A. Ittipiboon

Tunable dipole antennas

The concept of extending the operating bandwidth of a dipole has been investigated, and two bandwidth extension approaches were studied. The first approach involves the use of parasitic elements to physically change the length of the dipole. The second approach involves capacitively loading a dipole via varactor diodes to produce a travelling-wave dipole. Results are reported for the parasitic approach, which produces a broadband or multifrequency structure. The multifrequency antenna is controlled by an applied bias voltage. Simulations of the antenna showed the relationship between the diode junction capacitance and the isolation between the active and nonactive regions of the dipole.<<ETX>>

A dielectric rod antenna as a high efficiency feed for reflector antennas

Radiation characteristics of a short dielectric rod antenna excited by a circular waveguide is investigated experimentally. It is found that, by a proper selection of the waveguide radius and the exposed length of dielectric rod, the E and H plane radiation patterns, the side and back radiations and the bandwidth of the antenna can be controlled. Some experimental data are presented and the usefulness of this antenna as a feed for paraboloidal reflectors is also discussed.

The development of an active integrated microstrip antenna

The development of an active integrated antenna utilizing MMIC (monolithic microwave integrated circuit) amplifiers and microstrip transmission lines is presented. The design procedure is described and the antenna performance is discussed. An active integrated antenna has been verified experimentally. This active antenna configuration has several advantages over the passive configuration in terms of flexibility in size and gain.<<ETX>>

Holographic antenna efficiency

The holographic antennas efficiency is formulated to determine the holographic antenna dimensions and feed patterns that generate optimal performance. This efficiency comprises three main contributing sources of loss, namely, the spillover, taper, and, termination efficiencies. Antenna measurements using a linear tapered slot antenna as a holographic antenna feed confirm the accuracy of the efficiency derivation.

The application of a circuit simulator to the design and analysis of passive and active microstrip antennas

A procedure for the analysis and design of passive traveling wave antennas using a commercially available circuit simulator is presented. An example is presented showing its usefulness in providing insight into printed antenna operation. The procedure of analyzing traveling wave antennas using EMSim and calculating the resultant far-field radiation pattern has been applied to a circularly polarized rampart and four-arm traveling-wave antenna. Results look promising and a further extension to the analysis of integrated antennas is under way.<<ETX>>

A dual polarized line source feed for cylindrical reflector antennas

A line source feed for an offset cylindrical parabolic reflector antenna is described. The line source consists of two transverse and longitudinal dipole arrays over a ground plane. The elements of each array are fed separately from a feed waveguide through a coaxial line which is connected to a probe inserted into the waveguide. Within each waveguide these probes form a tapered array. The probes are treated as an N-port network to determine their penetration depth for a required Taylor distribution along the line source. The line source is then constructed and tested for both aperture field distribution and the input impedance. Its performance as a feed for an offset cylindrical parabolic reflector antenna is also studied.

A circular horn with dielectric coated flange

The similarity of a horn with corrugated flange and a horn with dielectric coated flange is discussed. Radiation characteristics of a circular horn with dielectric coated flange excited by a circular guide is then investigated experimentally. It is found that by a proper selection of the various elements-the waveguide radius, the distance from the waveguide aperture to the dielectric surface, the airgap thickness and the size of the flange-the E and H plane radiation patterns, the side and back lobe radiations and the bandwidth of the antenna can be controlled. These characteristics are found to be very similar to those obtained from a circular horn with corrugated flange. Some experimental data is given, as well as discussion of the usefulness of this antenna as a feed for paraboloidal reflectors.