Installation of DF antennas

Introduction to Direction Finding Antennas

Narda SignalShark Direction Finding (DF) antennas, such as the ADFA 1 and ADFA 2 models, when used with Narda SignalShark measuring receivers, offer high direction finding speed and accuracy. The ADFA 2 covers an extensive frequency range. While specifications are determined under ideal conditions, real-world installations may encounter environmental factors like reflections that can distort the measurement results. It is crucial to pay special attention to avoiding these field distortions.

Mobile direction finding, for instance, from a vehicle roof, can help avoid many interfering factors. Lightning protection is often unnecessary in such scenarios, and heatmap localization can mitigate reflections through constant location changes. However, fixed installations on masts require careful consideration.

Masts, buildings, other antennas, and lightning conductors can all affect the electromagnetic field, influencing measurement results. These effects are typically systematic and can be largely compensated for through calibration. Given the variability of installation locations, general installation instructions serve as recommendations. Meticulous planning and execution are essential for optimal direction finding results.

Installation Considerations

The ideal approach is to avoid influencing the field, thereby eliminating the need for later compensation. However, the surroundings of the measuring antenna are often fixed. Nevertheless, with some effort, significant improvements can be made.

1. The Mast

A non-conductive mast made of plastic is ideal. If metal masts are necessary, it's important to note that for frequencies above a few hundred MHz, the effects are relatively small. For the ADFA 2, specified from 10 MHz upwards, mounting on a metallic mast requires consideration:

• The ADFA 2 utilizes the Watson-Watt principle for direction finding at frequencies below 200 MHz, offering high sensitivity in a compact size with lower wind resistance.
• Far-field conditions, characterized by orthogonal electric and magnetic fields, are a prerequisite for accurate direction finding using the Watson-Watt principle.
• Metallic masts can experience strongly elevated fields at their ends when resonating, distorting the field at the direction finding antenna. Narda offers a non-metallic antenna adapter to fit between the metal mast and the ADFA to minimize this effect.

2. The RF and Control Cable

The RF cable can also significantly influence direction finding results. While the mast may be non-metallic, the cable cannot be. The influence of the cable can be reduced by using ferrites. Narda RF and control cables are equipped with ferrites as standard. For operation at the lower frequency limit of the ADFA 2, additional split ferrites should be added around the cable near the non-metallic part of the mast. Narda antennas and RF cables automatically factor in and compensate for antenna factors and cable attenuation.

3. Lightning Protection

Lightning protection is crucial for exposed locations, such as building roofs. A lightning conductor rod, from an RF perspective, acts as a monopole and reflects incident signals. To ensure safety during lightning strikes, the lightning conductor rod should be as RF-invisible as possible. Narda, in collaboration with DEHN SE + Co KG, has developed an RF-transparent lightning conductor rod using ferrites. This design minimally affects direction finding results while providing effective lightning protection. Figure 2 illustrates an ADFA installation with an optimized lightning conductor.

Corrective Measures

While the preceding information helps improve conditions for perfect direction finding, sometimes environmental factors like mast resonance or unavoidable structural elements (e.g., elevator machine rooms) necessitate subsequent correction. The ITU "Spectrum Monitoring" handbook details calibration and correction procedures for entire measuring stations. Narda provides a free facility for ITU-compliant correction of bearing values using a correction file within the SignalShark, starting from firmware version 1.7.5.

4. Omni Phase Correction

At very low frequencies and with long masts, ambiguities can occur, leading to a sudden 180° shift in the bearing result below a certain frequency. This systematic shift can be determined and largely compensated for, except at the exact point of the shift.

5. Azimuth Correction

Static effects from nearby objects, such as reflections from other antennas, building parts, or lightning conductors, are typically systematic and can be compensated for.

6. Generating a Correction File

Correction files can be created using an XML editor. Figure 3 shows the general layout of such a file. The root node is <Narda_3300_DF_CORR_DATA>. The <Info> child node contains general information about the correction file, including short and long names, frequency range (Fmin, Fmax), antenna part number (AntennaPN), serial number (AntennaSN), correction data date (CorrDataDate), and comments. Figure 4 illustrates the Info child node.

The <OmniPhaseCorrection> child node contains data for phase shift correction. The <OPCorrItems> sub-node lists pairs of values in <OPCorrItem>, each containing a <Freq> (frequency point) and <AzimuthCorrOffset> (offset angle). The list must have at least two entries, with the first specifying the frequency above which corrections apply. Interpolation is linear between entries. Figure 5 shows the Omni Phase Correction child node.

The <AzimuthCorrection> child node holds data for correcting systematic, non-directional bearing deviations. The <AzimuthCorrItems> sub-node contains a list of azimuth offset values (<AzimuthCorrItem>) independent of frequency and direction. Each <AzimuthCorrItem> includes a <Freq> and <AzimuthCorroffsets>, which is a list of azimuth offset angles at the given frequency. Intermediate values are interpolated linearly. The change in angle between entries is determined by the number of entries. For example, four entries imply a 90° angle change (360°/4). The antenna direction angle can be specified in the "azimuth" attribute of the <AzimuthCorrOffset> node for improved legibility. Figure 6 details the Azimuth Correction child node.

Application of a Correction File

In an AutoDF task, a correction file can be selected and applied via the Config menu in the Bearing View. Figure 7 illustrates this process. Activating the "DF Corr. File" button opens a selection menu listing available correction files from the directory "D:\Narda_SignalShark\Configuration\DFCorrection" (without the .xml extension). The option "None" is also available to disable corrections. "None" is the default selection. When a correction file is selected, all direction finding results are automatically corrected and displayed on the screen.

SignalShark Outdoor Unit, PoE and ADFA Installation Recommendations

The following recommendations are for stationary mast installations and should be adapted to specific site requirements. Consider individual component specifications for wind load, temperature, and mechanical load capacity.

General Installation Recommendations:

• No obstacles should be present within 50 meters of the antenna, except for the special lightning rod.
• Only low obstacles should be located between 50 and 100 meters from the antenna.
• Avoid large structures like high-tension masts or high-rise buildings within 400 meters.
• Ensure no obstacles exist outside a 60° cone below the Non-Conductive Antenna Mast.

Lightning Rod Position (regarding DF):

• The lightning rod should be positioned within or opposite the main direction of bearing.
• Consider the protection angle (α) according to local conditions and the desired lightning protection class.

Lightning Rod Spacer:

• The distance between the lightning rod and the antenna must be at least 1 meter.
• The lightning rod spacer should not be attached to the non-conductive mast adapter of the ADFA, as this adapter is made of glass fibre-reinforced plastic and could be damaged.

Diagram Description: The diagram shows a typical installation setup. A non-conductive antenna mast (approx. 2.5m) is mounted on a building roof. A special lightning rod with ferrites is positioned above the antenna. The ADFA antenna is mounted on the mast. A SignalShark Outdoor Unit with PoE is shown connected. A surge arrester (DEHNgate DGA G N) and a DEHNpatch Outdoor arrester for GBit Ethernet are indicated. A CAT.7 Ethernet cable suitable for outdoor use and a PoE++ Injector (60W) are also depicted.

All information on lightning protection and its components is exemplary. Installation must comply with national safety guidelines for lightning and personal protection and should only be performed by qualified personnel.