CARMA Memo #54

Circular Polarizers for the CARMA 1mm Receivers

Richard Plambeck, Greg Engargiola
U.C. Berkeley


  We describe the design of waveguide circular polarizers for the
  CARMA 1mm receivers.  A polarizer will be installed between the
  feedhorn and the orthomode transducer in each dewar, at a
  temperature of 4 K.  It will convert incoming R and L circularly
  polarized signals to X and Y linearly polarized signals, which
  will be separated by the OMT.

  The polarizer is a 2-section design, using half-wave and
  quarter-wave retarder sections rotated axially by 59.5 degrees
  with respect to each other to achieve broadband performance.  It is
  constructed in 0.047" diameter circular waveguide; the retarders
  are sections of reduced height, or `faceted,' circular waveguide.
  Network analyzer measurements of K-band scale models were used to
  verify the retarder design.

  The polarizer will be fabricated by electroforming on an aluminum
  mandrel.  Analysis shows that the diameter of the circular waveguide
  and the heights of the retarder sections must be controlled to
  +/- 0.0003", and the angular offset of the two retarders to 
  +/- 0.2 degrees, in order to achieve a polarization leakage of < 0.05
  across the 210-270 GHz frequency band.  Tapered transitions are used
  to reduce reflections from the faceted waveguide sections; the
  predicted return loss is < -20 dB.

  The CARMA receivers use Mylar beamsplitters to couple local
  oscillator power into the signal beam outside the dewar.  The
  beamsplitters have unequal transmission coefficients for signals
  polarized parallel and perpendicular to the angle of incidence,
  which will slightly increase the polarization leakage.