ei2400 applied antenna theory lecture 8: reflector antennas · reflector antennas kth school of...

School of Electrical Engineering

EI2400 Applied Antenna Theory

Lecture 8: Reflector antennas

Reflector antennas

KTH School of Electrical Engineering • www.ee.kth.se

• Reflectors are widely used in communications, radarand radio astronomy.

• The largest reflector antenna in the world is theradio telescope in Arecibo which has a sphericalreflector with a diameter of 300m.

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Parts of a reflector antenna


• Reflector antennas can have many different forms.

• Normally they consist of one or more reflectors whichare designed to collimate an incident plane wave byreflection and transmission via each reflector to a focalpoint at a convenient location.

- The first reflector on reception is the largest and is calledthe main reflector.

- The next reflector is called the sub-reflector.

• The feed antenna is located in the focal point.

- It is normally a horn antenna or a dipole above a ground

plane.

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Sketch of a reflector antenna


• The parts of a reflector antenna:

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Main reflector

Feed antenna

Main reflector

Feed antenna

Sub-reflector

If the sub-reflector is:

-Convex: Cassegrain.

-Concave: Gregorian.

Radiation characteristics


• Reflector antennashave often a fixedmain beam direction.

• However, they canalso steered beams bymechanicaldisplacement of thefeed or by rotation andtilting of the wholeantenna.

5

R.Yang; W. Tang, Y.Hao, "Wideband Beam-Steerable Flat

Reflectors via Transformation Optics," IEEE Antennas and

Wireless Propagation Letters, vol.10, 2011

Radiation


• Normally, the radiation is aimedto be a plane wave.

• It is also possible to generatecontoured beams, either byhaving one feed and shaping thereflector, or by using a feedarray, or both.

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Physical Optics (PO)


• Formulation:

7

)()( sa zzjk

sra erEE

aa EzH ˆ1

P.-S. Kildal, “Foundations of Antennas: A Unified Approach for Line-Of-Sight And Multipath”, 2014.

Parabola


• Parabola’s properties:

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The paraboloidal reflector (I)


• Our parabola is defined as follows:

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zzrsˆ)(ˆ),(

ysinxcosˆ

FFz

4)(

2

2/0for D

r)(),( fsfs rr

zcosˆsinr ff

ff

f

FFr

cos1

2

2/cos)(

2

0 0for f

The paraboloidal reflector (II)


• How to specify the parabola?

- D and F.

- D and θ0.

- F and θ0.

• θ0 is fixed to the half

beamwidth of the feed.

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0tan2 FD

Cassegrain antenna


• The classical Cassegrainantenna consists of aparaboloidal main reflectorand a hyperboloidal sub-reflector.

• We need four independentparameters in order touniquely describe thegeometry of the Cassegrain:d, D, θ0 and Ψ0.

11

P.-S. Kildal, “Foundations of Antennas: A Unified

Approach for Line-Of-Sight And Multipath”, 2014.

Influence of the feed


• There is a compromise between directivity andefficiency (spillover).

• Blockage at the center.

12

P.-S. Kildal, “Foundations of

Antennas: A Unified Approach for

Line-Of-Sight And Multipath”, 2014.

eeeee illpolspap

• Spillover efficiency.

• Polarization efficiency.

• Illumination efficiency.

• Phase efficiency.

Efficiencies (I)


• Spillover efficiency (esp):

- This is the power within the subtended angle θ0 (i.e., thepower hitting the reflector) relative to the total powerradiated by the feed.

-This should be reduced as much as possible in order toimprove the directivity.

-The spillover efficiency is typically between -0.05 dBand -0.5 dB, depending on the illumination taper of theaperture and the quality of the feed pattern.

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Efficiencies (II)


• Polarization efficiency (epol):

- This is the power of the co-polar field relative to the totalpower, both within.

- In most reflector antennas the polarization efficiency is veryhigh, typically better than -0.1 dB.

• Illumination efficiency (eill):

- It measures the illumination taper of our feed.

- Uniform illumination is the ideal case.

- The illumination efficiency is in a practical antenna typicallybetween -0.4 dB and -1.5 dB, for illumination tapersvarying between 10 dB and 20 dB.

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Efficiencies (III)


• Phase efficiency (eϕ):

- The phase efficiency is the only sub-efficiency whichdepends on the location of the phase reference point of thefeed, i.e., the location of the feed relative to the focal pointof the reflector.

- This fact can be used to uniquely define a phase center forthe feed, corresponding to the feed location whichmaximizes the phase efficiency.

- When the feed is located with its phase center in the focalpoint of the reflector, the phase efficiency is normally veryhigh, typically better than -0.1 dB.

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Aperture blockage


• In rotationally symmetricantennas the feed or thesub-reflector and theirsupport struts will causeaperture blockage.

• The center blockage can beaccounted for in theefficiency calculations by acenter blockage efficiency,which is obtained byremoving the central regionfrom the aperture integral.

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P.-S. Kildal, “Foundations of Antennas: A Unified

Approach for Line-Of-Sight And Multipath”, 2014.

Directivity


• It is coming from the side of our aperture and it isaffected by the efficiencies:

- The effects of tolerances and the blockage caused by thefeed and its support legs have not been included in thisversion of the efficiency.

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22

0

Deeee

DeD illpolspap

Edge diffraction efficiency


• The field incident on the main reflectorof the Cassegrain is reflected by GOfrom the sub-reflector.

• GO is only valid when the reflectorsare large in terms of wavelengths.

- The finite diameter of the sub-reflectorwill therefore cause edge diffractionlosses.

• These diffractions will be moreimportant when the reflector (and sub-reflector) is small in terms ofwavelength.

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Tailoring the radiation pattern


• The reflector can be used to tailor the radiation patternof our antenna.

• Not only a directive pattern in one direction can beachieved.

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Reflect-arrays


• A planar version of a reflector.

- Advantages: Low cost.

- Disadvantages: Narrow band.

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ei2400 applied antenna theory lecture 8: reflector antennas · reflector antennas kth school of...

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