EITN90 Radar and Remote Sensing Lecture 8: Radar antennas

EITN90 Radar and Remote SensingLecture 8: Radar antennas

Daniel Sjoberg

Department of Electrical and Information Technology

Spring 2018

Outline

1 Basic antenna concepts

2 Effect of the antenna on radar performance

3 Reflector antennas

4 Phased array antennas

5 Array architectures

6 Conclusions

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Learning outcomes of this lecture

In this lecture we willI Review basic antenna concepts, particularly gainI How the antenna affects the radar applicationI Study the two main high-gain antenna solutions:

I Reflector antennasI Phased array antennas

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Outline

1 Basic antenna concepts

2 Effect of the antenna on radar performance

3 Reflector antennas

4 Phased array antennas

5 Array architectures

6 Conclusions

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The isotropic antenna

Radiation intensity: I =Pt

4πW/steradians

Power density: Qt =Pt

4πR2W/m2

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Angular selectivity of an array

The received complex amplitudes may sum up constructively(normal incidence θ = 0, solid arrows), or destructively (obliqueincidence θ > 0, dashed arrows). The result is angular selectivity.

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Radiation pattern

Beam width: θ3 =αλ

L, α ≈ 1

Note that many results in the book are valid for large antennas,but not necessarily for small (where antenna dimension L λ). 7 / 52

Directivity, gain, and efficiency

The directivity and gain of an antenna are both measures ofradiation intensity I, but are differently normalized:

D(θ, φ)def=

I(θ, φ)

Prad/(4π), Prad = power radiated from antenna

G(θ, φ)def=

I(θ, φ)

Pacc/(4π), Pacc = power accepted by the antenna

The powers are related by Prad = Pacc − Ploss, where Ploss is thepower lost in the antenna, for instance resistive losses. Thus,G ≤ D, and the antenna radiation efficiency is η = G/D.

Dmax =ηa4πA

λ2≈ 4π(0.88)2

θ3φ3=

32 000

θ3[degrees]φ3[degrees]

The aperture efficiency is ηa.

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IEEE standard for definition of terms for antennas

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Sidelobes

Radiation in sidelobes is usually undesired. It is characterized bythe maximum side lobe level relative the main beam (dB) orrelative an isotropic antenna (dBi).

Dmax = 43 dBi, SLL = 36 dB, or SLL = 7 dBi.

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Average sidelobe power

Another characterization of sidelobe radiation is the average ratioof sidelobe power to that of an isotropic antenna with the sameinput power:

SLave =

PSLΩSL

Pt4π

=Pt − PMB

4π − ΩMB

4π

Pt=

1− PMBPt

1− ΩMB4π

≈ 1− PMB

Pt

I Pt = total radiated power

I PMB power radiated in main beam

I PSL = power in radiated in sidelobes

I ΩMB main beam solid angle (typically ΩMB 4π)

I ΩSL side lobe solid angle

The simplified version can be interpreted as power conservation.

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Aperture tapers

The sidelobe structure can be controlled by the spatial distributionof the electric field across the aperture, either by shaping areflector or controlling the elements of an array.

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Example: Taylor tapering

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Example: Taylor tapering

For lower side lobe levels, the beamwidth is increased.

Very low SLL:s, below −40 dB, are very difficult to realize, due tofinite tolerances in antenna components, reflector shape, thermaleffects, antenna alignment etc.

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Phase errors

De

D0= exp(−δ2

rms)δrms = 5 ⇔ 5

360 = 172 fraction of λ, requiring

control on scale 0.42 mm at 10 GHz.15 / 52

Outline

1 Basic antenna concepts

2 Effect of the antenna on radar performance

3 Reflector antennas

4 Phased array antennas

5 Array architectures

6 Conclusions

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Applications

The antenna has a very direct effect on the radar system.I The radar can only see within the antenna’s field of view

(FOV).I The maximum range is limited by the gain of the antenna:

Rmax =

[PtG

2λ2σ

(4π)3LsPmin

]1/4

I The spatial resolution ∆R = c/(2B) depends on bandwidthB, which is often restricted by the antenna function.

I Different combinations of average transmit power andeffective area may be important, PaveA

2e and PaveAe:

TrackPaveA

2e

λ2=

SNR · 4πkT0FLsR4 · PRF

σ

Search PavgAe =SNR · 4πkT0FLsR

4

σ

(Ω

Tfs

)17 / 52

Constant track/search performance

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Monopulse

A target’s angular location can be accurately determined in onepulse (no sweep) by using two closely spaced beams in the antenna.

verror(θ) =|∆(θ)||Σ(θ)|

cosβ β = arg(∆/Σ)

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Monopulse

∆θ =θ3

km

√2 · SNR

√1 +

(kmθ

θ3

)2

km = slope of curve

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Monopulse

Transmit with Σ pattern, receive in both Σ and ∆. Can add thesame functionality in elevation at the cost of more antenna ports.

