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Use of a Septum Feed Horn for Circular Polarization with Parabolic

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					Use of a Septum Feed Horn
for Circular Polarization
with Parabolic Dishes

by Edward R. Cole – KL7UW
                       Overview
• Credits:                      •   Feed horn design for parabolic
  This paper provides a brief       dishes has long been the focus
  compendium of papers              for many microwave engineers.
  addressing the application    •   Add the desire to incorporate
  of the Septum Polarizer to        circular polarization and design
  Amateur Radio antennas.           can get complicated.
                                •   Amateur Radio Astronomers
  Special Thanks to Zdenek          have few feeds to chose from.
  Samek and Paul Wade for       •   For Linear Polarization:
  permission to re-use certain      - dipole with reflector
  figures and tables from their     - open waveguide
  papers.
                                    - pyramidal horn
                                    - waveguide with coax probe
                      Overview
• For Circular Polarization:     • Many beginning radio
  - axial-mode helix (helical      astronomers have used the
  beam)                            helical beam, but it is limited to
  - crossed dipoles                one sense of circular
                                   polarization.
  - orthogonal probes in
  waveguide using a hybrid       • The helix does not produce the
                                   nicest illumination of the dish,
• There are slow wave methods:     with resulting edge spillover
  - dielectric slabs               and high side lobe energy.
  - posts at 45-degrees inside   • This degrades system noise
  waveguide                        temperature.
  - asymmetric waveguide         • The use of dishes by Amateur
  (squeeze tube)                   Radio for EME (Moonbounce)
  - septum                         shares many of these
                                   problems.
Overview
    • One of the most popular EME
      feed horns has been the
      “VE4MA” Feed.
    • It is a section of circular
      waveguide with orthogonal
      coax probes.
    • The Quadrature Hybrid that is
      required with this feed
      introduces loss ahead of the
      LNA which degrades noise
      temperature.
    • Zdenek Samek – OK1DFC
      introduced the Septum feed at
      the 2002 Int’l EME Conference
      in Prague.
    What is a Septum Feed Horn
• Four functions of feed horns:
  - Antenna Section: shapes beam for good illumination.
  - Waveguide Section
  - Polarizer
  - Excitation Section: waveguide or coaxial probe.
• Chen and Tsandoulas wrote of their exploration of the
  septum polarizer (IEEE, 1973). Their’s was an empirical
  study to find a design solution.
• Their design utilized a square waveguide with metal
  divider (or septum) separating this into two rectangular
  waveguide sections.
Analysis
                      Analysis
• The septum divides the excitation region into two
  rectangular waveguides where linear polarization exists.
• The size of the waveguide is chosen to be near the
  cutoff frequency so that as a signal enters the horn only
  one polarization component is passed into the
  rectangular sections.
• Thus one side receives RHCP and the other LHCP.
• Wave cancellation in the opposite sides offer
  considerable isolation of the coaxial ports.
• Detailed description can be found in the referenced
  papers listed in the footnotes.
Analysis
    • A sloped septum was found to
      be usable over only a 10%
      band width (though isolation
      was near 40 dB). Trial and
      error led to a stepped design
      with 25% band width, 24 dB
      isolation, and 21 dB of
      circularity.

    • Zdenek scaled Chen &
      Tsandoulas’ five step septum
      to 1296 MHz and later to other
      microwave frequencies.
Practical Designs
       • I used Zdenek’s online calculator to
         produce dimensions for 1420 MHz
         as shown in Appendix-A

       • Paul Wade, W1GHZ, has made a
         thorough analysis of the septum
         feed using Ansoft ANSS software.
       • He found that the square septum
         design has large rear lobes in the
         antenna pattern. They are only ten
         dB down from the main beam
         resulting in degrading antenna
         efficiency (60%) with higher noise
         temperature.
Practical Designs
         • Since side lobes are generated
           in the opening of the feed horn
           the addition of a flared horn
           improves side lobes with
           dishes of 0.5 to 0.7 F/D ratio.
         • Efficiencies of 65 to 70% are
           achieved.

         • A round choke ring is most
           effective reducing side lobes
           with dishes of 0.3 to 0.45 F/D
           ratio.
         • Efficiencies of 62% are
           achieved.
Super – VE4MA Feed Horn
          • This year Paul investigated use of
            the stepped septum in circular fed
            horns.
          • In the process he found increased
            efficiencies using deeper choke
            rings with a 0.77 wavelength
            diameter horn.
          • Increasing ring dimensions to 0.4
            by 0.6 wavelength, analysis
            showed theoretical efficiencies to
            80% with low side lobes.
          • Measurements of this horn by
            Tommy Henderson, WD5AGO,
            measured actual 65% efficiency!
KL7UW Septum Feed
       • Here is the Square Septum Feed
         that I built in fall 2005 and mounted
         on my 2.4 meter dish.
       • Operation over 1296 to 1420 MHz
         with good return-loss and isolation.
       • Sun noise Measurements
         produced 4-dB comparison with
         cold sky.
       • Cold Sky vs. Ground noise with the
         feed horn measured 4.5 dB without
         a choke ring and 7 dB with a ring.
       • Wide band noise measurements
         this spring were not successful due
         to microwave front-end problems.
KL7UW Septum Feed
    Radio Astronomy Application
• The Septum Feed Horn provides a much simpler and efficient way
  to make multi-polarity radio astronomy observations.
• Both RHCP and LHCP may simultaneously be observed using
  matched LNA’s on both ports of the septum feed.
• Addition of two-way signal splitters and a quadrature hybrid,
  orthogonal linear polarity can be recovered.
• Improved System Noise Temperature.
• Dual frequency observations over 25% separation.
• Normal radio telescope antenna efficiency is 40-45% in the effort to
  minimize side lobes below 20 dB.
• The cylindrical stepped-septum feed provides 65% efficiencies.
  This is equivalent to a 20% increase in effective antenna aperture!
Hamming it up on Microwave!
Appendix-A
         Appendix-B
OK1DFC Septum Feed Horn Performance
            Appendix-C
OK1DFC Septum with Round Choke Performance
Appendix-D
VE4MA Feed Horn

				
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posted:11/23/2011
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