Surface Waves Unique to Finite Periodic Structures

2 SURFACE WAVES UNIQUE TO FINITE PERIODIC STRUCTURES 

We have calculated the element currents only at / = 10 GHz—that is, close to the resonant frequency of the array. Let us now explore the situation at a frequency approximately 25% lower, namely at / = 7.8 GHz. From the SPLAT program we obtain the element currents shown in Fig. 1.3c, while the PMM program gives us element currents equal to 0.045 mA as shown in Fig. 1.3c, close to what would be expected based on the resonant value of 0.055 mA (see Fig. 1.3a). 

We observe in Fig. 1.3c that the element currents for the finite array not only fluctuate dramatically from column to column but also exhibit an average current that can be estimated to be somewhat higher than the currents even for resonance condition (0.055 mA). 

We shall investigate this phenomena in detail in Chapter 4. It will there be shown that the element currents are composed of three components  

The Floquet currents as observed in an infinite x infinite array—that is, currents with equal magnitude and a phase matching that of the incident plane wave. 

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Further support and interest in my work was shown by Dr. Brian Kent, Dr. Stephen Schneider, and Mr. Ed Utt from the U.S. Air Force. After completion of the development of the Periodic Method of Moments, the PMM code, the Hybrid radome, low RCS antennas, and more, the funding from the Air Force shifted into more hardware-oriented programs. Fortunately, the U.S. Navy needed our help in designing very broadbanded bandstop panels. Ultimately, this work resulted in the discovery of surface waves unique to finite periodic structures, which are treated in great detail in this book. The help and advice from Mr. Jim Logan, Dr. John Meloling, and Dr. John Rockway is deeply appreciated. 

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