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inversion conditions, a phenomenon
called ducting occurs, which enables
waves to travel unusually large
distances (the author has certainly
seen British TV programs in
Germany with this effect).
Ground waves are associated with
LF/MF waves, sky waves with HF
and direct waves with VHF frequencies
and above. In fact, the latter are also
called line-of-sight, meaning that
anything in the way, like hills or
buildings, will have a detrimental
effect (they will not bounce like HF).
You will get best reception if the
transmitter and receiver are in sight
of each other, but, in practice, you
can expect a little more than that,
actually to just beyond the horizon,
due to effects like refraction within
the troposphere (see VOR, below).
The actual figure is greater by a
factor of around 4/3.
HF frequencies need to be higher
during the day or when you are at
greater range from the station. At
night, you can use lower frequencies,
generally about half (that is, use
Double During Day). Because there is
a lot of noise, which makes them
uncomfortable to monitor for long
periods, SELCAL (Selective Calling) is
used to speak to particular aircraft. A
ground station transmits audio tones
which will be heard over the speaker
in the selected aircraft. Alternatively
a flashing light may be used.
Calculations
Electromagnetic energy travels at a
constant speed of 300,000,000
metres per second, abbreviated with
the letter C. The distance travelled
by a wave at that speed is called the
wavelength, or l for short.
Thus:
l =  C
F
Electricity & Radio 281
F is the frequency. For simplicity,
use metres and seconds when
playing with the above formula.
So, if you wanted to find the
wavelength of a wave with a
frequency of 300 KHz:
l =  300000000
300000
The answer is 1000 m.
Datalink
An advanced transponder system
based on Mode S that transmits your
speed, rate of climb and
acknowledgements direct to ATC.
VHF FM
Some systems use tones to restrict
the people on a particular frequency.
VOR
This stands for Very High Frequency
Omnidirectional Range, so is based on
VHF, using the phase difference
between two signals to signify your
direction from the transmitting
station. The frequency range is
between 108-112 MHz on even
decimals, plus 50 KHz (to prevent
confusion with the ILS), and 112-
118 on odd and even, plus 50 Khz.
VORs are identified on maps with a
compass rose around the station
aligned with Magnetic North. It is not
sensitive to heading, as is the ADF
(below) - it shows track.
The Station Identifier is transmitted in
Morse Code every 15 seconds, and
you must confirm the frequency and
ID before using a VOR for
navigation. If there is no ID, but
behaviour is otherwise normal, the
system is on maintenance.
The transmitter sends out a
reference signal in all directions,
frequency modulated at 30 Hz,
which is received by all stations at
the same phase, if they are the same
distance away. However, it is not
transmitted at the same strength all
the way round – the amplitude is
also varied to produce a polar
diagram called a limacon, which is
similar in shape to the cardioid used
by the ADF (below), but without an
absolute null point. It rotates 30
times per second. The phase of this
signal in the aircraft depends on the
bearing from the station (which is
probably why it's called a variphase
signal). Both signals are in phase at
0°, or North, 90° out when East, and
so on. For each degree moved, the
signal changes phase for the same
amount, in both frequency and
amplitude, which is how your
direction is determined. Because the
signal is frequency and amplitude
modulated, it is classed as an A9W
signal. Just to complicate matters,
Doppler VOR has its modulations
the other way round.
Overhead the beacon, you will be in
a cone of confusion that exists with any
antenna – this is an area where no
signal is received, so the TO/FROM
flags disappear and the alarm flag
comes up (in the case of the VOR,
the cone is 100° across). During this
station passage, you should ignore the
signal. There will also be ambiguities
abeam the beacon - at a point 90°
either side of the selected radial there
is a zone of ambiguity where the flag
will not show at all, and the
　
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