User manual: Installation and Operation of Direction Finder System RHOTHETA RT-600 / SAR-DF 517
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4.1.1
Influence of the antenna unit location and the environment on the bearing
accuracy.
Recommendations concerning the practical antenna unit location on helicopters and airplanes:
The bearing accuracy of the direction finder antenna unit is severely influenced (comparable to an airspeed
sensor) by the environment. For comparison, a perfect high quality speed sensor will not indicate the true
airspeed if this sensor is interfered by strong turbulences.
The direction finder antenna unit acts as a sensor for electromagnetic waves. In the near field of this antenna
unit, any conductive (e.g. metallic) obstacle will influence the incoming wave field and create reflections which
decrease the bearing accuracy. These are general physical limitations concerning all types of direction finder
systems.
To achieve the best bearing results, it should be considered that the antenna unit is installed as far away as
possible from any other vertically polarized conductive structure. If possible, there should be no reflectors within
an area of 2-3 feet around the antenna unit.
In this worst case example the direction finder will not work properly:
In this example, the antenna unit is located too close to the cable cutter of the helicopter. This cutter will produce
strong reflections, especially as the length is equal to Lambda/4 within the VHF airband.
