Ahmed Akgiray

Airborne Ku-Band Polarimetric Radar Remote Sensing of Terrestrial Snow Cover

Characteristics of the Ku-band polarimetric scatterometer (POLSCAT) data acquired from five sets of aircraft flights in the winter months of 2006-2008 for the second Cold Land Processes Experiment (CLPX-II) in Colorado are described in this paper. The data showed the response of the Ku-band radar echoes to snowpack changes for various types of background vegetation in the study site in north central Colorado. We observed about 0.15-0.5-dB increases in backscatter for every 1 cm of snow-water-equivalent (SWE) accumulation for areas with short vegetation (sagebrush and pasture). The region with the smaller amount of biomass, signified by the backscatter in November, seemed to have the stronger backscatter response to SWE in decibels. The data also showed the impact of surface hoar growth and freeze/thaw cycles, which created large snow-grain sizes, ice crust layers, and ice lenses and consequently increased the radar signals by a few decibels. The copolarized HH/VV backscatter ratio seems to indicate double-bounce scattering between the ground surface and snow or vegetation. The cross-polarized backscatter [vertical-horizontal (VH)] showed not only the influence of vegetation but also the strong response to snow accumulation. The observed HV/VV ratio suggests the importance of multiple scattering or nonspherical scattering geometry of snow grain in the dense-media radiative transfer scattering model. Comparison of the POLSCAT and QuikSCAT data was made and confirmed the effects of mixed terrain covers in the coarse-resolution QuikSCAT data.

Circular Quadruple-Ridged Flared Horn Achieving Near-Constant Beamwidth Over Multioctave Bandwidth: Design and Measurements

A circular quadruple-ridged flared horn achieving almost-constant beamwidth over 6:1 bandwidth is presented. This horn is the first demonstration of a wideband feed for radio telescopes which is capable of accommodating different reflector antenna optics, maintains almost constant gain and has excellent match. Measurements of stand-alone horn performance reveal excellent return loss performance as well as stable radiation patterns over 6:1 frequency range. Physical optics calculations predict an average of 69% aperture efficiency and 13 K antenna noise temperature with the horn installed on a radio telescope.

Noise Measurements of Discrete HEMT Transistors and Application to Wideband Very Low-Noise Amplifiers

The noise models of InP and GaAs HEMTs are compared with measurements at both 300 and 20 K. The critical parameter, Tdrain, in the Pospieszalski noise model is determined as a function of drain current by measurements of the 1-GHz noise of discrete transistors with 50- Ω generator impedance. The dc I-V for the transistors under test are presented and effects of impact-ionization are noted. InP devices with both 100% and 75% indium mole fraction in channel are included. Examples of the design and measurement of very wideband low-noise amplifiers (LNAs) using the tested transistors are presented. At 20-K physical temperature the GaAs LNA achieves 10-K noise over the 0.7-16-GHz range with 16 mW of power and an InP LNA measures 20-K noise over the 6-50-GHz range with 30 mW of power.

Design and measurements of dual-polarized wideband constant-beamwidth quadruple-ridged flared horn

A quad-ridged, flared horn achieving nearly constant beamwidth and excellent return loss over a 6∶1 frequency bandwidth is presented. Radiation pattern measurements show excellent beamwidth stability from 2 to 12 GHz. Measured return loss is > 10 dB over the entire band and > 15 dB from 2.5 to 11 GHz. Using a custom physical optics code, system performance of a radio telescope is computed and predicted performance is average 70% aperture efficiency and 10 Kelvin of antenna noise temperature.

A 75–116-GHz LNA with 23-K noise temperature at 108 GHz

In this paper we present the design and measurement results, both on-wafer and in package, of an ultra-low-noise and wideband monolithic microwave integrated circuit (MMIC) amplifier in the frequency range of 75 to 116 GHz. The three-stage amplifier packaged in a WR10 waveguide housing and fabricated using a 35-nm InP HEMT technology achieves a record noise temperature of 23 K at 108 GHz when cryogenically cooled to 27 K. The measured gain is 22 to 27 dB for frequency range of 75 to 116 GHz. Furthermore, the amplifier utilizes four-finger devices with a total gate width of 60 μm resulting in higher output power. Therefore, we consider that this amplifier achieves state-of-the-art performance in terms of bandwidth, noise temperature, gain, and linearity so far reported for cryogenically cooled amplifiers around W-band.

The Design and Performance of a Wideband Radio Telescope for the GAVRT Program

A wideband Radio Telescope was designed and built for use in the Goldstone Apple Valley Radio Telescope (GAVRT) program. It uses an existing 34-m antenna retrofitted with a tertiary offset mirror placed at the vertex of the main reflector. It can be rotated to allow using two feeds that cover the 0.5-14-GHz band. The feed for 4.0-14.0 GHz is a cryogenically cooled, commercially available, open-boundary quadridge horn from ETS-Lindgren. Coverage from 0.5 to 4.0 GHz is provided by an uncooled lower frequency version of the same feed that uses a cooled LNA. The measured aperture efficiency is greater than 40% over much of the band.

An MMIC low-noise amplifier design technique

In this paper we discuss the design of low-noise amplifiers (LNAs) for both cryogenic and room-temperature operation in general and take the stability and linearity of the amplifiers into special consideration. Oscillations that can occur within a multi-finger transistor are studied and verified with simulations and measurements. To overcome the stability problem related to the multi-finger transistor design approach a parallel two-finger unit transistor monolithic microwave integrated circuit LNA design technique, which enables the design of wideband and high-linearity LNAs with very stable, predictable, and repeatable operation, is proposed. The feasibility of the proposed design technique is proved by demonstrating a three-stage LNA packaged in a WR10 waveguide housing and fabricated using a 35-nm InP HEMT technology that achieves more than a 20-dB gain from 75 to 116 GHz and 26-33-K noise temperature from 85 to 116 GHz when cryogenically cooled to 27 K.

Ultrawideband square and circular quad-ridge horns with near-constant beamwidth

Two quadruple-ridged, flared horns, one with circular aperture and the other with square aperture, are presented achieving nearly constant beamwidth and excellent return loss over 6:1 and 4:1 frequency bandwidths, respectively, are presented. The circular and square quad-ridge horns are designed with nominal 10 dB beamwidths of 85 and 140 degrees, respectively. Radiation pattern measurements show excellent beamwidth stability in Φ = 0, 45 degree planes for both horns. Measured return loss is >; 10 dB over the entire band for the circular quad-ridge horn and much better for 85% of the frequency band. The square quad-ridge horn also has return loss ≥ 15 dB over much of the target frequency range. To the best knowledge of the authors, these horns are the first demonstrations of ultrawideband reflector antenna feeds with near-constant beamwidth which can accommodate widely different optical configurations.

Wideband feeds and low noise amplifiers for large arrays

The state of the art for feeds and LNAs with frequency ranges approaching one decade will be presented. These components reduce the cost of receivers required to cover a given frequency range and enable observations requiring large bandwidth such as spectral index measurements, search for spectral lines with unknown red shift, and detection of transient events. An example of a 2 to 12 GHz feed will be given and compared in efficiency with an octave band feed. Low noise amplifiers for the 0.5 to 12 GHz range utilizing HEMT and SiGe bipolar transistor will be described.