A 408‐MHz earth rotation synthesis telescope is described, which has been constructed as a second frequency channel on an existing 1420‐MHz telescope. Both frequencies are received simultaneously. The angular resolution at 408 MHz is 3.5 arc minutes, and the field of view is 7°. Sensitivity is 3 mJy/beam area (1 Jy = 10−26W m−2 Hz−1). A dual‐frequency, prime focus feed receives both hands of circular polarization at each frequency. The signal transmission system is designed so that the signal phase from its 408‐MHz input to its 30‐MHz output is entirely unaffected by changes in the length of the single coaxial cable which joins each antenna to the central processor. The functions of phase rotation, signal delay equalization, and correlation are performed in a digital signal processor, using microcomputer software rather than hardware. The quadrature output is also produced by computation in real time, using a band‐limited form of the Hilbert transform. Some observational results are presented.
This work analyzes the implications of noise coupling in arrays of antennas on the design of low-noise amplifiers (LNAs). To select LNA design parameters in a manner familiar to LNA designers, the effective noise temperatures T ef f of LNAs are presented. The effect of beamformer coefficients and antenna coupling on T ef f is analyzed in a manner similar to a stand-alone LNA analysis. This leads to the proposed LNA design strategy of selecting LNA Γopt near the reflection coefficient of antenna ports and selecting the largest practical |S11| with a proper phase. This strategy is based on the assumption that the antenna array may be used for various beamforming and nonbeamforming applications, unknown at the time of the design. Numerical simulations of a 38-and 41-element antenna arrays show that a global search of LNA S11 and Γopt, which result in the minimum average beamequivalent receiver noise temperature Trec, finds nearly identical results to the proposed LNA design strategy. For a 41-element array with high antenna coupling at low frequencies, the proposed method shows as much as 80% improvement in the Trec over conventional LNA design strategies that do not account for the intended use of the amplifier in an array.
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