Transcript Septum Feeds and Polarizers

Septum Feeds
and Polarizers
Paul Wade W1GHZ
[email protected]
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Septum feeds
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Zdenek, OK1DFC
used a septum polarizer to add
circular polarization
 Good results on EME
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Analyzed the feed performance
 Compare to other feeds
 Some variations
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FROM:
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OK1DFC Septum Feed
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Built by DL4MUP
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Built by DL4MUP
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G4DDK
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Circular Polarization
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Polarization rotates
 At
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Wave travels through space
 At
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frequency
speed of light
Wavefront like corkscrew
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Generating Circular Polarization
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Helical antenna is corkscrew
 Natural
circular polarization
 NOT reversible
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Combine horizontal and
vertical linear polarizations in
proper phase
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Circular Polarizers
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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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Inside
Septum
polarizer
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Imperfect Circular Polarization
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Amplitude imbalance or imperfect
phasing => elliptical polarization
Really circular plus some linear
Circularity or ellipticity is measure
of polarization quality
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Circularly Polarized
Dish Feeds
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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
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Reflector reverses polarization sense
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Septum feed
Open waveguide feed with septum
polarizer
 Septum polarizer works well with
many feeds
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OK1DFC Septum Feed
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Square guide with linear polarization
Square
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Diagonal
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Septum and diagonal horns
Identical with circular polarization!
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OK1DFC feed – circular polarization
2dB/color
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OK1DFC feed – circular polarization
LHCP
All
polarizations
45 degree
planes –
larger
sidelobes
LHCP
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OK1DFC feed performance
@f/D = 0.37
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Dish efficiency calculation
Including Crosspolarization
Circular
Polarization
only
(XPOL)
 Power
 Power
RCHP

reflector
 PowerRHCP
RCHP

 Power
AllPolarizations
Sphere
Sphere
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reflector
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Stepped and sloping septums
Non-linear taper
Linear taper
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Stepped and sloping septums
No difference
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Stepped vs. sloping – port isolation
Stepped septum
good over wider
bandwidth
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Flare horn for larger f/D
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Flare horn for larger f/D
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Septum horn with 1.1l
flare
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Septum horn with 1.4l flare
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Octagonal flare removes corners
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Octagonal flare
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Septum - 1.1l octagonal flare
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Septum - 1.4l octagonal flare
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Add choke to improve low f/D feed
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Round and square chokes
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Septum feed with round choke
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Septum feed with square choke
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VE4MA feed with septum
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VE4MA feed with septum
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VE4MA feed with septum polarizer
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Coffee-can feed with septum
Stepped septum
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Sloping septum
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Coffee-can feed with stepped septum
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Coffee-can feed with sloping septum
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W2IMU dual-mode feed with septum
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W2IMU dual-mode feed with septum
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W2IMU dual-mode feed with septum
NO
XPOL
LOSS
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Septum polarizer excitation
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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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Septum Polarizer Summary
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Works – simulation and on the air
Good circular polarization with no tuning
Works with variety of feed horns
Square waveguide
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Round waveguide
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stepped or sloping septum
stepped septum better
Dimensions fairly critical – not broadband
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For more information
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www.ok1dfc.com
www.w1ghz.org
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