A 77-118 GHz Resonance-free Septum Polarizer
  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. 
Yen-Lin Chen, Tzihong Chiueh, and Hsiao-Feng Teng
astro-ph.IM