  The Low Frequency Instrument (LFI) of the ESA Planck CMB mission is an array
of 22 ultra sensitive pseudocorrelation radiometers working at 30, 44, and 70
GHz. LFI has been calibrated and delivered for integration with the satellite
to the European Space Agency on November 2006. The aim of Planck is to measure
the anisotropy and polarization of the Cosmic Background Radiation with a
sensitivity and angular resolution never reached before over the full sky. LFI
is intrinsically sensitive to polarization thanks to the use of Ortho-Mode
Transducers (OMT) located between the feedhorns and the pseudo-correlation
radiometers. The OMTs are microwave passive components that divide the incoming
radiation into two linear orthogonal components. A set of 11 OMTs (2 at 30 GHz,
3 at 44 GHz, and 6 at 70 GHz) were produced and tested. This work describes the
design, development and performance of the eleven Flight Model OMTs of LFI. The
final design was reached after several years of development. At first, Elegant
Bread Board OMTs were produced to investigate the manufacturing technology and
design requirements. Then, a set of 3 Qualification Model (QM) OMTs were
designed, manufactured and tested in order to freeze the design and the
manufacturing technology for the flight units. Finally, the Flight Models were
produced and tested. It is shown that all the OMT units have been accepted for
flight and the electromagnetic performance is at least marginally compliant
with the requirements. Mechanically, the units passed all the thermoelastic
qualification tests after a reworking necessary after the QM campaign.

Instrumentation and Methods for Astrophysics