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night, it can easily be confused
with night effect, below
(promulgated ranges are not valid at
night for this reason).
· night effect, where the needle
swings erratically, at its
strongest just after sunset and
before sunrise. The loop is
designed to receive surface
waves – any others resulting
from sky waves will be out of
phase and confuse the system
entirely. If the ionosphere is not
parallel with the Earth's surface,
they will also arrive from a
different direction. Check for an
unsteady needle and a fading
audio signal. Promulgated
ranges are not valid at night.
· quadrantal error, variations caused
by the aircraft itself, in the same
way as it might affect a
compass. The signal is
reradiated by the airframe and
the receiver gets an additional
(much weaker) signal to
contend with. The greatest error
lies at 45° to the fore and aft
axis, hence quadrantal. Modern
systems have corrector boxes
for this.
· mountain effect, or variations
caused by reflections from high
ground, where two signals
might be received at the same
time from different paths.
· coastal refraction, from radio
waves in transit from land to
sea, or parallel to the coast,
because they travel slightly
faster over water. It is most
noticeable at less than 30° to the
coastline (exam question), and
at lower frequencies. The effect
is to make the aircraft appear
closer to the shore.
The most common error, though, is
failing to recognise station passage – if
you are directly over the beacon, it
will swing erratically and be
confused with one of the above
(exam question), or failure of the
instrument where the needle just
rotates to the right all the time. This
Electricity & Radio 129
is due to the same cone of confusion
that exists with the VOR (above).
The ADF is normally tuned with the
function switch in the ANT position
(it stands for antenna). This removes
the needle from the loop and saves
wear and tear as it tries to point at
every station you tune through –
here, the sense antenna is used by
itself to obtain the ID. Once there,
return the switch to the ADF
position (or COMP, on some sets).
As always, check – in this case,
ensure that the needle points vaguely
where you expect it to. The TEST
button spins the needle 90° away
from its tuned position, and return,
to indicate a good signal.
BFO means Beat Frequency Oscillator.
The BFO switch also uses the sense
aerial by itself to detect the
modulated Morse identifier. Hearing
this by itself helps you tell if there is
any fading (night effect) or noise
(thunderstorms, interference). The
tone you hear when this switch is
activated is actually put there by the
ADF receiver, since a carrier wave
by itself cannot be heard.
Aside from continuously listening to
the ID, the only way of knowing
there is a problem is seeing the
needle rotate to the right if the signal
is not received.
The fixed card display (goniometer)
consists of a compass rose with 0°
representing the nose of the aircraft
at the top of the instrument, and a
needle that points to where the
signal is coming from (including
thunderstorms if they are stronger
than what you are tuned into). Thus,
if a station is ahead, the needle will
point to 0°, or 180° if it is behind.
However, if you made no allowance
for wind, and just pointed the nose
of the aircraft at the station, you
would actually follow a curved path
towards it. Allowing for drift lets you
keep a straight track, which is
needed to keep on the airway (see
Tracking, below).
Unfortunately, working with fixed
cards involves a little maths. First of
all, though, here are a few
definitions:
· Magnetic Heading – the angle
between the aircraft’s
longitudinal axis and magnetic
North.
· Relative Bearing - the angle
between the aircraft’s
longitudinal axis and the NDB,
which is what you read directly
from a fixed card ADF.
· Magnetic Track or Bearing – the
angle between aircraft position
and the NDB, either to or from.
Take note of this formula (you will
need it in the exam):
MH + RB = BTS
In other words, the magnetic
heading plus the relative bearing
gives you the bearing to the station.
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