Septum Feeds and Polarizers - PowerPoint PPT Presentation

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Septum Feeds and Polarizers

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7/4/09. W1GHZ. 1. Septum Feeds and Polarizers. Paul Wade W1GHZ. w1ghz_at_arrl.net. 7/4/09. W1GHZ. 2. Septum feeds. Zdenek, OK1DFC ... – PowerPoint PPT presentation

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Title: Septum Feeds and Polarizers


1
Septum Feeds and Polarizers
  • Paul Wade W1GHZ
  • w1ghz_at_arrl.net

2
Septum 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

3
FROM
4
OK1DFC Septum Feed
5
Built by DL4MUP
6
Built by DL4MUP
7
G4DDK
8
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9
Circular Polarization
  • Polarization rotates
  • At frequency
  • Wave travels through space
  • At speed of light
  • Wavefront like corkscrew

10
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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

12
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13
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14
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15
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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

17
Inside Septumpolarizer
18
Imperfect Circular Polarization
  • Amplitude imbalance or imperfect phasing gt
    elliptical polarization
  • Really circular plus some linear
  • Circularity or ellipticity is measure of
    polarization quality

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 doesnt matter
  • Spillover reduces dish efficiency
  • Reflector reverses polarization sense

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

21
OK1DFC Septum Feed
22
Square guide with linear polarization
Square
Diagonal
23
Septum and diagonal horns
Identical with circular polarization!
24
OK1DFC feed circular polarization
2dB/color
25
OK1DFC feed circular polarization
LHCP
45 degree planes larger sidelobes
All polarizations
LHCP
26
OK1DFC feed performance
_at_f/D 0.37
27
Dish efficiency calculation
Including Cross-polarization (XPOL)
Circular Polarization only
   
28
Stepped and sloping septums
Non-linear taper
Linear taper
29
Stepped and sloping septums
No difference
30
Stepped vs. sloping port isolation
Stepped septum good over wider bandwidth
31
Flare horn for larger f/D
32
Flare horn for larger f/D
33
Septum horn with 1.1l flare
34
Septum horn with 1.4l flare
35
Octagonal flare removes corners
36
Octagonal flare
37
Septum - 1.1l octagonal flare
38
Septum - 1.4l octagonal flare
39
Add choke to improve low f/D feed
40
Round and square chokes
41
Septum feed with round choke
42
Septum feed with square choke
43
VE4MA feed with septum
44
VE4MA feed with septum
45
VE4MA feed with septum polarizer
46
Coffee-can feed with septum
Stepped septum
Sloping septum
47
Coffee-can feed with stepped septum
48
Coffee-can feed with sloping septum
49
W2IMU dual-mode feed with septum
50
W2IMU dual-mode feed with septum
51
W2IMU dual-mode feed with septum
NO XPOL LOSS
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

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

54
For more information
  • www.ok1dfc.com
  • www.w1ghz.org
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