Reflector design

1. Reflector antennas and their feeds
P. Hazdra, M. Mazanek, .
hazdrap@fel.cvut.cz
Department of Electromagnetic Field
Czech Technical University in Prague, FEE
www.elmag.org
v. 23.4.2015

2. Outline
• Simple reflector antennas (dipole above ground,
corner reflector)
• Parabolic reflector antenna
• Efficiency, feed considerations
• Examples
Katedra elektromagnetického pole 2

3. Reflector antennas
Katedra elektromagnetického pole 3

4. Reflector antennas
4

5. Dipole above ground
5
Self impedance
Mutual impedance
Input driving impedance
Input current for constant power P
For → 0 is → 0
/8
20 log 2.13 2.15 8.7
/8
8.95 Ω
Above /2 dipole!

6. Corner reflector
6
40 60 80 100 120 140 160 180
7
8
9
10
11
12
13
14
Max. directivity, S=0.257 
 []
D
[dBi]
D [dBi]
90∘

7. Corner reflector
7
1Ω

8. Trihedral Corner reflector antenna
8
2.5
1.6
0.74

9. Parabolic reflector
9
• Most widely used large aperture antennas
• High‐gain pencil beam with low side lobes and good cross‐polarization discrimination
characteristics
• Microwave links
• Widely used for low‐noise applications such as in radioastronomy
• Dual‐reflectors (Cassegrain 60 80%) hyperboloid
Slight shaping  almost uniform amplitude and phase  gain enhancement
50 70%

10. Parabolic reflectors and their feeds
Katedra elektromagnetického pole 10

11. Parabolic reflector geometry
11
0.3 1
Parabolic reflector transforms spherical
waves radiating at its focus into plane waves
2 tan
1
4
2
/ [deg]
0,25 90
0,3 79,6
0,33 73,7
0,4 64
0,5 53,1
1 28,1

12. Aperture efficiency of parabolic reflector
12
• power that is radiated by the feed, intercepted, and collimated by the reflecting
surface
• uniformity of the feed pattern over the surface of the reflector (taper)
• phase uniformity of the field over the aperture plane
• shadowing the reflector by feed itself
• non‐ideal reflections leading to phase error
4
cos
Model of typical feed pattern
Peak illumination efficiency
(for n=1 to 4) is near 82%. For
single reflector in practice
75%, simple feeds (open
waveguide 60%, dipole 50%)
/
⋅

13. Spillover and amplitude efficiency
13

14. Aperture efficiency of parabolic reflector
14
Phase errors:
• Displacement of the phase center of the feed antenna off the focal point (reflector
is defocused)
• Deterministic deviations of the reflector from design shapes (manufacturing
tolerances + external forces – wind, temperature gradients..)
• Imperfect feed antenna phase center
• Random surface error effects /
685.5 / dB
Phase center displacement of 0.64

15. Aperture efficiency of parabolic reflector
15
Feeds:
• Ideal feed produces uniform amplitude and phase distribution which compensates
for spherical spreading loss and does not have spillover (cannot be realized in
practice)
• The feed pattern should be rotationally symmetric (balanced feed)
• The feed pattern should be such that the reflector edge illumination is about ‐11 dB
• The feed should have a point phase center and the phase center should be
positioned at the focal point of the reflector
• The feed should be small in order to reduce blockage (it is usually on the order of a
wavelength in diameter)
• The feed should have low cross‐polarization, usually below ‐30 dB
• The above characteristics should hold over the desired operational frequency band

16. Radiation pattern – directional antenna
16
4
Ω
≅
4 41253
half‐power beamwidths
beam solid angle
Example 1 ⋅ 1  41253, 46.15 dBi

17. Parabolic reflector with circular WG
17
Open circular waveguide is not as
so bad as feeder. If 0.96 ,
pattern is quite symmetric and
≅ 117∘
 dish with
59∘
( / 0.44 needed
2 tan
1
4

18. Parabolic reflector with circular WG
18
Real edge taper is higher because of “spherical sphreading losses” at the aperture edge:
20 log 0 6 dB
10dB
12.4dB
Spherical wave vs. parabolic reflector

19. Spherical spread loss
Katedra elektromagnetického pole 19
/ [deg]
0,25 90
0,3 79,6
0,33 73,7
0,4 64
0,5 53,1
1 28,1

20. Parabolic reflector with circular WG
20
15
Co‐pol Cross‐pol

21. Parabolic reflector with circular WG
21
15
32dBi
10 /
⋅ ⋅ 100 71.4%
30
26.8

22. Offset D=60cm parabolic dish 2.45 GHz
22

23. Parabolic reflector
23
No cross‐polarization: Symmetrical Huygens source
X‐Pol components cancel
in main E/H planes
y‐oriented
dipole feed

24. Dual-mode feed
24
TE11
TE11+TM11
Phasing
section
,
, ,
1
Aperture field
0.78

25. Dual-mode feed
25
• Low sidelobes
• Symmetry
• Low X‐pol

26. Noise temperature, system consideration
26
L.. Insertion loss
Antenna noise temperature
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
Angle  [deg]
G/T
[dB]
Dual-Mode Feed
Skobolev Feed
measured data
Optimization of the feeder for highest /

27. Reflector antenna with dual-mode feed
27
0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7
0.7
20
30
40
50
60
70
80
80
f/D [-]
Aperture
efficiency
[%]
0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7
27
28
29
30
31
32
33
f/D [-]
D
[dBi]
15 @ 1.3 GHz
0.65

