Reflector antennas and their feeds P. Hazdra, M. Mazanek,…. [email protected] Department of Electromagnetic Field Czech Technical University in Prague, FEE www.elmag.org v. 23.4.2015
Reflector antennas and their feeds
P. Hazdra, M. Mazanek,…[email protected] of Electromagnetic FieldCzech Technical University in Prague, FEEwww.elmag.org
v. 23.4.2015
Outline
• Simple reflector antennas (dipole above ground, corner reflector)
• Parabolic reflector antenna• Efficiency, feed considerations• Examples
Katedra elektromagnetického pole 2
Reflector antennas
Katedra elektromagnetického pole 3
Reflector antennas
4
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
8.95Ω
Above /2 dipole!
Corner reflector
6
40 60 80 100 120 140 160 1807
8
9
10
11
12
13
14Max. directivity, S=0.257
[]
D [d
Bi]
D [dBi]
90∘
Corner reflector
7
1Ω
Trihedral Corner reflector antenna
8
2.5
1.6
0.74
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%
Parabolic reflectors and their feeds
Katedra elektromagnetického pole 10
Parabolic reflector geometry
11
0.3 1 Parabolic reflector transforms sphericalwaves radiating at its focus into plane waves
2 tan1
42
/ [deg]
0,25 90
0,3 79,6
0,33 73,7
0,4 64
0,5 53,1
1 28,1
Aperture efficiency of parabolic reflector
12
• … power that is radiated by the feed, intercepted, and collimated by the reflectingsurface
• … 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%)
/⋅
Spillover and amplitude efficiency
13
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
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
Radiation pattern – directional antenna
16
4Ω ≅
4 41253
half‐power beamwidths
beam solid angle
Example 1 ⋅ 1 41253, 46.15 dBi
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 tan1
4
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
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
Parabolic reflector with circular WG
20
15
Co‐pol Cross‐pol
Parabolic reflector with circular WG
21
15
32dBi
10 /⋅ ⋅ 100 71.4%
3026.8
Offset D=60cm parabolic dish 2.45 GHz
22
Parabolic reflector
23
No cross‐polarization: Symmetrical Huygens source
X‐Pol components cancel in main E/H planes
y‐oriented dipole feed
Dual-mode feed
24
TE11TE11+TM11Phasing section
,, ,
1
Aperture field
0.78
Dual-mode feed
25
• Low sidelobes• Symmetry• Low X‐pol
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 85 908
9
10
11
12
13
14
15
16
17
18
19
20
21
22
Angle [deg]
G/T
[dB
]
Dual-Mode FeedSkobolev Feed
measured data
Optimization of the feeder for highest /
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 0.70.720
30
40
50
60
70
8080
f/D [-]
Ape
rtur
e ef
ficie
ncy
[%]
0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.727
28
29
30
31
32
33
f/D [-]
D [d
Bi]
15 @ 1.3 GHz
0.65
Cassegrain antenna f=33GHz, Dpar=55λ, D=37.45 dBi
28
≅41253 41253
2.1 ⋅ 2.2 39.5
Hyperbolic subreflector
Cassegrain antenna f=33GHz, Dpar=55λ, D=37.45 dBi
29
E‐plane
H‐plane
The loop feed
30
0.25 0.275 0.3 0.325 0.35 0.375 0.4 0.425 0.45 0.475 0.50
10
20
30
40
50
60
70
80
90
100
Parabolic Dish f/D
Para
bolic
Dish
Effi
cien
cy %
Dish Antenna Efficiency
23 cm13 cm
Dish Diameter 1.5 m
OM6AA + HAZDRA
1296/2320MHz
S11 and S22 parameters for free space better than 25dB 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 foran antenna assembly with a dual band loop feed located atthe 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 configurationwas measured 45 dB at 23 cm band and 25 dB at 13 cmband. Only small changes in isolation between loops wereobserved, 19 dB @ 1296 MHz and 15.8 dB @ 2320 MHz.
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?
Prime-focus feed with backward radiation
32
teflon lens
circular waveguide
conical cap
reflecting metal plateteflon transition
(impedance matching)
Prime-focus feed with backward radiation
33
278.36log20 DDteor
Simulated directivity Ds = 33.87 dBi
Theoretical maximum directivity (constant dish illumination)
57% efficiency
E‐plane
H‐plane
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.
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]
Sxy
[dB
]
S11 sim.S21 sim.S21 meas.S11 meas.
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
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.jpRequirements and constraints:
Project “BIG-DISH”• 23cm feed utilizes septum polarizer prime focus feed and 2.3m diameter dish
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
Project “BIG-DISH”• 2m and 70cm 1λ loops
Loops placed under the hyperbolic subreflector
Project “BIG-DISH”The 23cm band presents system with unique triple reflector configuration!
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
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
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 integralsRadiation from x‐oriented current