A compact absorptive filtering patch antenna is presented. It consists of a filtering patch antenna and a bandstop filter (BSF), with their transfer functions being complementary to each other. A slot is fabricated in each of the patch and ground, giving a total of two radiation nulls for the lower bandedge. By using a dual-stub feed, two radiation nulls are also obtained for the upper bandedge. For the BSF, a λg/2 defected ground structure (DGS) and a λg/4 defected microstrip structure (DMS) are used in the design. It is terminated by a chip resistor. Since the filtering patch antenna and BSF have complementary transfer functions, the incident energy can be radiated effectively in the passband but largely absorbed by the resistor in the stopbands. As a result, only little energy will be reflected over a wide frequency range, giving a reflectionless characteristic. To demonstrate this idea, an absorptive filtering antenna operating at 5.8 GHz was designed, fabricated, and tested. Its impedance is matched from 5 to 6.5 GHz, with the measured out-of-band suppression being higher than 17 dB and 20 dB for the lower and upper stopbands, respectively. The measured peak realized gain is 7.28 dBi.

This paper presents a novel design of cylindrical modified Luneberg lens antenna at millimeter-wave (mm-wave) frequencies in which no dielectric is needed as lens material. The cylindrical modified Luneberg lens consists of two air-filled, almost-parallel plates whose spacing continuously varies with the radius to simulate the general Luneberg's Law. A planar antipodal linearly-tapered slot antenna (ALTSA) is placed between the parallel plates at the focal position of the lens as a feed antenna. A combined ray-optics/diffraction method and CST-MWS are used to analyze and design this lens antenna. Measured results of a fabricated cylindrical modified Luneberg lens with a diameter of 100 mm show good agreement with theoretical predictions. At the design frequency of 30 GHz, the measured 3-dB E- and H-plane beamwidths are 8.6° and 68°, respectively. The first sidelobe level in the E-plane is -20 dB, and the cross-polarization is -28 dB below peak. The measured aperture efficiency is 68% at 30 GHz, and varies between 50% and 71% over the tested frequency band of 29-32 GHz. Due to its rotational symmetry, this lens can be used to launch multiple beams by implementing an arc array of planar ALTSA elements at the periphery of the lens. A 21-element antenna array with a -3-D dB beam crossover and a scan angle of 180° is demonstrated. The measured overall scan coverage is up to ±80° with gain drop less than -3 dB.

A filtering dielectric resonator (DR) antenna (DRA) that does not require any filtering circuits is investigated. It deploys a rectangular DR and three slots fabricated in the ground plane. The antenna is excited in the fundamental TEy1δ1 DRA mode and two slot modes. By making use of these three modes, two tunable radiation nulls near the edges of the passband can be obtained, giving filtering effects without the need of any filtering circuits. In this paper, the design, with minor modifications of the slots, is extended to a frequency-reconfigurable filtering DRA by using varactors. Its stopband can be extended to the third harmonic. By varying the voltages of the varactors, the antenna has a continuous frequency agility of 27.7% (2.15–2.84 GHz), with almost the same harmonics suppression level. The antenna has a gain of ~5.0 dBi and a rejection level of over 16.1 dB across the stopband. Its total antenna efficiency in the passband is ~81%, whereas it is generally lower than the average value of 1.8% in the stopband (up to 7.0 GHz). Comparing with the reported frequency-reconfigurable filtering antennas, our design has a better frequency agility, more stable filtering response, and weaker radiation in the stopband.

The cylindrical dielectric resonator (DR) antenna (DRA) is excited in its omnidirectional TM01δ mode by a planar shorted microstrip cross. With this non-intrusive feed, the DRA can be fabricated without the need of drilling a hole in the DR as required in the probe feed method. This DRA is applied to the first omnidirectional circular polarized (CP) diversity DRA. To generate omnidirectional CP fields, the TM01δ and TE011+δ modes are excited simultaneously. The TE011+δ mode is excited by four microstrip arcs. They provide a pair of equivalent magnetic dipoles that generate fields that are orthogonal to those of the TM01δ mode. Omnidirectional CP fields can be obtained when the (orthogonal) fields of the TM01δ and TE011+δ modes are equal in amplitude but in phase quadrature. In our two-port CP diversity design, phase differences of +90° and −90° are obtained in Ports 1 and 2 to generate right- and left-hand CP fields, respectively. Prototypes at ~2.4 GHz were designed, fabricated, and measured for WLAN applications. The S-parameters, radiation patterns, antenna gains, and efficiencies are studied. For the diversity design, the axial ratio, envelope correlation coefficient, and mean effective gain are also obtained. The measured and simulated results are in reasonable agreement.

A new omnidirectional circularly polarized (CP) cylindrical dielectric resonator (DR) antenna (DRA) is excited by four open-ended logarithmic spiral slots in the ground plane. Radiated E _θ and E _φ components are obtained from the TM _01δ mode of the DR and the slotted ground plane, respectively. Omnidirectional CP fields can be obtained when E _θ and E _φ are equal in magnitude but different in phase by 90°. This CP technique does not require any special shape or treatment of DR, greatly facilitating the design. To demonstrate the idea, a prototype using a glass DR was fabricated. It is a dual-function DRA that also serves as the cover of light sources. The prototype has a measured bandwidth of ~6.2%, covering the entire 2.4-GHz WLAN band.