Antenna Adjustments - GE BC-375-E Preliminary Instructions

The side-tone control is a four-position switch, two
positions of which are used for high-level and low-level
side-tone signals for a 2000-ohm load, and the other
two positions are used for the high-level and low-level
side-tone signals for a 150-ohm load. With the side-
tone control set in the 2000-ohm high-level position
(4), the output voltage will be between 17% and 30
volts. In the 2000-ohm low-level position (3), the
output is approximately 10 volts. The output in the
150-ohm high-level position (2) will be between 4.5
and 7.5 volts. While the 150-ohm low-level position
(1) will give approximately 2.5 volts, the operator
should determine the headset resistance and select
the correct side-tone switch position at the time of
installation.
f . Calibration Reset: Due to necessary manu-
facturing tolerances, the inter-electrode capacity of a
vacuum tube varies between limits fixed for each
particular type of tube. Since the tube capacity is an
appreciable part of any master-oscillator circuit, it is
impossible to make up a calibration chart which
will be exactly accurate for all tubes whose capacities
are within the allowable limits. For this reason
Radio Transmitter BC-375-E is provided with a
calibration reset capacitor which enables the operator
to reset the radio transmitter frequency to correspond
with the calibration chart when the set is first placed
in operation and thereafter whenever the master-
oscillator tube is changed. A heterodyne frequency
meter or other standard of frequency is required.
The procedure is as follows:
(1). Allow radio transmitter to warm. The
operator will obtain the most accurate results by
allowing the radio transmitter to "warm up" on
key locked CW for a period of at least 20 to 30
minutes before set Ging the calibration reset capac-
itor or checking the calibrated transmitter
frequency.
(2). With the transmitter tuning unit for the
highest working frequency placed in the radio
transmitter, tune the transmitter for CW operation
on one of the calibrated frequencies at the high
frequency end of the band. (Approach the cali-
brated point by proceeding from a lower dial
reading to a higher one.)
(3). Place the frequency meter in operation and
adjust it to the frequency indicated on the trans-
mitter calibration in accordance with the operating
instructions and calibration chart furnished with
the frequency meter. The calibration accuracy of
this frequency meter should be 0.01 per cent, or
better.
(4). Open the calibration reset port, located on
the front panel, between the TEST KEY and
TONE-CW-VOICE switch, insert a screwdriver,
and rotate the calibration reset capacitor until the
transmitter frequency coincides with that of the fre-
quency meter or standard.
(5). Close the calibration reset 'port.
The transmitter calibration is now reset for any
tuning unit of the same order number and serial
number as the radio transmitter and the accuracy
of calibration will be within 0.05 per cent plus the
accuracy of the standard. The calibration must be
checked in this manner each time the m-o tube is
changed.
15. ANTENNA ADJUSTMENTS
The antenna is, in general, a system of conductors
which, when excited by a radio-frequency voltage,
sets up an electromagnetic and electrostatic field
causing a component of this field to travel away
from the antenna with the velocity of light.
The radiation characteristics of antennas, direc-
tivity, and radiation efficiencies are determined by
their physical shape and location with respect to other
bodies. In aircraft, since there is no wide latitude in
physical design of fixed antennas, the problem be-
comes mainly one of determining the best methods of
operating available designs.
It is necessary first to determine how the antenna
impedances vary at the operating frequencies in order
that the loading facilities may be properly used.
The reactance of an antenna may be approximated
from the theory of transmission lines in which uni-
formly distributed inductance and capacity are
assumed, resistance and insulator leakage being
neglected.
Selecting a certain length of antenna and plotting
its reactance variation with frequency, we obtain
repeating cotangent curves. It may be seen that at
certain frequencies the reactance becomes zero.
Under these conditions, the antenna is "resonant,"
analogous to a simple-series circuit turned to reso-
nance.
It may be noted that the "series" resonant points
occur at all odd multiples of the first resonant fre-
quency, which is called the fundamental frequency.
The fundamental frequency, the third harmonic,
and the fifth harmonic, all points of zero reactance,
correspond to a voltage distribution along a simple
vertical wire of "quarter-wave," "three-quarters
wave," and "five-quarters wave." Antennas of this
type, operated at zero or low reactances, are com-
monly called "current fed"; that is, they require low
driving voltages for their operation.
At even multiples of the fundamental frequency, it
may be seen that the antenna reactance is very high.
Operation at the second and fourth harmonics corre-
sponds to "half-wave" and "full-wave" operation.
Under these condicions, antennas are "voltage fed" ;
that is, they require high driving voltages.
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