  Aperture arrays have been studied extensively for application in the next
generation of large radio telescopes for astronomy, requiring extremely low
noise performance. Prototype array systems need to demonstrate the low noise
potential of aperture array technology. This paper presents noise measurements
for an Aperture Array tile of 144 dual-polarized tapered slot antenna (TSA)
elements, originally built and characterized for use as a Phased Array Feed for
application in an L-band radio astronomical receiving system. The system noise
budget is given and the dependency of the measured noise temperatures on the
beam steering is discussed. A comparison is made of the measurement results
with simulations of the noise behavior using a system noise model. This model
includes the effect of receiver noise coupling, resulting from a changing
active reflection coefficient and array noise contribution as a function of
beam steering. Measurement results clearly demonstrate the validity of the
model and thus the concept of active reflection coefficient for the calculation
of effective system noise temperatures. The presented array noise temperatures,
with a best measured value of 45 K, are state-of-the-art for room temperature
aperture arrays in the 1 GHz range and illustrate their low noise potential.
