  A pre-requisite for the design of wireless systems is the understanding of
the propagation channel. While a wealth of propagation knowledge exists for
bands below 6 GHz, the same can not be said for bands approaching
millimeter-wave frequencies. In this paper, we present the design,
implementation and measurement-based verification of a re-configurable
27.5-29.5 GHz channel sounder for measuring dynamic directional channels. Based
on the switched array principle, our design is capable of characterizing
128$\times$256 dual-polarized channels with snapshot times of around 600 ms.
This is in sharp contrast to measurement times on the order of tens-of-minutes
with rotating horn antenna sounders. Our design lends itself to high angular
resolution at both link ends with calibrated antenna arrays sampled at
2$^\circ$ and 5$^\circ$ intervals in the azimuth and elevation domains. This is
complemented with a bandwidth of up to 2 GHz, enabling nanosecond-level delay
resolution. The short measurement times and stable radio frequency design
facilitates real-time processing and averaging of the received wavefronts to
gain measurement signal-to-noise ratio and dynamic range. After disclosing the
sounder design and implementation, we demonstrate its capabilities by
presenting dynamic and static measurements at 28 GHz over a 1 GHz bandwidth in
an office corridor environment.
