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TM 11-5825-270-10
b. The receiver is a crystal-controlled, single- conversion superheterodyne unit, providing high degree of
selectivity and sensitivity. The receiver is fixed-tuned to the same frequency as that of the associated NDB
transmitter, over the range of 190 kHz to 535 kHz. In operation, the receiver continuously monitors the rf carrier
level and modulation of the NDB transmitter signal. As long as the amplitude of the rf carrier and modulation
level of the signal being received are above a predetermined threshold, the operation is considered to be
normal. However, if some abnormal operating conditions cause the received carrier level and/or modulation to
fall below the predetermined threshold, the receiver generates the following alarm outputs:
(1) Visual alarm (red light on front panel)
(2) Aural alarm (3 kHz tone from built-in loudspeaker).
(3) Alarm contacts (form-C contacts for remote monitoring).
1-8. Description
The receiver assembly is designed for installation in a standard 19-inch rack and operates from 115/230 vac (47
to 63 Hz) primary power source. The antenna assembly is normally secured to a pole or a post with a U-clamp
outside the building/shelter where the receiver is installed. Interconnections between the receiver and the
antenna are made with a single, 50-ohm coaxial cable.
1-9. Difference Between Models
The monitor receiver does not have different models.
1-10. System Applications
a. A typical system tie-in diagram for the monitor receiver is shown in figure 1-2. The NDB transmitter is
a self-contained facility (not part of the monitor receiver) and it may be physically several miles from the
monitor receiver site. The NDB transmitter continuously transmits navigation information for use by aircraft
flying within the designated range of the transmitter.
b. The monitor receiver continuously monitors the presence of NDB transmitter signal. Monitoring by the
receiver includes signal level check as well as the presence or absence of the modulation. If a malfunction
occurs and the radiated power output and/or keyed modulation drops below a predetermined level, the receiver
provides an alarm condition.
c. In addition to aural and visual alarm outputs, the monitor receiver provides two additional outputs for
use by the remote monitoring facility. These outputs are shown in figure 1-2.
1-11.  Considerations for Optimum Performance
For best overall system operation, due consideration should be given to the following:
a. Although the whip antenna is one meter long, its effective length in terms of a wavelength of the signal
fed to the receiver is only 0.2 meter. The receiver requires a minimum input of 10 uV; therefore the signal
strength at the antenna must be at least 50 uV/meter. This requirement must be considered when selecting the
monitor receiver site in relation to the NDB transmitter site. In practice, the receiver site is chosen so that the
signal strength at the whip antenna is several times the minimum requirement of 50 uV/meter.
b. The maximum output from the 600-ohm line is 250 mW, measured at the receiver.
c. The remote alarm Form-C contacts are rated at 1 ampere.
d. If all monitoring functions of the receiver are to be carried out from the remote monitoring site, switch S1
on audio amplifier PCB should be left in the off position to inhibit the local aural alarm.
1-12. Tabulated Data
Table 1-1 details the physical and electrical Characteristics of the NDB monitor receiver.

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