Frequency (GHz)

Figure 7.133 Square spiral antenna geometry and gain with respect to frequency ([53], copyrightC 2005 IEEE).

7.2.2.1.4.6.3 Archimedean spiral antennas

Another example of spiral antenna is the Archimedean spiral described in [53], in which feeding technique, single resistance loading, tapered dielectric superstrate, and so forth, are discussed and the performances are introduced. The antenna is shown in the inset of Figure 7.133. The geometrical parameters of the antenna are; slot width w = 0.0762 cm, conductor width S = 0.2286 cm, and aperture dimensions are 5.715 × 5.715 cm, while the actual spiral has dimensions of 4.6482 × 4.9530 cm. To achieve unidirectional radiation, a square cavity having an inner dimension of 5.08 cm and a depth of 2.54 cm is used. The square spiral slot is printed on a 0.06096 cm thick substrate (εr = 4.25 – j0.0595). A 0–180◦ broadband hybrid is employed for feeding and 15 resistors using Klopfenstein taper [64] are used for the termination, instead of an infinite coaxial balun embedded on the surface of the spiral elements and feeding the spiral at its center. Figure 7.133 illustrates measured and simulated gain. In order to terminate with only a single resistor, the slot line is extended to the aperture edge, resulting in use of a 47resistor that can provide an effective termination in terms of both axial ratio and impedance matching. By this means gain is improved and is shown in Figure 7.134, which also shows the gain when resistive taper is used as a comparison.

Further improvement of gain is achieved by using high-contrast dielectric loading [53]. Photos of a tapered dielectric layer placed as the superstrate on the slot spiral are shown with cross sectional view in Figure 7.135(a), and the measured gains for various values of the superstrate εr = 9, 16, 30, and 90 are illustrated in Figure 7.135(b). Lesser thickness of the dielectric is used as εr becomes larger. It was found that with higher εr, the gain tends to be higher. When –15 dB gain is taken as a reference, lowering frequency is observed from 884 MHz to 564 MHz as εr increases from unity (no dielectric) to 90.