The Dual-Ridged Horn Antenna

The classic horn antenna undergoes some tough analysis and emerges with improvements that increase the frequency band to 3 GHz.

The dual-ridged horn antenna (DRHA) is a workhorse in EMC and antenna pattern measurement laboratories. Its broad band allows the user to test over a large range of frequencies without changing antennas.

However, these antennas came under scrutiny when Burns, Leuchtmann, and Vahldieck showed the pattern problems of these antennas at the upper end of the band.¹ Their work illustrated that these types of horns have problems with the integrity of the main lobe of the pattern.

While efforts to improve these horns have been concentrated primarily on the 1- to 18-GHz models, DRHAs also cover other frequency ranges.² The 200-MHz to 2-GHz range DRHA is required by MIL-STD-461E for both emissions and immunity in the 200-MHz to 1-GHz range.³

Measured gain data suggests there also are pattern issues with these types of horns. Figure 1 shows the typical gain for a 200-MHz to 2-GHz DRHA. At the upper end of the frequency range, the gain drops from a 9-dB average to 3.5 dB between 1,900 and 2,000 MHz. This is similar to the drop in gain seen in the 1- to 18-GHz models between 16 and 18 GHz.

Figure 1. Gain of a Typical Model 3106 Antenna

To get a better idea of what causes the drop in gain, a 200-MHz to 2-GHz DRHA was placed in an antenna pattern measurement taper chamber. The antenna pattern then was measured using EMQuest Data Acquisition and Analysis Software.

Using this software, the antenna under test is rotated inside the quiet zone of the chamber, and the radiation is measured with a receive antenna. The software measured both principal polarizations simultaneously with the aid of a dual-polarized antenna and then generated a 3-D view of the radiation pattern.

Figure 2 shows a notch or split where the main beam breaks into four separate beams of the pattern. This notch causes the drop in gain at the upper end of the frequency range.

Figure 2. Pattern of a Typical 200-MHz to 2-GHz DRHA at 2 GHz

Numerical Modeling
Many computational electromagnetic (CEM) tools are on the market today. They allow the antenna engineer to design antennas and check the effects of changes to the structure without building any prototypes.

For this application, MICROWAVE STUDIO^ï¿½ (MWS) from Computer Simulation Technology was chosen to simulate the DRHA. This software takes a time-domain approach that offers solutions over large frequency spans after only one execution.

The software provides accurate predictions of the performance of these horns.^4,5The first step is to generate a model within MWS that will recreate the issues shown in Figure 2.

The metal parts of the antenna are modeled as perfect electric conductors. The structure is fed via a coaxial cable. To reduce the number of unknowns in the simulation, a symmetry plane was used in the E-plane of the antenna. This is the plane along the ridges. The structure was simulated and the pattern at 2 GHz plotted.

At this point, a study of the feed was performed. The behavior that the pattern exhibits is caused by multimoding. At certain frequencies, more than one mode are excited at the feed. As they radiate, they produce the split patterns observed in Figure 2.

The feed was modified by adding some reflective structures to eliminate some of the additional modes. The antenna model then was computed, and after a series of iterations, a configuration was found that corrected the pattern issue.

Figure 3 shows the new pattern at 2 GHz with no split showing. The next step was to fabricate prototypes to verify the predictions of the model.

Figure 3. Computed Pattern to the Improved DRHA at 2 GHz

An additional test was performed in which two DRHAs were placed facing each other at a distance of 1 m as in the SAE ARP 958 calibration approach. For this case, an additional plane of symmetry was used to reduce the unknowns needed to solve the problem. The horns are fed with a gap source at the location of the coaxial probe. Figure 4 shows the computed gain and compares it with the actual measurements.

Measurements
Although the model demonstrates that we have a solution, there are limitations to the numerical model, mainly from the assumptions made in the creation of the model. The model results can be validated by measuring the performance of the prototypes.

Figure 4 shows the comparison of the gain measured per SAE ARP 958 and the computational results. The plot illustrates a difference between the computed and the measured values at the lower frequencies where it may be explained by the limits of the boundary condition. But overall, there is <2-dB difference between the measured gain and the computed gain from the MWS simulation.

Figure 4. Comparison of Near-Field Gain Measurements Against Computations

The final step is to verify that the pattern has improved as predicted by the software. For that purpose, the antenna was measured in the taper chamber.

Figure 5 shows the measured pattern created by the EMQuest Data Acquisition Software. As predicted by the software, the pattern remains in a single lobe at 2 GHz.

Figure 5. Measured Pattern of a 200-MHz to 2-GHz Improved DRHA at 2 GHz

From the pattern data, EMQuest can extract the beam width of the antenna as well as directivity, the side lobe level, and any other pattern-related parameters. Both the computed and measured data have shown that the pattern has improved at 2 GHz, and with it should have come an improvement in the gain of the antenna.

The improvement can be seen in Figure 4 where there is no drop in gain as we approach 2 GHz. Figure 6 presents the actual improvement in gain when compared with the traditional 200-MHz to 2-GHz DRHA. The plot demonstrates a 6-dB improvement in gain at the 2-GHz point. The result is an antenna that makes better use of amplifier power when generating fields for immunity purposes.

Figure 6. Gain Comparison of the Improved and Traditional 200-MHz to 2-GHz DRHA

Conclusion
The measured and computed results agree that, with a change to the feeding cavity, the performance of the antenna has improved significantly. Not only have we managed to get a much higher gain and better illumination pattern at the 2-GHz point, but the improvement at 2 GHz also opens the possibility for this 200-MHz to 2-GHz DRHA to be used to higher frequencies. An extended measurement of the gain shows that it is fairly stable and that the antenna is usable up to 3 GHz without significant deterioration of the radiation pattern.

References
1. Burns, C., Leuchtmann, P., and Vahldieck, R., ï¿½Analysis and Simulation of a 1-18 GHz Broadband Double-Ridge Horn Antenna,ï¿½ IEEE Transactions on Electromagnetic Compatibility, Vol. 45, No. 1, February 2003, pp. 55-60.
2. Rodriguez, V., ï¿½A New Broadband Double Ridge Guide Horn With Improved Radiation Pattern for Electromagnetic Compatibility Testing,ï¿½ 16th International Symposium on Electromagnetic Compatibility, Zurich, Switzerland, February 2005.
3. MIL-STD-461-E, ï¿½Requirements for the Control of Electromagnetic Interference Characteristics of Subsystems and Equipment,ï¿½ Department of Defense, August 1999.
4. Rodriguez, V., ï¿½Design of an Open-Boundary Quad-Ridged Guide Horn Antenna Using a Finite Integration Time Domain Technique,ï¿½ 22nd Annual Review of Progress in Applied Computational Electromagnetics, Miami, FL, March 2006.
5. Rodriguez, V., ï¿½An Open Boundary Quad-Ridged Guide Horn Antenna for Use as a Source in Antenna Pattern Measurement Chambers,ï¿½ IEEE Antennas and Propagation Magazine, Vol. 48, No. 2, April 2006, pp. 157-160.

About the Author
Vicente Rodriguez, Ph.D., is senior principal antenna engineer at ETS-Lindgren. He received a B.S.E.E., an M.S.E.E., and a doctorate from the University of Mississippi and formerly was on the electrical engineering and computer science faculty at Texas A&M University. ETS-Lindgren, Field Generation Group, 1301 Arrow Point Dr., Cedar Park, TX 78613, 512-531-6436, e-mail: Vince.Rodriguez@ets- lindgren.com

August 2006