Reflector antennas and their feeds

P. Hazdra, M. Mazanek,….hazdrap@fel.cvut.czDepartment 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