June, 1969

Dielectric-filled waveguide, traveling-wave arrays can be designed for main beams pointing between broadside and forward endfire or between broadside and rear endfire. This beam pointing direction is a function of frequency, the waveguide width, the spacing and phasing of the radiating elements, and the dielectric constant of the material used in the array.

The curves in Figs. 1-11 relate these parameters. A waveguide filled with dielectric material reduces the waveguide wavelength and thereby reduces the size of the waveguide. The use of dielectric-loaded waveguide therefore provides not only a flexibility of design through several design options, but reduced fabrication costs as well.

Each curve represents a different dielectric constant, of integer value (K = 1 through 9). Graphs have been included for fused silica and Pyroceram brand glass-ceramic with dielectric constants of 3.78 and 5.55 respectively. These two ceramics have been found to be especially suited for waveguide loading applications, having a negligible dielectric constant variation over extended temperature ranges. For other specifically desired dielectric constants, parameters can be obtained by interpolation from the available graphs.

The curves show the maximum normalized interelement spacing without gain reducing grating lobes or secondary main beams appearing. Given specifications such as beam position and frequency, the antenna designer can immediately obtain parameters from the graphs provided.

The curves are intended to present all the possibilities and trade-offs available in evaluating a traveling-wave array rather than a specific design procedure.

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Use of curves and their relation to parameters

The dependence of the main beam pointing direction on the various parameters of a dielectric-filled waveguide array may be described as follows:

where

ϕ = the main beam pointing direction from forward endfire

K = dielectric constant of the material investigated

λ = free space wavelength

α = inside width of the rectangular waveguide

m = 1 or 3 when slots are staggered

m = 0 to 2 when slots are collinear (in line)

d = spacing between adjacent elements (slots) as measured along the waveguide axis.

Figures 1 through 11 are graphical representations of Equation 1. If forward endfire is the direction of the waveguide propagation and rear endfire is the direction of the source or feed with respect to the radiating elements (Fig. A), then the angle of the main beam pointing direction from forward endfire is the ordinate of the curves. The other ordinate is the maximum normalized interelement spacing not allowing grating lobes or secondary main beams to appear. All parameters defined in Equation 1 are presented on Figs. 1 through 11. When m is equal to zero, the beam position is independent of the interelement spacing; therefore, the dotted line representing this case is applicable to all interelement spacings that do not allow grating lobes to appear in the radiation pattern.

Antennas filled with certain types of these dielectric materials can generate either of two beams (corresponding to m equals 0 or 2 for collinear slots or to m equals 1 or 3 for staggered slots). Ambiguities do not exist since the two beams appear over non-overlapping bandwidths.

Example problem considering various dielectrics

A beam position of 60 degrees from forward endfire at a frequency of 5 GHz is desired. An interelement spacing close to 0.65λ will be investigated. Table 1 gives the possible parameters for this problem. As can be seen, many trade-offs exist. The waveguide width and frequency sensitivity vary depending on the dielectric material available. Generally, the beam position is less sensitive to waveguide tolerances for wider widths. The solution to this problem is thus dependent on the maximum waveguide width allowable, the tolerable beam shift due to the required frequency bandwidth, and the dielectric materials that are obtainable.

Example problem showing interpolation technqiues

It is required to design an array with a beam pointing 70 degrees from forward endfire at 10 GHz. If other considerations dictate usage of a material whose dielectric constant is 2.5, which is not covered by any of the curves of Figs. 1 through 11, a design could be obtained by interpolating between the curves representing the two integer dielectric constants closest to 2.5. The results in Table 2 follow from an interpolation of the curves in Figs. 2 and 3.

The second design is chosen because it is less frequency sensitive even though it is wider.

Example problem using a specific dielectric material

An antenna is assumed to be mounted on a missile parallel to the missile axis; the antenna is to be fed from the forward missile end. A beam angle of 50 degrees from the forward missile axis at a frequency of 6 GHz is desired. (ϕ = 180-50 deg.) Due to the insensitivity of the dielectric constant of Pyroceram brand glass-ceramic to temperature, it is chosen as the loading material. Table 3 presents the solution to this problem. The second design is chosen because it is less frequency sensitive.

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