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Simple antenna characterisation using multiple VNAs

Simple antenna characterisation using multiple VNAs

Technology News |
By Jean-Pierre Joosting



Introduction

Antenna measurements are usually expensive and complicated, and take a long time for results to be displayed. While the VNA is today’s standard tool for measuring frequency-dependent parameters, it does have the disadvantage that it is virtually impossible to extend the VNA ports to the desired antenna locations without incurring problems such as cable losses.

However, as the project described in this article shows, a VNA such as the Anritsu MS46121A single-port analyser can serve as a valuable and inexpensive tool for simple antenna measurements, with measured antenna characterisation close to the manufacturers’ specifications.

 

Student project

The background of this project is a student work at RWTH Aachen University’s Institute of High Frequency Technology that was supported by Anritsu GmbH.
The student task was to design and develop a stepper motor hardware concept and control software for the azimuthal rotation of the antenna under test. An additional requirement was for the visualisation of the antenna radiation pattern and antenna gain in a MATLAB environment.

Anritsu supported this project with the necessary VNA hardware, a stepper motor, anantenna tripod and a Schwarzbeck USLP 9142 antenna. RWTH Aachen University was responsible for the design, development and realisation of the project. After a successful demonstration at the 2015 German Microwave Conference, it was agreed to extend the project to produce some real results.


Experimental set-up

The MS46121A is a full-featured single-port USB vector analyser module including time-domain measurement capabilities. By adding a second module, the test engineer can carry out scalar transmission measurements such as S21 testing. In such a set-up, either of the modules can be used as the stimulus source, with the other behaving as a fully vector corrected (calibrated) receiver. For the antenna measurement concept, two of these modules are sufficient. For complex applications such as multi-band antenna test, it is possible to employ up to 16 VNA modules, with one acting as a stimulus and the remaining 15 as receivers.

The system is controlled from user an external PC or laptop with the Windows 7 or higher operating system and the ShockLineTM VNA software available free of charge from Anritsu. MS46121A option 021 enables the scalar measurement (|S21|,|S12|) capability, in which each attached module appears as a separate physical measurement channel. The active selected channel is always the stimulus source. The receive port(s) can be selected via the response menu (S11, S22 … S16 16).

A further option (002) offers bandpass and lowpass time-domain measurements with time gating capability for measuring parameters such as distance to fault or impedance.

For antenna measurements the VNA benchmarks are dynamic range, stimulus power and measurement sweep. One of the main drivers limiting dynamic range is the use of long VNA test port cables with their inherent high insertion loss and the limited VNA stimulus power.

The ideal set-up is for the VNA hardware to be linked directly to the antennas without any cable and for measurement data to be transferred by means of a low-cost USB hub extended with an Ethernet LAN link to a PC. In this way, undesired losses can be avoided and the money for expensive long test port cables can be saved. As a result, the dynamic range is improved. With a stimulus power of +3 dBm (>23.2 MHz to 4 GHz), antenna side lobes down to a level of -30 dB up to frequencies of 3 GHz with an antenna distance of up to 10 m can be characterised.


Antenna characteristics

The key antenna characteristics to be measured (Figure 1) are:

  • Radiation pattern;
  • Antenna half power beamwidth;
  • Antenna sidelobes;
  • Antenna gain.
Figure 1: Key antenna characteristics.

Gain measurements require essentially the same environment as their corresponding pattern measurements. To measure the gain of antennas operating above 1 GHz, anechoic chambers are usually used. Between 0.1 GHz and 1 GHz, ground-reflection ranges are used.

Within the scope of this project, three different gain-measurement techniques are available. The first two are so-called “absolute gain” measurements: the two-antenna method and the three-antenna method, while the third is the gain-transfer (or gain-comparison) method.


The two-antenna method is based on the Friis transmission equation and requires two identical samples of the antenna under test antenna: one acting as the radiating antenna, and the other as the receiving one.

The three-antenna method is used when only one sample of the test antenna is available. Then, any other two antennas can be used to perform three measurements, which allow the calculation of the individual gains of all three antennas. All three measurements are made at a fixed known distance between the radiating and the transmitting antennas.

 

Antenna impedance

The input impedance of an antenna is calculated via the reflection coefficient at its terminals, which are connected to a transmission line of known characteristic impedance. If the magnitude and the phase of the reflection coefficient are known, it is possible to calculate the antenna input impedance.

Because of real matching conditions, the gain of an antenna is reduced by the losses due to the mismatch of the antenna input impedance to the characteristic impedance. The gain obtained after this reduction is called realised gain.

In our case, the gain was measured using the VNA directly connected to the USLP 9143 log-periodic antenna under test, which was attached to a tripod on a lawn. The frequency range was calibrated between 700 and 2000 MHz in steps of 100 MHz with an intermediate-frequency bandwidth of 100 Hz and a stimulus output of +3 dBm.


