What Radiation from Surface Waves

In our EM upbringing we have been inundated with numerous facts of Ufe. One of these is, Surface waves do not radiate  

However, once the total radiation is determined, it is of course a simple matter to assume that the radiation emanates entirely from the ends of the structure and then obtain some magic coefficients that potentially could be labor-saving, was it not for the fact that these are not invariant but change considerably with frequency and angle of incidence (see Section 4.9). 

these magic coefficients should be traded among scientists with great reservation. And not like small boys trade baseball cards  

Much of the spotlight in this chapter has been focused upon surface waves on passive periodic structures. It appears that at this point in time we have two distinct groups, one of which is associated with the presence of a stratified medium placed in the immediate neighborhood of the periodic structure. It always requires a stratified medium to exist but is independent upon whether the structure is finite or infinite. It readily shows up in programs based on the infinite array approach like, for example, the PMM program. 

The other group can exist whether a dielectric is presented or not, but the structure must be finite. Thus, it shows up only in programs based on finite array theory like, for example, the SPLAT program and not when using the PMM program. 

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This title will undoubtedly raise a few eyebrows. As stated in many respectable textbooks, surface waves do not radiate—period. What is not always emphasized is the fact that the theory for surface waves in general is based on a two-dimensional model like for example an infinitely long dielectric coated wire. And as discussed in this chapter infinite array theory may reveal many fundamental properties about arrays in general but there are phenomena that occur only when the array is finite. The fact is that surfaces waves are associated with element currents. They will radiate on a finite structure in the same manner an antenna radiates, namely by adding the fields from each column in an end-fire array. Numerous examples of this kind of radiation pattern will be shown in Chapter 4. They are typically characterized by having a mainbeam in the direction of the X axis that is lower than the sidelobe level. The reason for this abnormality is simply that the phase delay from column to column exceeds that of the Hansen-Woodyard condition by a considerable amount [29]. They also have a much lower radiation resistance. 

In the case of an aperture such as that shown in Fig. 13.1(e), the primary currents are flowing on the wave-guide and horn surfaces and are too difficult to find. In this case, the standard practice is to introduce what are known as equivalent currents. For the rectangular aperture at the mouth of the horn, equivalent electric and magnetic currents can be found from the equivalence principle so that the radiation from the equivalents is the same as from the primary currents. In any event, the current must be known before proceeding with the analysis of an antenna. 

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