- sectors for the interfering signal;
- Look for the interfering signal from a nearby rooftop or top floor. In an urban area, this may be the best way to direction find;
- Move to higher ground;
- Investigate nearby valleys, swales, or other low spots;
- Use in-instrument mapping techniques to plot signal strength versus location.
Once the signal has been spotted at ground level, the RF source needs to be located using the following method:
- Import a geo-referenced map onto your spectrum analyzer;
- Select an antenna;
- Setup the spectrum analyzer;
- Go to mapping mode;
- Find the signal direction;
- Repeat the direction finding process from several locations.
A Geo-referenced map has GPS latitude and longitude information embedded in it. This allows a GPS-enabled spectrum analyzer to locate the user’s current position when plotting signals on the map.
Traditionally, a Yagi antenna is used for direction finding because it has good directivity, good front-to-back ratio, and generally low side lobes. Its biggest disadvantage is it usually has a fairly narrow frequency band. Other options include a Log Periodic antenna, which has broad frequency coverage but less directivity, a panel antenna (best to ward off reflections), or omni-directional (appropriate when seeking the strongest signal).
Anritsu has developed the MA2700A Handheld InterferenceHunter (figure 2) that accepts many types of antennas with standard N-(f) connectors. The handle also contains a magnetic compass and a GPS receiver, which report position and direction to the spectrum analyzer. This simplifies taking directional bearings. The InterferenceHunter also has a built in pre-amp to ease the process of getting a sufficiently strong signal to the spectrum analyzer.
Users then load the map into the spectrum analyzer and enter mapping mode. Next, the InterferenceHunter is installed and enabled. The spectrum analyzer will show the current location on the map and the direction that the antenna is pointing.