Title: Septum Feeds and Polarizers
1Septum Feeds and Polarizers
- Paul Wade W1GHZ
- w1ghz_at_arrl.net
2Septum feeds
- Zdenek, OK1DFC
- used a septum polarizer to add circular
polarization - Good results on EME
- W1GHZ
- Analyzed the feed performance
- Compare to other feeds
- Some variations
3FROM
4OK1DFC Septum Feed
5Built by DL4MUP
6Built by DL4MUP
7G4DDK
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9Circular Polarization
- Polarization rotates
- At frequency
- Wave travels through space
- At speed of light
- Wavefront like corkscrew
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11Generating Circular Polarization
- Helical antenna is corkscrew
- Natural circular polarization
- NOT reversible
- Combine horizontal and vertical linear
polarizations in proper phase
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16Circular 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
17Inside Septumpolarizer
18Imperfect Circular Polarization
- Amplitude imbalance or imperfect phasing gt
elliptical polarization - Really circular plus some linear
- Circularity or ellipticity is measure of
polarization quality
19Circularly 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 doesnt matter
- Spillover reduces dish efficiency
- Reflector reverses polarization sense
20Septum feed
- Open waveguide feed with septum polarizer
- Septum polarizer works well with many feeds
21OK1DFC Septum Feed
22Square guide with linear polarization
Square
Diagonal
23Septum and diagonal horns
Identical with circular polarization!
24OK1DFC feed circular polarization
2dB/color
25OK1DFC feed circular polarization
LHCP
45 degree planes larger sidelobes
All polarizations
LHCP
26OK1DFC feed performance
_at_f/D 0.37
27Dish efficiency calculation
Including Cross-polarization (XPOL)
Circular Polarization only
28Stepped and sloping septums
Non-linear taper
Linear taper
29Stepped and sloping septums
No difference
30Stepped vs. sloping port isolation
Stepped septum good over wider bandwidth
31Flare horn for larger f/D
32Flare horn for larger f/D
33Septum horn with 1.1l flare
34Septum horn with 1.4l flare
35Octagonal flare removes corners
36Octagonal flare
37Septum - 1.1l octagonal flare
38Septum - 1.4l octagonal flare
39Add choke to improve low f/D feed
40Round and square chokes
41Septum feed with round choke
42Septum feed with square choke
43VE4MA feed with septum
44VE4MA feed with septum
45VE4MA feed with septum polarizer
46Coffee-can feed with septum
Stepped septum
Sloping septum
47Coffee-can feed with stepped septum
48Coffee-can feed with sloping septum
49W2IMU dual-mode feed with septum
50W2IMU dual-mode feed with septum
51W2IMU dual-mode feed with septum
NO XPOL LOSS
52Septum 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
53Septum 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
54For more information
- www.ok1dfc.com
- www.w1ghz.org