1. Septum Feeds and Polarizers
Paul Wade W1GHZ
11/24/2018
W1GHZ

2. Septum feeds Zdenek, OK1DFC W1GHZ
used a septum polarizer to add circular polarization
Good results on EME
W1GHZ
Analyzed the feed performance
Compare to other feeds
Some variations
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W1GHZ

3. FROM:
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4. OK1DFC Septum Feed
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W1GHZ

5. Built by DL4MUP
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6. Built by DL4MUP
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7. G4DDK
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8. 11/24/2018
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9. Circular Polarization
Polarization rotates
At frequency
Wave travels through space
At speed of light
Wavefront like corkscrew
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W1GHZ

10. 11/24/2018
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11. Generating Circular Polarization
Helical antenna is corkscrew
Natural circular polarization
NOT reversible
Combine horizontal and vertical linear polarizations in proper phase
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W1GHZ

12. 11/24/2018
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13. 11/24/2018
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14. 11/24/2018
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15. 11/24/2018
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16. Circular Polarizers Phased dipoles or probes
Slow-wave structures – delay one polarization 90 degrees
Dielectric slab at 45 degrees
Posts or screws at 45 degrees
Asymmetrical waveguide (squeezed pipe)
Septum
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W1GHZ

17. Inside Septum polarizer
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18. Imperfect Circular Polarization
Amplitude imbalance or imperfect phasing => elliptical polarization
Really circular plus some linear
Circularity or ellipticity is measure of polarization quality
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W1GHZ

19. Circularly Polarized Dish Feeds
Circular polarization must be good over whole reflector for good efficiency
Side and back lobes often have poor or reverse polarization
Spillover polarization doesn’t matter
Spillover reduces dish efficiency
Reflector reverses polarization sense
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W1GHZ

20. Septum feed Open waveguide feed with septum polarizer
Septum polarizer works well with many feeds
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W1GHZ

21. OK1DFC Septum Feed
11/24/2018
W1GHZ

22. Square guide with linear polarization
Diagonal
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23. Septum and diagonal horns
Identical with circular polarization!
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W1GHZ

24. OK1DFC feed – circular polarization
2dB/color
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W1GHZ

25. OK1DFC feed – circular polarization
LHCP
45 degree planes – larger sidelobes
All polarizations
LHCP
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26. OK1DFC feed performance
@f/D = 0.37
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W1GHZ

27. Dish efficiency calculation
Including Cross-polarization
(XPOL)
Circular Polarization only
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W1GHZ

28. Stepped and sloping septums
Non-linear taper
Linear taper
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W1GHZ

29. Stepped and sloping septums
No difference
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W1GHZ

30. Stepped vs. sloping – port isolation
Stepped septum good over wider bandwidth
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W1GHZ

31. Flare horn for larger f/D
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32. Flare horn for larger f/D
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33. Septum horn with 1.1l flare
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34. Septum horn with 1.4l flare
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35. Octagonal flare removes corners
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36. Octagonal flare
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37. Septum - 1.1l octagonal flare
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38. Septum - 1.4l octagonal flare
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39. Add choke to improve low f/D feed
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W1GHZ

40. Round and square chokes
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41. Septum feed with round choke
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42. Septum feed with square choke
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43. VE4MA feed with septum
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44. VE4MA feed with septum
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45. VE4MA feed with septum polarizer
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46. Coffee-can feed with septum
Stepped septum
Sloping septum
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47. Coffee-can feed with stepped septum
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48. Coffee-can feed with sloping septum
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49. W2IMU dual-mode feed with septum
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50. W2IMU dual-mode feed with septum
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51. W2IMU dual-mode feed with septumNO
XPOL
LOSS
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52. Septum polarizer excitation
OK1DFC shows both probe and waveguide excitation
Calculated performance is identical
OK1DFC uses thin probes at 1296 with tuning
Fatter probes (1/4” at 1296) better match
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W1GHZ

53. Septum Polarizer Summary
Works – simulation and on the air
Good circular polarization with no tuning
Works with variety of feed horns
Square waveguide
stepped or sloping septum
Round waveguide
stepped septum better
Dimensions fairly critical – not broadband
11/24/2018
W1GHZ

54. For more information
11/24/2018
W1GHZ