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power of the NDB.
d) It has to be noted, moreover, that, while with overland paths of good conductivity, night effect will only be serious
at somewhat greater distances than those indicated over paths of poor conductivity, night effect may become
pronounced at much shorter ranges. This will also depend to some extent upon the characteristics of the radiation
system.
6.4 Considerations affecting operations of NDBs
6.4.1 Depth of modulation
6.4.1.1 In specifying that the depth of modulation should be maintained as near to 95 per cent as is practicable, it must
be noted that, at the frequencies used for NDBs, the small antennas generally in use can affect the effective modulation depth
of the NDB system due to attenuation of the sidebands.
6.4.1.2 At this order of frequency, the antennas are normally only a small fraction of a wavelength long; they are
therefore highly reactive and tend to have a high Q.
6.4.1.3 The effect is illustrated in Figure C-19, which was compiled from measurements made by one State. The
modulating frequency in these measurements was 1 020 Hz. If a lower modulating frequency were used, the effect would be less.
ATT C-65 23/11/06
Annex 10 — Aeronautical Communications Volume I
Antenna
characteristics
A 500
F 10 ohms
B 200 F 10 ohms
C 150 F 10 ohms
D 200 F 2 ohms
E 150 F 2ohms
Percentage effective modulation depth for 100% current modulation of P. A. stage
Frequency in kHz
Note.— The modulating frequency in these measurements was 1 020 Hz.
A
B
C
D
E
200 300 400 500
100
80
60
40
20
0
Figure C-19. The effect of antenna Q on the depth
of modulation of the radiated signal
23/11/06 ATT C-66
Attachment C Annex 10 — Aeronautical Communications
6.4.1.4 In order to reduce the attenuation, attempts should be made to reduce the Q of the antenna. This can be done in
two ways, by increasing either its capacity or resistance.
6.4.1.5 Inserting additional resistance in an antenna wastes power, whereas increasing the capacity does not.
Additionally, the effect of increasing the capacity is to reduce the voltage across the system and hence to reduce the
insulation problems.
6.4.1.6 For these reasons, it is considered desirable to increase antenna capacity by the use of a top load as, for example,
in the so-called umbrella top capacity.
6.4.2 Earth systems
Frequency planning is done on the assumption that the field strength will be maintained at the correct value. If the earth
resistance is high (i.e. an insufficient earth system), not only will the radiation efficiency be low but the power radiated will
be sensitive to changes in climatic conditions and other factors affecting the earth loss. In all cases, the earth system needs to
be the best possible, taking into account all local circumstances.
6.5 Considerations affecting the choice of the
modulating frequency for NON/A2A NDBs
Recognition of the fact that modern narrow band ADF receivers have improved selectivity characteristics requires
consideration of the fact that, in so far as attenuation of the audio sidebands by these receivers results in a reduction of the
effective depth of modulation of the signal, the distance at which satisfactory identification is obtained is consequently
reduced. In such circumstances, it is considered that 400 Hz would provide a better identification service than 1 020 Hz.
There is some evidence, however, that under conditions of high atmospheric noise, the higher frequency of 1 020 Hz may
provide a more easily readable signal.
6.6 Guidance material on the testing of NDBs
6.6.1 In the following paragraphs an indication is given of the desirable periodicity for various tests. These figures are
intended as only general guidance, it being recognized that many local circumstances will influence a State in establishing its
test programmes.
6.6.1.1 Carrier frequency
6.6.1.1.1 The carrier frequency should periodically be tested against an accurate standard to ensure that the tolerance
specified in Chapter 3, 3.4.4.2, is maintained. A standard such as a crystal controlled frequency meter having an accuracy of
0.001 per cent of the operating frequency is recommended for this purpose.
6.6.1.1.2 This test of carrier frequency stability should be carried out:
a) at least semi-annually for crystal controlled transmitters;
b) at least once a month for non-crystal controlled transmitters.
6.6.1.1.3 If practicable, tests at shorter intervals should be made.
ATT C-67 23/11/06
Annex 10 — Aeronautical Communications Volume I
23/11/06 ATT C-68
6.6.1.2 Frequency of the modulating tone
6.6.1.2.1 The identification frequency requirements are specified in Chapter 3, 3.4.5.4. An audio signal generator
　
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