28. Cassegrain antenna f=33GHz, Dpar=55λ, D=37.45 dBi
28
≅
41253 41253
2.1 ⋅ 2.2
39.5
Hyperbolic subreflector

29. Cassegrain antenna f=33GHz, Dpar=55λ, D=37.45 dBi
29
E‐plane
H‐plane

30. The loop feed
30
0.25 0.275 0.3 0.325 0.35 0.375 0.4 0.425 0.45 0.475 0.5
0
10
20
30
40
50
60
70
80
90
100
Parabolic Dish f/D
Parabolic
Dish
Efficiency
%
Dish Antenna Efficiency
23 cm
13 cm
Dish Diameter 1.5 m
OM6AA + HAZDRA
1296 / 2320 MHz
S11 and S22 parameters for free space better than 25
dB at both bands.
Isolation (S21) between loops for free space is 17.4 dB
@ 1296 MHz and 15.7 dB @ 2320 MHz respectively.
Impedance and isolation have also been measured for
an antenna assembly with a dual band loop feed located at
the focus of a 1.4 m dish antenna with an f/D ratio of 0.5.
Impedance match on the 23 cm band for this configuration
was measured 45 dB at 23 cm band and 25 dB at 13 cm
band. Only small changes in isolation between loops were
observed, 19 dB @ 1296 MHz and 15.8 dB @ 2320 MHz.

31. Prime-focus feed with backward radiation
• Linear/circular polarization capabilities
• Good axial ratio if CP used
• Low cross‐polarization losses
• ‐13dB dish edge taper for optimal G/T illumination (subtended angle 2x 85°)
• Suitable radiation pattern for minimalization of shadowing effects (blockage)
60cm dish f/D = 0.285, f = 10.368 GHz
Feed requirements
Ø60cm
?
Feed structure?

32. Prime-focus feed with backward radiation
32
teflon lens
circular waveguide
conical cap
reflecting metal plate
teflon transition
(impedance matching)

33. Prime-focus feed with backward radiation
33
278
.
36
log
20 


D
Dteor
Simulated directivity Ds = 33.87 dBi
Theoretical maximum directivity
(constant dish illumination)
57% efficiency
E‐plane
H‐plane

34. Circularly polarized prime focus feeds
with septum polarizer
• Circularly polarized feed with septum polarizer for 1.296GHz EME band (23cm)
• Septum optimalization by Mode Matching Technique (Mician Microwave
Wizard) and FIT (Microwave Studio)
z
rc
lc
Hazdra, P. - Galuščák, R. - Mazánek, M.: Optimalization of the Septum Polarizer Feed for 1.296 GHz EME. In Proceedings of The European Conference on
Antennas and Propagation: EuCAP 2006 [CD-ROM]. Noordwijk: ESA Publications Division, 2006, ISBN 92-9092-937-5.

35. Circularly polarized prime focus feeds
with septum polarizer
-150 -100 -50 0 50 100 150
-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
 []
AR
[dB]
=0
=90
LHC pattern, LHC port excited RHC pattern, LHC port excited
Axial Ratio
1 1.05 1.1 1.15 1.2 1.25 1.3 1.35 1.4 1.45 1.5 1.55
-40
-35
-30
-25
-20
-15
-10
-5
0
f [GHz]
S
xy
[dB]
S11 sim.
S21 sim.
S21 meas.
S11 meas.

36. Circularly polarized prime focus feeds
with septum polarizer and choke
• Circularly polarized feed with septum polarizer for 1.296GHz EME band (23cm)
• Using chokes to improve system (feed+dish) efficiency

37. Project “BIG-DISH”
• A proposal for using the KDDI 32‐meter Cassegrain reflecting dish antenna for amateur radio EME (Earth‐Moon‐
Earth microwave communication utilizing the Moon as a passive reflector) was initiated in 2006 when a group of
Japanese amateur radio enthusiasts met for their special meeting at KDDI‐Ibaraki Satellite Communication
Center in Takahagi City, Japan.
• 2m, 70cm and 23cm bands covered with one antenna
• drilling, milling, edging, etc. not allowed
• not allowed to remove or move the hyperbolic subreflector
• High gain ‐ good efficiency
• Vertical polarization for 2m and 70cm bands
• RHC & LHC polarization for 23cm band
• Prompt band‐switching without requiring tuning
• Minimum possible reciprocal influence between feeds
32m
http:// 8N1EME.jp
Requirements and constraints:

38. Project “BIG-DISH”
• 23cm feed utilizes septum polarizer prime focus feed and 2.3m diameter dish

39. Project “BIG-DISH”
• 23cm feed utilizes septum polarizer prime focus feed and 2.3m diameter dish
• FEKO simulation shows 50dBi directivity with ~ 53% system efficiency

40. Project “BIG-DISH”
• 2m and 70cm 1λ loops
Loops placed under the
hyperbolic subreflector

41. Project “BIG-DISH”
The 23cm band presents system with unique triple reflector configuration!

42. Project “BIG-DISH”
• 23cm directivity ~ 50dBi, 53% system efficiency
• 70cm directivity ~ 34dBi, 13% system efficiency
• 2m directivity ~ 29dBi, 31% system efficiency
• 8N1EME: 154 stations on the 2m band, 67 on 70cm and 71 stations on 23cm
Project BIG‐DISH summary:
Due to mechanical limitations it was
not possible to place loops exactly
into the parabola’s focus

43. Literature
• C. A. Balanis, Antenna Theory and Design, Wiley, 2005
• W. L. Stutzman, G. A. Thiele, Antenna Theory and Design, Wiley 2012
Katedra elektromagnetického pole 43

44. Far field solution – BOR type antennas
44
Assume field radiated by feed to be
, cos sin
, ,
1 cos
2
cos sin
, ,
1 cos
2
cos sin
Remember radiation integrals
Radiation from x‐oriented current