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enroute. If there is no ID, but the
system otherwise appears to behave
normally, the NDB is undergoing
calibration or maintenance.
The primary function of the ADF
receiver is to determine the bearing
of an incoming NDB signal, which is
vertically polarised. To do this, it
uses a loop aerial. When the loop is
square to the beacon, the signal
reaches both sides of the loop at the
same time and there is no signal
detected. When the loop is sidewayson,
however, the signal reaches one
part of the loop first:
and the second part will be out of
phase, so a current will be generated,
which drives an electric motor to
continually seek the null position. It
is phase sensitive, so it can always
turn the shortest way. Various stages
of magnification inside the receiver
help this along, but that need not
128 Canadian Private Pilot Studies
concern us here. The point is that
the detected signal is not actually
used to determine the bearing, but
the null signal point, since the
current flow is slow to build up and
break down, and is a bit on the
woolly side. The null point is much
sharper and easier to find.
Because the current flows in the
opposite direction depending on the
position of the loop antenna, you
also need some way of determining
which end is what, otherwise you
could be 180° out. A single vertical
aerial called a sense antenna helps here
– both signals are combined
algebraically and the magnitude and
polarity of the sense aerial arranged
to be identical to the loop. The result
is a polar diagram called a cardioid,
which has only one null point:
On one side of the loop the polar
diagrams are positive and combine,
but on the other, one is positive and
the other negative. They cancel each
other out, hence the null point on
one side.
The modern (and more stylish)
equivalent is a small housing with
two coils at right angles to each
other, wound on ferrite cores. They
are connected to the stator coils of a
goniometer which points the needle.
Limitations of the system include:
·  static.
·  station overlap, where more than
one NDB has the same
frequency. Because this is more
pronounced at night, it can
easily be confused with night
effect, below (the CFS has
promulgated ranges, which 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.
Electricity & Radio 129
·  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
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
　
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