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inversion conditions, a phenomenon
called ducting occurs, which enables
Electricity & Radio 123
waves to travel unusually large
distances (the author has certainly
seen British TV programs in
Germany with this effect).
Fading is due to multi-path propagation,
amongst other things, where signals
may be received from many sources
and be out of phase with each other
at the aerial. Sometimes, under such
circumstances, waves will cancel
each other out.
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.
Air-ground transmissions are limited
to 25 nm in the UK, up to 4,000 feet
for tower frequencies and 10,000
feet for approach.
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).
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.
124 JAR Private Pilot Studies
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
indications should therefore not be
relied upon.
Inside your aircraft will be a large
black box somewhere in the back,
with a remote indicator in the
cockpit:
Once you select a radial by turning
the Omni Bearing Selector (the small
knob just under the dial), the Course
Deviation Indicator needle will be in
the centre, or either side of the
centreline, up to 10° away from the
radial, so each dot left or right
represents 2°, if there are 4 dots on
your display (2 ½ ° if there are 3).
When in the middle, you will be on
the radial, which traditionally is from
the station when on the same side,
　
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