The FSS Case

When a periodic structure is intended to work as a wire FSS, it would lead to unacceptably high reflection loss if each element was loaded with resistors 

One approach is to simply have no resistors anywhere over the entire surface, with the exception of a few colnmns at the edges. An example is shown in Fig. 1.5b, where the two outer colnmns have been loaded with 200 ohms, the next ones toward the center with 100 ohms, and finally the third column with 50 ohms. We observe a significant rednction of the ripple amplitudes as compared to the nnloaded case in Fig. 1.3c. It shonld be noted that no parametric study was done on the resistive values of the loads at this point. More in Chapter 4. 

We also show in Fig. 1.5c a case where each element over the entire surface has been loaded very lightly, namely with 20 ohms. We observe a strong reduction of the ripples from column to column—in particular, in the right half of the array. 

The transmission loss at resonance dne to the 20-ohm load resistors is obtained from the equivalent circuit in Fig. 1.6b. The reduction of current is equal to Za Za -h Zi) = 200/(200 -t- 20) = 0.9, or about 1 dB (just barely permissible). 

Alternatively we may instead of the 20-ohm loss resistors obtain a moderate loss by simply using a slightly lossy dielectric next to the elements or simply a resistive sheet close to the elements. 

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In this chapter we shall smdy the FSS case in more detail and in particular how to condol the surface waves. The phased array case will be discussed in Chapter 5. 

These resistive components cause significant attenuation of potential surface waves along the structure. In fact, they will in general be so weak that the surface wave radiation from active arrays can be ignored in contrast to the FSS case discussed in Chapter 4. However, they may be strong enough to produce jitter of the terminal impedance. 

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