  Measurements of the polarized radiation often reveal specific physical
properties of emission sources, such as strengths and orientations of magnetic
fields offered by synchrotron radiation and Zeeman line emission, and the
electron density distribution by free-free emission. Polarization-capable,
millimeter/sub-millimeter telescopes are normally equipped with either septum
polarizers or ortho-mode transducers (OMT) for the detection of polarized
radiation. While the septum polarizer is traditionally conceived to be limited
to a significantly narrower bandwidth than the OMT, it does possess
advantageous features for astronomical polarization measurements unparalleled
by the OMT. Challenging the conventional bandwidth limit, we design an
extremely wideband circular waveguide septum polarizer, covering $42\%$
bandwidth, from 77 GHz to 118 GHz, without any undesired resonance. Stokes
parameters constructed from the measured data in between 77 GHz and 115 GHz
show that the leakage from $I$ to $Q$ and $U$ is below $\pm 2\%$ and the $Q-U$
mutual leakage below $\pm 1\%$. Such a performance is comparable to other
modern polarizers, but the bandwidth of this polarizer can be at least twice as
wide. This extremely wide-band design removes the major weakness of the septum
polarizer and opens up a new window for future astronomical polarization
measurements.