Outdoor antenna ranges

Antenna measurement sites or antenna ranges can be categorised as outdoor ranges or indoor ranges (anechoic chambers). According to the principle of measurement, they can also be categorised as reflection ranges, free-space ranges, and compact ranges. For this type of project, a reflection-free propagation free-space range like the so-called elevated or slanted range is applicable.

For measurements, the antenna is placed at the Fraunhofer distance, which approaches far-field conditions. Separating the antenna under test and the instrumentation antenna by this distance reduces the phase front variation of the received wavefront enough for a plane wave approach.

 

Antenna test setup

Based on the given outdoor environment, a slanted range was adapted based on an equilateral triangle having a side length of 10.35 m with the antenna under test located at the downhill apex. One of the single-port VNA modules was directly connected to the USLP 9143 log-periodic antenna, which was installed on a Zaber rotary stage, and the other was connected to a tripod-mounted TDK precision log-periodic antenna (acting as the illuminator), also fixed on a tripod.

The aim was to verify antenna pattern and gain at four individual frequencies: 700, 800, 1000 and 2000 MHz.

The antenna under test was mounted in the apex of the triangle, with the “illuminator” on one opposing side and a standard gain horn antenna on the other. This arrangement ensures that no mechanical changes are required for the later planned gain measurements. The distance and therefore the free space loss (FSL) is constant and just a re-alignment with a laser is necessary to switch from antenna pattern to antenna gain measurements.

A MATLAB script was used to control the ShockLine VNA modules through the ShockLine GUI software and the Zaber stepper motor.


Measurement results

Typical gain results are shown in Tables 1 and 2, while the radiation patterns in Figures 2 and 3 are normalised to the maximum value in bore sight direction and smoothed with an average moving filter with a span of 3.

Table 1: Qualitative difference between the applied gain calculation methods for USLP 9143.
Table 2: Qualitative difference for SAS-571 between specified and calculated gain.
Figure 2: Relative gain of antenna under test (USLP 9143).
Figure 3: Antenna radiation pattern results.

Comparing the results with USLP 9143 supplier specification shows that the form and shape are matching, but the results are more fringed than and not as smooth as those given by the supplier. This is due to reflections resulting from the non-ideal environment (distance to earth, lighting masts close to the measurement area, buildings etc.).


Conclusion

The aim of this measurement campaign was to prove the viability of free-space antenna measurements “on the lawn” using Anritsu ShockLine single-port USB VNA MS46121A modules that are linked over a long distance using an active USB-to-LAN converter.

The planned result was a qualitative comparison between the suppliers’ USLP 9143 specification and measured results of the antenna’s radiation pattern and realised gain. It has been possible to show that a limited investment in test & measurement hardware can deliver reasonably accurate results even in an environment that is far from optimal for antenna characterisation. The experimental setup is ideally suited to VHF and UHF applications where engineers are interested in quick results in order to prove antenna designs.

 

Acknowledgment

The content of this article is based on a student work at RWTH Aachen University’s Institute of High Frequency Technology that was supported by Anritsu GmbH. A full version of the original White Paper is available from Anritsu.

 

References

  1. Anritsu, ShockLine™ 1-Port USB VNA MS46121A, Available online at https://www.anritsu.com/en-US/test-measurement/products/ms46121a
  2. Jeffrey A. Fordham: “An introduction to antenna test ranges, measurements and instrumentation”, Microwave Instrumentation Technologies, LLC: https://cuminglehman.com/wp-content/uploads/Introduction_to_Antenna_Test_Ranges_Measurements_Instrumentation.pdf
  3. S. Burgos, M. Sierra-Castañer: “Introduction to antenna measurement systems”, Technical University of Madrid: https://ocw.upm.es/teoria-de-la-senal-y-comunicaciones-1/antenna-design-and-measurement-techniques/contenido/MaterialCursoAthensUPM26/intro-antenna-meas_athens09_def2.pdf
  4. N.K. Nikolova, “Lecture 8: Basic Methods in Antenna Measurements”, Canada Research Chair in High-frequency Electromagnetics, 2014: https://www.ece.mcmaster.ca/faculty/nikolova/antenna_dload/current_lectures/L08_Measure.pdf
  5. Sergiy Pivnenko, “Antenna Measurements – Fundamentals and Advanced Techniques”, 24th International Travelling Summer School on Microwaves and Lightwaves, Technical University of Denmark, 2014: https://www.itss.ems.elektro.dtu.dk/~/media/Subsites/ITSS/Forside/Programme(1)/Professors/L17%20%20Antenna%20Measurements_ITSS2014.ashx
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