The engineers at Narda Safety Test Solutions (STS) have successfully developed an automatic direction-finding antenna (ADFA) that, when used with the powerful SignalShark real-time receiver, is capable of delivering incomparably stable bearing results extremely quickly. Insensitive to reflections, which are ever present in urban environments, it reliably and accurately determines the location of a detected signal. A complete bearing cycle takes just 1.2 milliseconds. This high measurement speed contributes towards the unprecedented stability of the bearing results. A long list of further advantages makes the ADFA 1 the ideal solution for automatic direction-finding of signals in the frequency range between 200 MHz and 2.7 GHz. As well as for cellular network providers and the military, it is primarily aimed at the users of PMR (professional mobile radio or trunked radio) and BOS (security and safety authorities and organizations) that have applications that involve safety and security, such as the police, fire and ambulance services, frontier defense, and intelligence services.

The sources of its reliability

At the heart of the ADFA 1 is an array of nine antenna elements around a central omnidirectional reference antenna optimally arranged to achieve such robust measurement results. The nine antenna elements are measured against the central reference element, which can receive signals from all directions and which allows test engineers to observe the broadband spectrum at the same time as determining the direction of the signal being traced. This feature is particularly useful when the test engineer is tracking a suspect who is broadcasting in a particular channel and who then suddenly changes to a different channel. If the spectrum is not monitored, the signal will have disappeared, so ending the search. However, with the information from the reference antenna, the test engineer can see where the signal has gone and resume observation and tracing of the interference signal.
Another detail that ensures the reliability and precision of the results is the ability of the new Narda STS ADFA 1 automatic DF antenna to determine the elevation angle of the signal bearing, thereby utilizing an additional factor to increase the reliability of the measurement results. This is particularly useful in an urban setting where localization to an individual floor level may be needed.

Automatic direction-finding with single-channel receivers

The principle of automatic direction-finding is based on the measurement of the phase difference of a signal between several antenna elements. Multi-channel systems are naturally ideal for such measurements, but the disadvantage here is that the cost of such receivers is totally off the scale. Direction-finding can also be successful using a single channel system, however, particularly when intelligent antenna elements are used, as in the ADFA 1. An ingenious array of antennas, phase shifters, and complex algorithms makes it possible for the ADFA 1 to reliably and extremely precisely determine the signal direction fully automatically using only a single-channel receiver. The phase difference between the antenna elements is measured for this. The greater the distance between them and the greater the recorded phase difference, the more precise and reliable the bearings. Weighing just 5.6 kg and with a diameter of 480 mm, the ADFA 1 is the ideal symbiosis of precision and compactness. In this context, size does matter, so there is no compromise here.

The ADFA 1 in practical use

In practice, the ADFA 1 is attached to the roof of a normal vehicle with the aid of a magnetic mount. There is absolutely no need to drill holes in the vehicle bodywork or make any other modifications. The high immunity of the antenna to influences from the mounting surface means that correction tables for the specific vehicle type are no longer needed. This independence means that the ADFA 1 can be used flexibly and universally. For other applications, the ADFA 1 can also be mounted on a tripod and used to trace interference from a semi-fixed location as is commonly required in military applications. The secret of the stable bearing results lies in the combination of the large aperture of the ADFA 1, the use of a reference element and the clever processing of the bearing results by the SignalShark software. Finally, the high measurement speed also plays a part. The chances of obtaining a perfect bearing result, of getting a line of sight result when driving past a row of buildings naturally increase with the speed of the measurement. The greater the number of measurements that indicate a certain direction or location, the more stable the result will be. In the end, that is nothing more than statistics.

An additional hand-held antenna can be used with the SignalShark to determine the exact location of a signal source over the final few meters. This allows the test engineer to enter a previously localized building on foot in order to determine the floor and the exact room where the source is located.