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Outline

1 Basic antenna concepts

2 Effect of the antenna on radar performance

3 Reflector antennas

4 Phased array antennas

5 Array architectures

6 Conclusions

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Reflector antennas

Often rotational symmetric. Note the subreflector (or antennafeed) needs to be supported in the aperture.

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Parabolic reflector

The reflector is often characterized by the focal length to diameterratio, f/D.

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Feed

The higher f/D, the more directive feed horn is necessary.

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Subreflector (Cassegrain configuration)

The size requirements can be relaxed by using a subreflector.

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Offset feed

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Outline

1 Basic antenna concepts

2 Effect of the antenna on radar performance

3 Reflector antennas

4 Phased array antennas

5 Array architectures

6 Conclusions

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Phased array

With a phased array, the antenna beam can be scannedelectronically. Much faster than mechanical steering, but morecomplex electronic implementation.

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Array factor

AF(θ) =1

N

N∑n=1

exp

[−j

(2π

λn∆x sin θ − φn

)]30 / 52

Uniform phase shift: φn =2πλ n∆x sin θs

AF(θ) =1

N

N∑n=1

exp

[−j

(2π

λn∆x(sin θ − sin θs)

)]31 / 52

Phase shifters

The phase shift in each antenna element can be controlled by aphase shifter.

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Phase shifters, quantization error

More bits give higher resolution and less gain loss, but also higherinsertion loss (there are losses in each stage of the phase shifter).Phase shifters are typically narrow-band.

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Grating lobes

From the array factor

AF(θ) =1

N

N∑n=1

exp

[−j

(2π

λn∆x(sin θ − sin θs)

)]it is seen that it is maximized when

∆x

λ(sin θ − sin θs) = 0,±1,±2, . . .

The zero is the intended main beam θ = θs, but with large enoughspacing ∆x other angles θ can correspond to the non-zerointegers. These are called grating lobes.

If scanning is restricted to the region θ ∈ [−θs, θs], grating lobesare absent if

∆x ≤ λ

1 + | sin θs|For ∆x < λ/2, there are no grating lobes regardless of θs.

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Grating lobes, example for ∆x = λ

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Grating lobes

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Gain loss

When scanning electrically, the physical aperture has a fixedorientation. The width of the main beam (directivity) isdetermined by the projected aperture. Typically scanning with aplanar array would be constrained to ±60 or less.

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Array element pattern

E(θ) = Ee(θ) AF(θ) =Ee(θ)

N

N∑n=1

exp

[−j

2π

λn∆x(sin θ − sin θs)

]38 / 52

Array element influence on beam steering

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Typical effect when scanning

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Wideband phased arrays

The narrow bandwidth of phase shifters may cause the beam topoint in different angles depending on frequency. The tolerablefractional instantaneous bandwidth is Bi = θ3

2 sin θs. Problem for

wideband radars with narrow beamwidth.41 / 52

Wideband phased arrays

Instead of phase shifting, the time delay unit (TDU) implements afixed time delay in each chain. However, they are still bulky andcostly. 42 / 52

Wideband phased arrays: subarrays

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Outline

1 Basic antenna concepts

2 Effect of the antenna on radar performance

3 Reflector antennas

4 Phased array antennas

5 Array architectures

6 Conclusions

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Passive array architecture

High power handling in all array components.

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Active array architecture

Individual T/R units behind each antenna element, phase andamplitude can be controlled. Costly, but extremely flexible.

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Trade-off power-aperture

Power requirements in each element can be traded with aperturesize: low power requires larger area and more elements, but eachelement may be cheap and there is less need for cooling.

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Subarray architecture

Breaking down the array into subarrays may simplifymanufacturing and maintainance. Digitizing the signal at subarraylevel allows many simultaneous beams (achieved by processing).

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Simultaneous beam operation

Since the centers of the subarrays are far apart, grating lobesappear when scanning the subarray AF, but are suppressed by thefixed subarray pattern.

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Overlapped subarrays

The subarray distance can be reduced by overlapping subarrays, atthe price of a more complicated feeding network (since eachelement will be connected to more than one subarray).

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Outline

1 Basic antenna concepts

2 Effect of the antenna on radar performance

3 Reflector antennas

4 Phased array antennas

5 Array architectures

6 Conclusions

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Conclusions

I Basic antenna concepts: directive radiation pattern, sidelobes, aperture tapering.

I Antenna effects on radar function: FOV, gain and range,power-aperture tradeoff.

I Improved angular localization with sum and differencepatterns.

I Reflector antennas provide high gain in differentconfigurations, but little scan possibility.

I Phased arrays are extremely flexible, but also costly. Variouslevels of analog/digital solutions.

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