Tapered periodic surfaces

A tapered periodic surface (TPS) is an array of wire or slot type elements with element sizes and interelement spacings that vary as a function of position over the array. Many different appUcations of TPS come to mind. Most of these applications involve a smooth electrical transition to terminate the finite edge of an FSS or phased array. A TPS could also be used to transition from one FSS design to another in a radome with multiband window areas. Another application is as a building block for broad band and low side lobe antenna designs. Tapered Periodic Surfaces exhibit two fundamental properties ... 

Fig. 9.18 Parallel type Tapered Periodic Surface of linear wire segments.

Functionally, a tapered periodic surface performs in a fashion similar to a tapered resistive film (sometimes referred to as an R-card or edge card). Ideally,... 

Fig. 9.19 An edge-view of a Tapered Periodic Surface used to gradually transition from metal to free space.

Fig. 9.21 RCS spectrum of an 18-inch wide metal edge terminated with a 12 inch long broad band Tapered Periodic Surface, 30° above grazing, E-parallel, pare metal edge case also shown).

Fig. 9.22 The equivalent conductive length of a slot Tapered Periodic Surface decreases with increasing frequency.

However, the most discussed subject was the Broadband Array Concept It was set in motion by two of the author s oldest friends, namely Mr. Wilham Croswell and Mr. Robert Taylor from the Harris Corporation. This relationship resulted in many innovative ideas as well as support. So did my cooperation with Mission Research (home of many of the author s old students). My deep-felt thanks goes to all who participated in particular Errol English who wrote Section 9.6 about Tapered Periodic Surfaces, and Peter Munk who supplied Section 3.7 investigating Periodic Surfaces with arbitrary oriented elements. 

Frequency Compensation Frequency compensation is a very important property of the TPS that is not shared by the tapered R-Card. Since the TPS is a periodic surface, it is frequency sensitive, a property that may be exploited on a variety of broad band antenna applications. 

In their study of branched PSA, Maniar et al. (1990) found that the molecular architecture of branched polymers affects the release kinetics in a variety of ways. They found that the branched polymers degraded faster than linear PSA of comparable molecular weight (Maniar et al., 1990). They also noted that drug (morphine) release profiles were more characteristic of bulk erosion than surface erosion An initial lag time during which very little drug was released was associated with the time required for water to swell the polymer. This was followed by a period of relatively fast release, which tapered off as the device disintegrated. The polymer matrix lost its mechanical integrity before the release experiment was complete (Maniar et al., 1990). Despite the increase... 

Capillary forces offer a coherent explanation for the drying periods of many materials. If a tapered capillary is filled with water and exposed to a current of air, the meniscus at the smaller end remains stationary while the tube empties from the wider end. A similar situation exists in a wet particulate bed and the phenomenon is explained by the concept of suction potential. A negative pressure exists below the meniscus of a curved liquid surface which is proportional to the surface tension, X, and inversely proportional to the radius of curvature, r. (The meniscus is assumed to be a part of a hemisphere.) This negative pressure or suction potential may be expressed as the height of liquid, expressed by Eq. (25),... 

These squares of mortar with the glass cloth over each, are allowed to "cure" undisturbed for no less than seven days at a surface temperature of 70°F (14 days if the surface is at 60°F-the minimum cure temperature permissible). At the end of that period, the 2" wide open strips between the squares are mortared and cloth covered in the same manner, lapping the cloth 2" over the previously placed squares. (For this part of the work, 6" wide strips of glass cloth are used, and the mortar is extended from the 2" wide bare strip, 2" over the tapered edges of the squares.)... 

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