By dielectrically embedding monopole elements in low loss material, compact electronically steerable antennas can be fabricated with good mechanical stability.
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. Kumar1-3 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:
λo = the wavelength in air medium (εo) and
εr = 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. (εr)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 (0.75λo).The length of the monopole is 0.25λo(4.84 cm) with a diameter of 0.03λo (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 (εr) 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 - j002.4Table 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 λr(11.48 cm). Eight passive (parasitic) monopole elements are located around this active element at a radius 0.33λr (5.05 cm) from the active monopole. The length of the parasitic elements is 0.25λr(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.
The metal outer and inner cylinders form a coaxial line that is electrically shorted at the top by a shorting plate. A polytetrafluoroethylene (PTFE) spacer is placed between the outer and inner to maintain the spacing between the two. A feedthrough capacitor is mounted on the ground plane, which holds the parasitic element perpendicular to the metallic plate. One end of the feedthrough capacitors center conductor is connected to the inner conductor of the coaxial line. A PIN diode is connected between the outer conductor of the coaxial line and the ground plate.
When the PIN diode is biased for conduction, the associated parasitic element is strongly excited by the incident electromagnetic (EM) field and reradiates the energy. When the PIN diode is in its nonconducting state, the associated parasitic element behaves as an isolated short dipole and is virtually transparent. By applying suitable bias voltages to the appropriate parasitic elements, it is possible to generate a number of different radiation patterns of variable directivity and orientation in both azimuthal and elevation planes.
A printed-circuit board (PCB) designed to accommodate the electronic circuits for the parasitic elements was placed beneath the antenna's metallic plate. This antenna controller PCB uses high-speed digital switching techniques to control the parasitic elements. The antenna controller circuit provides appropriate bias to the parasitic elements to achieve different azimuth and elevation patterns. Together with specially developed software, a microprocessor is used in the controller circuit to control the switching of the parasitic elements.
Table 2 shows the measured return losses for a one-ring antenna with and without dielectric embedding in the presence of a radome cover. For both cases, the measured return loss is less than 16 dB from 1.52 to 1.67 GHz.
Figure 3(b) shows the bias configuration of the antenna that generates the radiation pattern of Figure 3(c) at 1.55 GHz. Five PIN diodes are switched on for conduction. These conductive diodes provide the reflecting property of the antenna in the direction of maximum radiation. Three PIN diodes control the nonconductive state and these are transparent in the direction of maximum radiation. The antenna beam can be moved in any direction by making PIN diodes reflecting or transparent in the direction of maximum radiation. The software simplifies the switching of the PIN diodes for precise beam control.
Figure 3(c) shows the measured radiation patterns for the antennas with and without dielectric embedding. Dielectric embedding resulted in a gain increase of 1 dB at 1.55 GHz, i.e. from 4 to 5 dBi. This type of antenna can be used for base-tracking in mobile communications by switching parasitic elements according to requirements.
Figure 4(a) shows a diagram of a two-ring monopole antenna, with an active monopole surrounded by two rings of monopoles. The antenna is embedded in a dielectric medium with permittivity (εr ) of 1.6. The diameters of the inner and outer rings are 0.33λrand 0.66λr, respectively. The antenna's inner and outer rings contain 8 and 16 parasitic monopoles, respectively. The inner conductor of the monopole is connected to the feedthrough capacitor and the outer cell is grounded via a PIN diode. The feedthrough capacitor forms the base of the parasitic element. As with the other antenna design, all parasitic elements are connected to an antenna controller, which provides suitable bias current to generate different radiation patterns in both the azimuthal and elevation planes.
Figures 4(b) and 4(c) show biasing arrangements for generating low and high elevation beams, respectively. The low and high beams are optimized for elevation angles of 30 and 45 deg., respectively. For the low beam, five parasitic elements in the outer ring and one in the inner ring are biased (activated) by switching the PIN diodes for conduction. The conductive diodes are shown by small heavy (black) circles while the nonconductive diodes are shown by small empty circles.
Figure 4(d) shows the measured radiation pattern in the azimuthal angle versus relative power (in dB) at elevation angles of 30 and 45 deg. Curve 1 (solid line) is the measured low-beam pattern for the two-ring monopole antenna in air at 1.55 GHz. Curve 3 (solid line) shows the high-beam pattern in air. Curves 2 (dashed line) and 4 (dotted-dashed lines) show the low-and highbeam patterns, respectively, for the dielectric-embedded monopole antenna. For the two-ring monopole design, dielectric embedding yields about 0.5 dB higher gain that without. The measured return loss is better than 15 dB for the low-and high-beam cases from 1.52 to 1.67 GHz. Preliminary vibration tests shows that mechanical stability is increased due to dielectric embedding. This type of antenna is suitable for flight-based mobile satellite-communications systems. Further development work will explore the addition of parasitic rings to increase antenna gain.
Author would like to thank Angel Kumar (graduate of Computer Science, McGill University, Montreal) for the development of software and measurement of radiation patterns.