The septum feeds are impressive - the septum polarizer provides good circular polarization performance with no adjustments. The other common ways to achieve circular polarization use two orthogonal probes phased by 90°; the phasing is achieved with an external 3-dB hybrid or a phasing section in the guide, often a series of screws, to provide a slow-wave structure for one polarization. The screws require careful adjustment to achieve good circularity and good VSWR, while the external hybrid and cabling adds some losses. The septum polarizer offers the possibility of good, low-loss, circular polarization with no adjustments. The septum polarizer is applicable to a variety of feedhorns The simple square cross-section described by OK1DFC is ideal for low blockage on small deep dishes, while a choke may be added for better performance on larger dishes. A flare section to increase the aperture will better illuminate shallow and offset dishes. In square waveguide, the step and slope septums both appear to work, but only the step septum polarizer works well in cylindrical horns like the VE4MA feed and the W2IMU dual-mode feed. Best performance is found on feeds with low sidelobes, since sidelobes contain unwanted polarizations and increase cross-polarization loss.
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