Misconceptions, Mistakes and Pitfalls

Aural Null Depends on a Nearly Circular Radiation Pattern

The aural null actually depends on a nearly circular radio horizon.

Aural Null Depends on a Nearly Circular Area of Equal Signal Strength

The aural null actually depends on a nearly circular radio horizon.

Perpendicular Bisectors are Lines of Position

Perpendicular bisectors are not lines of position. The lines of position in an aural search would be circles, with a radius equal to the distance to the radio horizon of the search aircraft, drawn about the aural null points. With two points, the two circles of position would intersect at two locations. The perpendicular bisector is simply a line drawn through those to intersections. Because they are not lines of position, one must be careful about drawing conclusions from the scatter of intersections or cocked hat that forms when multiple bisectors are drawn.

Flying Towards Higher Signal Strength will Close on Transmitter Location

Usually the strongest signal will be in close proximity to the transmitter. However the aircraft will not be in close proximity to the transmitter unless it is nearly directly overhead. When directly overhead the nominally toroidal radiation pattern will reduce the signal strength somewhat. More importantly, when the transmitter is activated as the result of an accident it is possible for debris to reduce the signal strength in any direction, including towards a nearby aircraft.

Most important of all though, the received signal strength may have local high and low points that will result in an aircraft seeking the strongest signal to home to one of the local high points which may be a long way from the location of the transmitter.

Off Tuning Provides a Method of Measuring Signal Strength and Distance to the Transmitter

See also Flying Towards Higher Signal Strength will Close on Transmitter Location.

Off tuning from the design frequency of the transmitter may cause the receiver to listen to a spurious transmission that is lower in strength than the main signal. How much lower may not be accurately knowable due to relaxed regulations that apply to emergency transmitters. The radio propagation effects on the spurious frequency may be different than those affecting the main signal received at the aircraft location. Destructive interference on one frequency may be replaced with constructive interference on another frequency resulting in a stronger received signal strength when the receiver is off tuned. This effect becomes more likely the further the receiver is off tuned because the amount of reduction of signal strength decreases as the receiver is tuned further off frequency.  

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