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November, 1967

Historically, slot antennas have taken several shapes—rectangular, ellipse, dumbbell, bowtie, cruciform, “U,”, “H,” and annular. To prevent radiation from both sides of the slot, a backup cavity is used on one side. But, space, missile and aircraft vehicle packaging design demands that small-sized cavities and slots be used. This restrains the antenna designer because small sizes are not amenable to broadband, high-power reasonable-gain designs.

The problem is that for matched bandwidths, antenna impedance and input VSWR have a direct dependence on slot length. Also, the self-impedance of the rectangular slot is highly reactive with small real impedances if the slot length is very small in terms of wavelengths. In addition, the backup cavity will only sustain certain modes above a cutoff frequency which is dependent on the cavity internal dimensions.

The luna-slot antenna was thus developed using both empirical and theoretical methods. As a measure of bandwidth improvement for the same volume, a cross-slot dual-channel antenna was used. The configuration of this antenna is shown in Fig. 1 together with its VSWR characteristics. The following design goals were established based on specific volume constraints (4-1/2 in. dia by 1-1/2 in. high) imposed by vehicle design: weight, less than one pound; power handling, 10 W; VSWR, less than 2:1 over a 5-Mc band centered at 442 Mc; and a broad radiation pattern.

Antenna impedance

An electrically short rectangular slot and a shallow backup cavity result in a high-Q antenna because of the high reactances and low resistances. These are predominant parameters that limit the bandwidth and VSWR of a basic slot antenna.

A series feed, in which the cavity becomes a transformer from the coaxial-line TEM mode, and a stretching of the slot into a lunar shape, largely overcome the described limitations. Such a feed is shown in Fig. 2, where the cavity field excitation is the radially symmetric TEM mode which has no low cutoff frequency. This is one of the modes used in the luna-slot antenna development to avoid cutoff restrictions.

A basic model was constructed of a cylindrical cavity. The resulting annular slot was used as the initial radiating aperture geometry as shown in Fig. 3. The model had a 4-1/2-in. dia, 1-1/2-in. deep cavity. A coaxial center conductor, soldered to a circular disc placed in the plane of the cavity aperture, resulted in a flush disc antenna.

Impedance measurements were made of the flush disc antenna at uhf frequencies, but because of the small sizes (relative to a wavelength) the VSWRs were exceptionally high (30:1). A re-analysis of the slot region was made using a rectangular slot-aperture analogy with its known current and field distribution. The rectangular slot was then progressively evolved into a luna-slot by stretching it into a crescent or lunar shape shown in Fig. 4. The radial asymmetry of this new geometry was also considered to fill in the null normal to the symmetrical annular slot aperture.