Description - TM-11-5825-270-100007

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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