Multi-frequency Dual-Polarized Cloud Radar Antenna
of Massachusetts Microwave Remote Sensing Laboratory is building a multi-frequency
radar to measure particle sizes and density of clouds as part of global
warming and climatology studies. Three radars operating at 13, 35 and 94
GHz will use a single antenna system mounted on a large truck for transportability.
The antenna was designed, assembled and tested by the Antenna Laboratory.
A single, dual-polarized, two-frequency feed was designed for 13 and 35
GHz. The feed illuminates the subreflector of a dual-offset Gregorian antenna.
The 2-meter main reflector is fully utilized for the 13 GHz beam, yielding
0.75-deg beamwidth H and V polarized beams with identical shapes and boresight
directions. At 35 GHz, only the central portion of the main reflector is
used to produce 0.75-deg H and V polarized beams that are aligned with the
13-GHz beams. The design of the feed insures that the four beams (two frequencies
+ two polarizations) are matched for all look directions. The 95-GHz beam
requires only a 30-cm aperture, which is provided by a small dielectric
lens antenna mounted securely to the main reflector.
The new antenna system is smaller and lighter than a multiple-antenna system
with similar performance, and it eliminates problems of parallax and alignment
that have plagued previous multiple-antenna systems.
Wide Bandwidth Phased Arrays
The Antenna Laboratory
at UMass has studied wide bandwidth Tapered Slot Antenna antennas and
arrays, often called Vivaldi antennas, for two decades. Efficient computational
electromagnetic (CEM) codes have been developed and used to design wide
bandwidth scanning arrays and to uncover key phenomenology that governs
the behavior of these antennas. The early work of the laboratory provided
quantitative estimates of the radiation characteristics of single Tapered
Slot Antennas (TSAs). Our frequency domain CEM codes for infinite arrays
provided the first antenna designs with bandwidths exceeding 5:1 and scan
volumes exceeding 450 from broadside. For several years, UMass has supported
the efforts of international radio astronomers to design a radio telescope
with one square kilometer of collecting area. The antenna should operate
0.2 – 2.0 GHz, providing 1 square degree field of view and 0.1 arcsec
resolution at 1.4 GHz. Many visitors and collaborators contributed to
our understanding of wide bandwidth antenna arrays and enriched the educational
experience of our students.
Time-Domain Integral Equation Analysis
methods are particularly useful for studying wide bandwidth radiation
and scattering because the computations are performed without matrix inversion
and Fourier transformation of the transient response yields the entire
frequency range at once. The UMass Antenna Lab has developed an efficient
time-domain integral equation code exploiting the efficiencies of surface
integral formulations and incorporating several techniques from advanced
signal processing to maintain stable solutions even for large structures
that require a very long time history to accurately characterize their
behavior. Using TDIE, ordinary desktop computers can analyze the input
impedances of a 16x16 array of Vivaldi tapered slot antennas in a couple
of hours. A convenient user interface permits entry of many common antenna
types with a few mouse clicks and the post-processor provides easy to
interpret graphical displays of key antenna and scattering parameters.
Ultrawide Bandwidth Antenna Systems
In collaboration with
the UMass Wireless Communication Center, the University of Southern California,
and UC Berkeley, the Antenna Laboratory is developing antennas and analysis
techniques for UltraWide Bandwidth (UWB) communication, tagging and location
systems. UWB systems use extremely wide bandwidth signals with low power
spectral density so that they can coexist with narrow bandwidth signals
without significant degradation of those signals. Typically, UWB antennas
must effectively radiate or receive signals with more than an octave of
frequency bandwidth, and they should be small and inexpensive to fulfill
objectives of many proposed applications. Time-domain analysis of UWB
antennas yields necessary information for the ultrawide bandwidth signals.
Easy to use computer codes based on Marching-in-Time algorithm for Maxwell's
equations permit quick and accurate characterization of existing antennas
and helps to design new antennas with improved performance.