Orthomode Junction
Understanding Orthomode Junctions
Orthomode junctions enable dual-polarization operation, which doubles the capacity of a communication link by carrying independent signals on orthogonal polarizations through the same antenna and waveguide.
OMJ Types
- Turnstile junction: Four symmetrically placed probes in a circular waveguide. Wideband.
- Boifot junction: Asymmetric branching with compensating elements. Very wideband (40%+ bandwidth).
- Septum: A stepped septum plate in the waveguide separates polarizations. Compact.
Specifications
- Isolation: > 30-40 dB between the two polarization ports.
- Return loss: > 20 dB at each port.
- Insertion loss: < 0.2 dB per polarization path.
Frequently Asked Questions
What is an orthomode junction?
An OMJ separates or combines two orthogonally polarized signals in the same waveguide. It enables dual-polarization operation, doubling capacity by carrying independent signals on H and V (or RHCP and LHCP) polarizations.
How much isolation does an OMJ provide?
Good OMJs provide 30-40 dB isolation between polarization ports. This limits cross-pol interference to -30 to -40 dBc. Higher isolation is needed for higher-order modulation schemes in dual-pol systems.
Where are OMJs used?
Satellite earth station feeds (dual-pol operation), radio astronomy (simultaneous H/V observation), radar (dual-pol weather radar), and military communications (polarization diversity).