Use of a Septum Feed Horn
for Circular Polarization
with Parabolic Dishes
by Edward R. Cole – KL7UW
• 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
- pyramidal horn
- waveguide with coax probe
• 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
- 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
- asymmetric waveguide • The use of dishes by Amateur
(squeeze tube) Radio for EME (Moonbounce)
- septum shares many of these
• One of the most popular EME
feed horns has been the
• It is a section of circular
waveguide with orthogonal
• The Quadrature Hybrid that is
required with this feed
introduces loss ahead of the
LNA which degrades noise
• Zdenek Samek – OK1DFC
introduced the Septum feed at
the 2002 Int’l EME Conference
What is a Septum Feed Horn
• Four functions of feed horns:
- Antenna Section: shapes beam for good illumination.
- Waveguide Section
- 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
• 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
• 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.
• 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
• Zdenek scaled Chen &
Tsandoulas’ five step septum
to 1296 MHz and later to other
• 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
• 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
• A round choke ring is most
effective reducing side lobes
with dishes of 0.3 to 0.45 F/D
• Efficiencies of 62% are
Super – VE4MA Feed Horn
• This year Paul investigated use of
the stepped septum in circular fed
• In the process he found increased
efficiencies using deeper choke
rings with a 0.77 wavelength
• 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 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!
OK1DFC Septum Feed Horn Performance
OK1DFC Septum with Round Choke Performance
VE4MA Feed Horn