Antenna Measurements with Drones: All About Raster Scans

With an increasing number of users reliant on the finite radio spectrum, it’s more important than ever that the spectrum is used efficiently. This means ensuring that antennas are optimized and functioning as they should be. Measuring the radiation pattern is a fundamental feature of antenna testing as it gives an indication of antenna performance, allowing for error identification, design reviews and verification of compliance. 

A raster scans is a measure of the radiation diagram on a 2D plane. It provides a clear view of the antenna's main beam and the first few sidelobes. The measured data is captured by measuring the received signal power on a sequence of equally spaced lines, which is afterwards interpolated to be represented as a heatmap or a 3D image. This method provides a great deal of data points, and it is critical that users are able to interpret and use the data.

The Benefits of Raster Scans

Before going on to explore the benefits of raster tests, let us first consider in more detail how raster scans differ from cuts. When you only make principal cuts, perhaps an azimuth or elevation cut, you get just a partial view of the sidelobes so potential raised level on diagonals could easily get missed.

So, rasters are a good way to quickly view full radiation patterns of what matters the most in the antenna, the main beam and the first few sidelobes. And that with confidence that potential anomalies aren´t hidden. It´s almost like the analogy of having a dimmed light in a room and with rasters you get to open the window blinds and let the full light through. 

With the primary metrics extracted from a raster, such as the antenna beamwidth, the sidelobe levels, the null level as well as the shape of the main beam, it is then very easy to make an assessment of the antenna quality in various instances, such as during different design phases, during site acceptance testing, or even during maintenance and troubleshooting. 

The measurement, being done in the operational environment, gives the engineer onsite the possibility to quickly evaluate and utilize the results to optimize the antenna. Raster scans help identify problems with feed misalignments, or manufacturing tolerances, allowing for necessary adjustments to be made to optimize performance. In addition to evaluating an antenna’s performance, raster scans also play a vital role in allowing users to track changes in the antenna pattern when equipment modifications are made. This is a crucial aspect of ongoing maintenance and improvements in the field because it ensures that any adjustments made are beneficial.

Another use for rasters, as they show the full main beam, is to evaluate the pointing mechanism of antennas. This is especially useful for tracking antennas with electromechanical systems that have pointing capabilities. With our ability to precisely establish a local coordinate system, we can evaluate if the antenna will really point where the user wants or if there´s an offset – which can be measured and then corrected in the system. Measuring the receive and transmit beams simultaneously will provide the operator with the difference in pointing between the RX and TX chains. This is especially useful in the case of ESAs (electronically steerable antennas). Similar use cases of alignment in between beams can be also referred to the case of multi-feed systems, where one would want to know if the change of the feed system results in any change in the pointing accuracy.

Figure 1. Raster scan used to measure antenna pointing accuracy. Here, the antenna pointing was offset with 0.3 degrees in Azimuth direction and 0.14 degrees in the Elevation direction.

A raster scan is not a measurement unique to the main beam and first side lobe assessment, it can also be used to measure other directions, for example in cases of power flux density measurements for regulatory compliances, where such a comprehensive view will give the operator a clear indication of the power levels being radiated towards a critical protected structure, such as a road.

Who Should Be Conducting Raster Scans

Raster scans are a versatile testing method suitable for a range of stakeholders in the RF and satellite industry. Manufacturers can use raster scans to validate the actual performance of antennas in the design process or during Factory Acceptance Tests (FAT). It's also valuable for testing performance at different stages of product development.

Installers can employ raster scans during Site Acceptance Tests (SAT) and for calibrating the antenna sub-reflector or feed. These serve as a final check to ensure everything is working as it should be.

Satellite constellation owners can use raster scans to evaluate different antenna models for their systems. This allows them to make informed choices when selecting their equipment, leading to an optimized network with peak bandwidth performances and thus having a direct impact on the cash flow.

Analyzing the Data

In order to be useful, the raster scan data obviously needs to be correctly analyzed. Firstly, the contour lines show the power levels. The user can add contour lines at any desired dB levels to evaluate the antenna against specification. The contours are also useful in showing the 3dB and 10 dB beamwidth, from which one can get an indication of the antenna estimated gain. In contrast, if only relying on cuts in the azimuth and in elevation, you might completely miss the shape which indicates the error, particularly if antenna has an elliptical, 3dB beamwidth.

Figure 2. Applying the ITU-R S.580 mask on top of the measurement. Area going above the mask, highlighted with black (left), and level above mask (right)

Furthermore, the standard mask compliance can be applied over the raster and the regions that are above can be highlighted and quantified.

Another method we use to analyze the data is delta plots. Essentially, delta plots allow us to compare the data between two different raster scans. This is useful when optimizing antennas, so you can compare the measurements taken before (state A) and after (state B) an adjustment. In these cases, you first take an initial measurement of the antenna pattern to identify any problems, make the necessary adjustments, and then carry out a further scan. With the raster scan, you can look at the plot to see how the energy has shifted from state A to state B. 

It’s also possible to do other types of analysis such as the side lobe level plot, which involves creating a polar plot, in order to compare the level of the side lobe to the main beam. In the example below, you can see that the side lobes are lower than the main beam all around.

Figure 3. Extrapolated plane cuts from a raster scan measurement

After making the necessary adjustment, we can create another plot to assess the shift in energy balance on the side lobe between state A and state B.

Figure 4. Analysis and Comparison of the Sidelobe level from two different measurements

This method is not only useful for comparing a before and after state, but is also useful for comparing and contrasting two different antennas. In this scenario, you make one measurement on each antenna, and then evaluate the difference between the two antennas. This can also be used for before and after the installation of radome, to evaluate the effect of the radome on the energy distribution.

The image shows the whole shape of the 3dB beam width. In contrast, if only relying on cuts in the azimuth and in elevation, you might completely miss the shape which indicates the error, particularly if antenna has an elliptical, 3dB beamwidth.

In this example, the contours show the 3dB drop which is half of the power and an important metric in the antenna. 

As well as plotting the data on a heat map, as shown in the images above, it’s also possible to generate a 3D image, as shown below, which makes it even easier to see where the energy is being distributed.

Figure 5. Raster scan view in 3D

Performing Raster Scans

The following steps outline how we perform raster tests using drones. 

1. First, we define the size of the scan, which determines the drone’s flight path. This involves specifying both the width of measurement in azimuth and elevation, as well as the granularity of measurement lines. The drone will fly on the edge of a sphere so next we decide on the measurement radius. We then know the flight total distance and the estimated flight time.
   
   Figure 6. Visualisation of the flight path example for capturing a raster scan measurement
   
2. When the drone is ready for flight, the test can begin. The drone flies autonomously maintaining constant pointing and polarization alignment with the antenna under test.
3. The amplitude levels are merged with the computed angular position of the drone compared to the Antenna Under Test (AUT).
4. The collected data is presented as a heatmap or as a 2D or 3D diagram. This visual representation makes it easier to interpret the results.

Testing Efficiency

One of the key advantages of raster scans is their efficiency. A typical test takes somewhere between 5 and 15 minutes of flight time, to capture the main beam and the first two sidelobes on any antenna. Time also varies depending on the granularity of lines. The results are generated "on-the-fly”, allowing the operator to take immediate performance optimization decisions and continue the measurements if necessary.

At QuadSAT, we have extensive experience in performing measurements for a wide range of antennas, from C and up to Ka-band, and for antennas sizes ranging from 40cm to 17m and measurement distances from the antenna from 50m up to 12km.

Raster scans are an invaluable tool for the satellite industry that can help us to optimize antenna performance, use the spectrum efficiently and ultimately improve service performance and maintain seamless connectivity.