Jar J. Lee

Reconfigurable aperture antennas using RF MEMS switches for multi-octave tunability and beam steering

The requirements for increased functionality within a confined volume will place greater burdens on electromagnetic platforms for air, space, and sea over the next few decades. An important piece of the any solution to these new requirements are transmitting and receiving apertures that can handle multi-octave bandwidths with beam steering capability. The ability of an aperture to be reconfigured for a particular mission will become essential. New types of devices are being developed which will enable the realization of these reconfigurable apertures. This paper presents a discussion of how one of these new devices, the RF MEMS switch, can be utilized to change the phase and frequency characteristics of conventional antenna elements to perform beam steering over a wide range of microwave frequencies.

A wideband beam switching antenna using RF MEMS switches

Reconfigurable aperture antenna technology has been enabled by the advent of RF MEMS switches. Typical measured insertion losses of less than 0.25 dB up to 40 GHz with superb broadband isolation means that reconfiguration of the antenna for multi-band operation can now occur with a few MEMS switches rather than multiple PIN diode or FET switch circuits. In this paper, we describe a reflectarray antenna approach that can perform true time delay beam steering by using cascaded RF MEMS switches/coplanar strip transmission line sections. Measured RF MEMS switch characteristics in coplanar strip transmission lines are presented and the modeled reflected phase of five cascaded switch/transmission line sections is presented.

Array antennas using low loss MEMS phase shifters

A line source of sixteen 5-bit MEMS phase shifters was used for beam scan of a novel X-band CTS (continuous transverse stubs) antenna over +/- 45 degrees in the H-plane. The 5-bit phase shifter has 1 dB loss on average over 6-10 GHz. The building block of these phase shifters is a cantilevered MEMS switch developed by HRL Laboratories. The metal-contact switch is characterized by 0.15 dB loss and 40 dB isolation at 10 GHz.

A UHF wide-band SAR antenna

This UHF wideband antenna was developed for DARPAs synthetic aperture radar (SAR) applications. The small array could be mounted at the belly of an aircraft. The operating frequency covers a 4:1 bandwidth. The antenna including the feed and a contoured ground plane was designed to fit in a shallow radome (50 cm/spl times/100 cm/spl times/100 cm). The depth of the radome is only one quarter of a wavelength at the low end of the UHF band. The small volume imposes a severe constraint in the electrical design and the packaging of the antenna, which is required to produce an elevation (EL) beam pointed at 30 degrees depression angle from the horizon (60 degrees from the nadir). The 3 dB beam at mid-band is about 50 degrees in both EL and azimuth (AZ) planes. Since the beam is broad enough to cover a region from 10 to 60 degrees depression angle, no steering in the EL plane is required. Furthermore, the antenna must provide dual linear polarizations over a 4:1 bandwidth with VSWR less than 2:1 over 90% of the band. With the beam peak steered to 60 degrees from the broadside, it is equivalent to imposing a wide scan in the EL plane. To suppress grating lobes at the high end of the band, the element spacing was kept less than 25 cm. This spacing is about one-eighth of a wavelength at the low end of the band, which makes it difficult to match the input impedance of the antenna because the element is electrically small.

Performance of an ultra wide band UHF antenna

A 4:1 bandwidth UHF antenna has been developed for radar applications. The small array could be mounted at the belly of an airborne platform. The antenna and the feed network were designed to fit in a shallow radome (50 cm/spl times/100 cm/spl times/100 cm). The depth of the radome is only one quarter of a wavelength at the low end of the UHF band. The small volume imposes a severe constraint in the electrical and mechanical design of the antenna. The antenna produces a broad beam in the elevation (EL) plane, with its beam peak pointed at the 30-degree depression angle (60 degree from the nadir).

Backscatter measurements of Hi-Z ground plane

Backscatter measurements in monostatic and bistatic modes were made on a 25 cm /spl times/ 40 cm Hi-Z ground plane designed for S-band antenna array experiments. Compared to a regular plate, the Hi-Z ground plane exhibits a 3 dB dip in the backscattered power in the monostatic mode at the band gap frequency (2.8-2.9 GHz). In the symmetrical bistatic mode, a 20 dB dip was observed in the reflected power at the same frequency at 70 degree incident angle.

Ultra wide band cylindrical array and 360-degree beam scan system

An ultra wideband cylindrical array consisting of 48 radial columns of end-fired elements is described. A 5-column mini-sector test array was built and tested. Test results for 100 to 900 MHz were recorded. Interleaved with the cylindrical array at the outer rim is a 96-element wideband (500-2000 MHz) ring array for IFF operation.

A multibeam array using RF mixing feed

Multiple beams can be produced and independently scanned with a single photonic series feed without using multiple sets of phase shifters. This is accomplished by mixing two counter-propagating signals to generate the required phase distribution for each beam. The linear progressive phase is proportional to the sum of two control frequencies, but the output frequency is equal to the constant beat frequency controlled by a phase locked loop. This design can be extended to 2-D scan by cascading two such feeds in the two orthogonal planes. Test results of a 2-element (X-band) transmit array constructed to demonstrate the operation of two simultaneous beams are reported.