be set so that flashover occurs just beyond
the point of 100% sine wave modulation.
If the gaps are properly spaced, the gaps at time
of discharge present a low impedance path to
ground, and will carry directly to ground any cur-
rent caused by the lightning striking the tower.
Although there is a second path to ground, through
the tuning equipment or transmitter output, the
higher impedance of this second path usually pre-
vents excessive discharge under normal conditions.
In instances where the tuning house is located under
the tower or directly adjacent to it, the ratio of these
two impedances may not be sufficiently high to pre-
vent appreciable discharge current through the tun-
ing equipment to ground, with consequent destruc-
tion of coupling equipment.
To increase this ratio,
a one- or two-turn loop should be installed in the
antenna lead from the tower to the tuning house.
No such loop is required where the tuning house is
several feet from the tower.
The longer lead in the
latter instance provides the necessary higher im-
pedance.
Antenna Current Meters
Where thermocouple-type instruments are used
for antenna current metering, the meter circuits
should be equipped with make-before-break knife
switches such as are incorporated in RCA line tun-
ing units. This type switch enables one side of the
meter circuit to be opened when readings are not
being taken.
WIRING
Wiring to and from the transmitter should be car-
ried in conduit or a trench terminating below the
unit. The base plan indicates where this wiring
should enter the unit.
Table 1 lists the external con-
nections that should be made to the transmitter.
TABLE 1
EXTERNAL CONNECTIONS TO BTA-250M UNIT
POINT OF
CONNECTION
EXTERNAL CIRCUIT
TBl
Ground
TB2
1
TB3 \
AF input
TB4
-
TB5
-
TB6
-
TB7
R e m o t e antenna ammeter
TB8
(when used)
TB9 /
I 1 7 volts, 50/60 cycles for
TBlO 1
crystal heater
TBll )
TB12 1
I
17 volts, 50/60 cycles power
input
Frequency monitor
Modulation monitor
JlOl
5102
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