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We present the design, fabrication and measured performance of a hierarchicalsinuous-antenna phased array coupled to transition-edge-sensor (TES) bolometersfor measurements of the cosmic microwave background (CMB). To efficiently covera broader range of frequencies, many CMB experiments have begun to deploymultichroic pixels. For a given cryogenic and optical design, the pixel sizethat optimizes the mapping speed is frequency-dependent and increases withwavelength. If a multichroic pixel is the same size at all frequencies, thenthe mapping speed will be optimal in at most one frequency band. To achieveoptimal mapping speed at all frequencies, a pixel size that varies withfrequency is needed. This can be accomplished by creating phased arrays fromneighboring pixels, where the size of each phased array is chosen independentlyfor each frequency band. As the array unit, we choose a lenslet-coupled sinuousantenna on account of its wide bandwidth, dual polarization and constant beamwaist. We find that the array factor tends to compensate for beamnon-idealities, so that the sinuous antenna can be substantially undersizedrelative to the single-element case; this frees up valuable focal-plane areafor bolometers, bandpass filters, summing networks, readout circuitry, etc. Anadditional benefit of a hierarchical phased array is the reduced readoutrequirement: since the low frequencies are arrayed to increase their mappingspeeds, the detector count scales approximately logarithmically with the numberof frequency bands. Here we present measurements from a prototype device, inwhich hierarchical triangular arrays are used at 90, 150 and 220 GHz to keepthe effective pixel size and, therefore, the beam width roughly constant acrossthe entire frequency range.

Hierarchical sinuous phased array for increased mapping speed of multichroic focal planes