RadiMation Application Note 103

How to determine the antenna diagram of an antenna

This [[::Category:RadiMation Application Note|Application Note]] will explain how measurements can be done in RadiMation^® to determine the antenna diagram of an antenna on one or more frequencies. RadiMation^® is not designed to perform antenna diagram measurements, but due to its flexibility and versatility it is possible to perform this type of measurement.

Overview

To determine the antenna diagram of an antenna, the effectiveness of the antenna should be measured depending on the angle of the antenna. For clarity we will call this antenna the EUT-Antenna in this application note, as it very often is the antenna that is part of an EUT.

To measure the effectiveness of the EUT-antenna a second antenna is needed. One of the two antennas is used as a transmit antenna, and the other antenna is used as a receive antenna and connected to a spectrum analyzer to measure the amplitude of the received signal. It is not important if the EUT-antenna is used as a transmitting antenna or as a receiving antenna. The measurement procedure to measure the antenna diagram of the EUT-antenna is the same in both situations. It is however important that the EUT-antenna is positioned in the center of a turntable, and that the second antenna is positioned on a fixed location relative to the turntable.

In this application note we assume that the EUT-antenna is used as a transmitting antenna. A fixed and stable signal should be applied to the antenna. This can be a signal generator, or the EUT that contains the EUT-antenna that generates a stable signal. The second antenna is then connected to a spectrum analyzer, which is used to measure the variation in the measurement result caused by the rotation of the turntable angle and thus the EUT-antenna angle. As the EUT-antenna diagram is often used as a relative measurement, the absolute measurement value is often not important. The most important information is the variation (in dB) depending on the angle of the EUT-antenna.

Hardware configuration and preparations

The EUT-antenna should be placed on the turntable, and it should be connected to a signal generator that generates a stable signal on the desired frequency. The receiving antenna can be placed on another location outside the turntable area, and should be connected to the spectrum analyzer.

It is required that the used RadiMation^® license contains the Radiated Emission module.

In RadiMation^®, a testsite should be used that contains the following devices

• Turntable: the turntable on which the EUT-antenna is positioned
• Antenna: the second antenna (not being the EUT-antenna) that is used in the measurement setup.
• Spectrum analyzer: the spectrum analyzer that is connected to the receiving antenna
• (optional) cables: to correct for cable loses between the receiving antenna and the spectrum analyzer
• (optional) preamplifier: an external preamplifier between the receiving antenna and the spectrum analyzer to amplify the received signal

All corrections that are selected in the selected devices will be taken into account when the measurement results are shown. The EUT-antenna is not included in the testsite, because the EUT-antenna is the component that is being tested, and this is is represented by the EUT in RadiMation^®.

In this application note we used a virtualized setup, which uses the 'Virtual Spectrum Analyzer' and the 'Virtual Turntable'. This virtualized setup has a signal on .... MHz, which fluctuates depending on the turntable angle. This virtualized setup can be used to demonstrate the capability of RadiMation^® to determine the antenna diagram. This same virtualized setup and the following measurements can be performed on any RadiMation^® installation.

Frequency measurement to determine the frequency

Create a new EUT that represents the antenna that is being measured. This EUT file is also used to store and maintain the measurements results.

Multiband emission, applicable frequency range Measure on the turntable angle on which the highest antenna gain is expected. No need to specify a limitline Detect a single peak. Ensure that it is listed in the table of peaks Only necessary to use the peak detector. Also for final measurement. Short measure time for final measurement.

- screenshot of TSF setup -

Verify if the correct peak is selected. Select a single peak that should be measured. A peak can be selected by clicking with the mouse on the signal in the graph. RadiMation^® will then automatically mark the clicked signal as a new peak to the list of detected peaks.

- screenshot with frequency graph with selected peak -

Performing the angle measurement

Switch to the Polar graph. It will show the amplitude and the turntable angle on which the value was measured. Ensure that only the peak final measurement detector is activated. Activate the continuous measurement mode.

- screenshot of polar graph. No polar trace yet -

Press the Turn CW and Turn CCW buttons. Pressing the Turn CW or Turn CCW button again while it is pushed, will stop the turning. Can have benefit to first turn to a start angle (for example -180 degrees), and then start the 'continuous measurement mode' by pressing the Continuous measurement mode button. Then press the Turn CW which will start a complete turntable rotation. During the turntable movement, the continuous measurement on the selected peak will be performed and the measured data will be updated in the polar graph. The antenna diagram will be determined and shown in the polar plot dynamically.

Multiple rotations of the turntable can be performed, however due to the speed of the turntable and spectrum analyzer this very often results in measurements at slightly different turntable angles. If also the amplitude of the signal generator has changed, this can cause that the measurements are shown as a triangle wave. In this situation it is best to press the Clear Results button, which clears the measurement data of the selected peak. This will cause that the polar plot becomes empty again, and a new measurement can be performed.

After the measurement is finished, press the Continuous measurement mode button to stop the continuous measurement.

- Screenshot of polar graph, showing a complete antenna diagram of the peak -

Multiple frequencies

Measurement of multiple frequencies and the antenna diagrams for those frequencies is also possible within the same test. Just select additional peaks in the frequency graph. The polar plot only can show the measurement results of a single peak. The information of the peak that has the selected checkbox in the list of detected peaks will be shown. When multiple peaks are added to the list of peaks, the rotation of the turntable with an activated 'Continuous measurement mode' should be repeated for each peak.

Automated measurement of multiple frequencies

If multiple frequencies should be measured, it is also possible to perform the measurement automatically. The TSF can be configured to perform measurements on multiple angles. The measurement results at these different angles are also used to determine the results as it is shown in the polar plot. It is for example possible to perform a measurement on every 10 degrees of the turntable. The peaks can be automatically detected by RadiMation^® or manually selected in the frequency graph. For all of the determined peaks a polar plot with a 10 degrees accuracy will be available. This polar plot of the peaks can then always be further improved by performing the 'Continuous measurement mode' manually, for the relevant turntable range.

- screenshot of TSF with multiple angle measurement -

Reviewing the measurement results

The measurement results are automatically saved when the test is closed. When the test is reopened from the EUT File, it is possible again to switch to the polar plot graph on the Polar tab. The antenna diagram of the antenna for the selected peak frequency will then be shown again.

Equipment Under Test

Decibel

Test Setup File

Carrier Wave: An unmodulated RF signal

Continuous wave: An unmodulated RF signal