The conclusion of this two-part article will provide guidance on testing broadband Vivaldi antennas and how to apply those measurement results into improved performance when used in a CAE simulator.
Vivaldi antennas can provide excellent directional propagation at microwave frequencies. As introduced in Part 1 of this article (July Microwaves & RF), the Vivaldi antenna can be a simple design based on a tapered-slot-antenna (TSA) architecture. This concluding installment in this two-part article will compare measurements of a designed and fabricated X-band antenna with simulations from the Advanced Design System (ADS) software from Agilent Technologies (www.agilent.com).
The eight-point design and simulation process using the Agilent Momentum EM analysis tool from Agilent's ADS software suite was detailed in Part 1. The antenna was fabricated on RO4003C material and an SMA end-launch connector was affixed to the microstrip line of the antenna. The outer ground pins of the SMA receptacle was shorted to the slotline ground planes of the antenna while the SMA's center pin was soldered to the stripline transmission line. Figure 5 shows a Vivaldi antenna board fabricated in this manner, ready for testing and evaluation.
Measurements include generating S-parameters and radiation patterns. Figure 6 shows an example test setup for evaluating the S11 parameters of the Vivaldi antenna. Before making measurements, the vector network analyzer (VNA) must be calibrated. Then, connections are made from the Vivaldi antenna's 50-ohm connector and the VNA's port 1 connector by means of a 50-ohm low-loss coaxial cable. The VNA is set for measurements from 8 to 12 GHz.
Once the S11 log magnitude results are obtained from the VNA, they can be compared with the simulated results from the Momentum analysis. By this comparison, an optimum frequency of 9.20 GHz was found for the radiation pattern measurements. The test setup used for the radiation pattern measurements is shown in Fig. 7. To perform these measurements, a microwave signal generator is connected to the Vivaldi antenna's SMA connector by means of a highquality 50-ohm coaxial cable. The signal generator is tuned to the antenna's optimum frequency of 9.20 GHz, and its output power level is set to +10 dBm. If this is the proper optimum frequency for the antenna, measured radiation patterns should match the simulated results.
As the measurement receiver, a microwave spectrum analyzer with appropriate frequency range is connected to another Vivaldi antenna by means of a low-loss 50-ohm coaxial cable. The analyzer is then tuned to the range of frequencies used by the signal generator (8 to 12 GHz).
For testing the Vivaldi antenna transmitter and receiver together, the distance between the transmitter and receiver is set to 1 m or more. The receive antenna is rotated at various angles while the transmit antenna is set with a fixed angle. This setup is useful for measuring the power received by a Vivaldi antenna and to determine the polar field radiation patterns for the antenna. The receive antenna is rotated from 0-to-360-deg. angles in 10-deg. steps, with power levels measured on the spectrum analyzer in units of dBm. The power transmitted by a Vivaldi antenna can be found by replacing the horn and parabolic transmit antennas with a Vivaldi antenna as the transmitter.
The gain measurement test setup is similar to that used for the radiation pattern measurements. A horn antenna is connected to the signal generator and used as the transmitter, connected to the generator through a precision 50-ohm coaxial cable. The generator is set to 9.20 GHz at a power level of 0 dBm. The Vivaldi antenna is connected to the spectrum analyzer by means of a coaxial cable, and the horn antenna is pointed toward the Vivaldi antenna. The power received by the Vivaldi antenna is shown in the spectrum analyzer's display, from which the gain can be calculated.
As mentioned earlier, the main measurements of interest for the Vivaldi antenna include S-parameter, radiation-pattern, and gain measurements. As proposed by Langley et al. in 199612, the S11 log magnitude should be below -15 dB for optimal antenna performance. The radiation patterns of a properly designed Vivaldi antenna should show an end-fire radiation pattern with directional propagation. Moderate gain is expected for the antenna to perform effectively in microwave imaging applications.
The measured S11 results (log magnitude in dB versus frequency) were plotted and compared with the results obtained from the ADS simulation (Fig. 8). As can be seen, the simulation and measured values match closely. Any differences in the two plots could be due to mismatch between the cable and the antenna input since this was not accounted for in the simulation. During the simulation, the input port is at the edge of the substrate. But in the actual measurements, the SMA connector is soldered inside the antenna substrate, resulting in an actual transmission line that was shorter than the one used in the simulation.
Table 2 shows a comparison of the Vivaldi antenna's S11 log magnitude performance from measurements and simulation. The frequency of interest is 9.20 GHz, where the S11 log magnitude is below -30 dB for both the simulated and measured results. By comparing the simulation and measurement results at the minimum point, both show S11 log magnitude performance that is below -40 dB. This indicates low reflection losses with the transmitted signal being almost completely received by the antenna. Any differences at 9.20 GHz may be the result of fabrication quality, although these slight differences are acceptable for this design.
Table 3 offers a comparison of simulated and measured S11log magnitude data for the frequency range at levels below -15 dB.12 The data reveal a wide frequency range for the antenna, and that the design can operate well at 9.20 GHz since the S11 for both simulation and measurement are less than -15 dB.
Some differences were found between the simulated and measured S11 phase for the Vivaldi antenna design (Fig. 9). The simulation only reveals one phase change, while the measured data shows three phase changes. A slight phase shift occurs in the range 9.10 to 9.30 GHz. At 9.20 GHz, the simulated phase result is -77.738 deg. while the measured result at that frequency is 49.710 deg., a difference of 127.448 deg.
The simulated and measured radiation patterns for the Vivaldi antenna are compared in Fig. 10 at 9.20 GHz, with some significant differences. Simulation results show that the antenna does not radiate in a directional manner while measurements indicate directional radiation, with the radiation high when the antenna and transmitter are facing each other. The measured radiation pattern is similar to an end-fire pattern, thus it is theoretically more accurate.
Many factors can affect the measurements made on the Vivaldi antenna, including:
- The extra solder that connected to the Rogers RO4003C dielectric material with a SMA type connector. The extra solder may raise conductivity and alter surface currents.
- Some interference and obstruction may occur between the Vivaldi antenna and the transmitter during radiation pattern measurement s which could prevent effective transmission of radiated energy.
- Since the measurements were not performed in free space, environmental elements may have absorbed transmitted energy rather than the antenna.
- Any compromise in fabrication quality of the RO4003C dielectric can impact test results.
- Measurement instability can also result due to cable or coaxial adapters used with the spectrum analyzer.
By calculating gain in dB, evaluation can be made of not just the antenna's radiating efficiency but the efficiency of the overall antenna system. Table 4 shows a comparison of gain obtained from the simulation and measurement. Many factors can impact gain, including the materials used to build the antenna, the physical size and properties of the materials and antenna components, and how they are assembled.
In short, the simulations and measurements show that the fabricated Vivaldi antenna provides good performance at X-band frequencies, particularly at 9.20 GHz. The antenna's design requirements were met and have been summarized in Table 5.
12. J. D. S. Langley, P. S. Hall, and P. Newham, "Balanced antipodal Vivaldi antenna for wide bandwidth phased arrays," IEE Proceedings Microwaves, Antennas and Propagation, 1996, Vol. 143.
13. Agilent Technology (www.agilent.com), Momentum planar EM simulator, ADS 2005.