Electronically steerable antennas can provide beam scanning in azimthal and elevation planes without moving parts, adding to their reliability under severe weather conditions. Such an antenna has been developed based on a monopole center feed, an eight-monopole main ring, and 16 monopoles in two parasitic rings. The monopoles are embedded in dielectric to achieve a size reduction. The parasitic elements are connected to a controller circuit via a PIN diode and feedthrough capacitor. The controller switches between conductive or nonconductive behavior for the parasitic elements to steer the antenna beam.

Demand has increased for reliable two-way data-and voice-communication systems to support public safety, emergency-response teams' security due to terrorism, and various government and nongovernment agencies. Kumar^1-3 has reported several low-profile microstrip antenna array designs suitable for meeting the mobile satellite demand at L-band. These antennas rely on mechanical steering for beam control in the elevation plane.

Unfortunately, under severe weather conditions, such mechanical steering mechanisms can breakdown. Therefore, an adaptive antenna is a good candidate for application of electronic scanning. The physical structure is simple and requires relatively little power to operate compared to its phased arrays. By electronically controlling parasitic-element biasing, directional beams and nulls can be formed and steered throughout the azimuthal and elevation planes.

Electronically beam-steerable adaptive arrays have existed for some time.³ Design refinements, however, can yield increased performance and reduced size. The size reduction is achieved by embedding the feed antenna, including the parasitic elements, in a homogeneous dielectric material. The adaptive antenna reported here uses monopole elements, with a height of about one-quarter wavelength in a dielectric media. The use of dielectric embedded monopole elements reduced the size of the antenna by (ε _r)^0.5 The approach can also enhance performance. When antenna elements are placed in a high-dielectric-constant medium of infinite extent, the wavelength of the antenna in the dielectric medium is:

where:

λ_o = the wavelength in air medium (ε_o) and

ε_r = the permittivity of a dielectric medium in which antenna is embedded.

The size of the monopole array antenna-is reduced by the factor of the square root of the permittivity of the dielectric media, i.e. (ε_r)^1/2.

Figure 1 shows a photograph of a monopole antenna on a small circular metallic disk. The antenna was designed at 1.55 GHz for operation in the frequency range of 1.52 to 1.67 GHz. The disk diameter is 14.52 cm (0.75λ_o).The length of the monopole is 0.25λ_o(4.84 cm) with a diameter of 0.03λ_o (0.58 cm). The method of moments (MoM) and the uniform theory of diffraction have been combined to find the radiation patterns of a monopole on a finite ground plane.

This hybrid analysis technique has proved to be very useful for a small-size ground plane.

The antenna's radiation patterns were measured in an anechoic chamber at 1.55 GHz (Fig. 2) . Figure 2 shows the measured (dotted lines) and calculated (heavy dots) radiation patterns. The monopole antenna is embedded in dielectric material with permittivity (ε_r) of 1.6 which reduced the required length of the monopole from 4.84 cm to 3.83 cm and the diameter of the ground plane from 14.52 cm to 11.48 cm. Figure 2 also shows calculated and measured radiation patterns as a function of the elevation angle. For both antennas (with and without dielectric embedded monopole), good agreement was achieved between the calculated and measured results. Using the embedded dielectric material resulted in an improvement in the peak elevation angle of about 20 deg. compared with the value obtained from a monopole antenna without the dielectric embedding. Consequently, antenna gain increased from 2 to 3 dB due to embedding the dielectric to the monopole. The length of the monopole and diameter of the ground plane were reduced by 1.265 times due to dielectric embedding. The measured return loss with and without dielectric embedded antennas is better than 16 and 15 dB, respectively.

Figure 3 shows a nine-element monopole antenna with small ground plane. The holes in the metallic plate between the center monopole and parasitic elements are required to embed the dielectric material using a special mold. A low-loss liquid dielectric material is used in the mold to cover all the monopoles. The liquid solidifies into one piece without damaging the feedthrough capacitor. The dielectric material has a complex permittivity of 1.6 - j002.⁴ Table 1 summarizes the dimensions of one-and two-ring antennas.

In the case of the one-ring antenna, an active monopole element is placed in the middle of the structure, positioned at the center of a perfect conducting ground plane of radius 0.75 λ_r(11.48 cm). Eight passive (parasitic) monopole elements are located around this active element at a radius 0.33λ_r (5.05 cm) from the active monopole. The length of the parasitic elements is 0.25λ_r(3.83 cm); they are connected to the ground plate via a PIN diode. The parasitic element incorporates a PIN diode and a RF bypass capacitor.