Overview of Antennas for UAVs

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Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

Overview of Antennas for UAVs

Prof. David Jenn833 Dyer Road, Room 437

Monterey, CA 93943(831) 656-2254

[email protected]://web.nps.navy.mil/~jenn

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Antenna Systems for UAVs Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

• Antennas are required for a wide variety of UAV systems • Antenna requirements depend on the specific platform and mission:

> Radar/Electronic Warfare> Communications> Data links> GPS/geolocation> Other sensors (biological, chemical, etc.)

• Ground station antennas not addressed here

UAV

GROUNDSTATION

OBSTRUCTIONS

RANGE

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UAV Antenna Issues Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

• For airborne applications:> Size, weight, power consumption> Power handling> Location on platform and required field of view (many systems compete for limited real estate)> Many systems operating over a wide frequency spectrum> Isolation and interference> Reliability and maintainability> Radomes (antenna enclosures or covers)

• Accommodate as many systems as possible to avoid operational restrictions• Signatures must be controlled: radar cross section (RCS), infrared (IR), acoustic, and visible (camouflage)• New architectures and technologies are being applied to UAVs

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Antenna Performance Measures Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

0

MAXIMUM SIDELOBE

LEVEL

PEAK GAIN

GA

IN (d

B)

θsPATTERN ANGLE

θ

SCAN ANGLE

HPBW3 dB

• Gain, rule of thumb: > A = area, λ = wavelength > e = efficiency (0 < e < 1)• Field of view or beamwidth > usually half power, HPBW, • Polarization• Sidelobe level

> maximum > average

• Antenna noise temperature,• Operating bandwidth

> instantaneous > tunable

• Radar cross section > in band > out of band

AT

Bθ

2/4 λπAeG =

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“New” Antenna Technologies for UAV Applications

Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

• Some “new” concepts have been around since the 1960s, but have only recently become practical due to advances in computers and micro devices• New technologies and architectures include: > Solid state (active antennas) > Adaptive > Conformal > Reconfigureable > Smart antennas > Multiple beams

(“smart skins” or “living skins”) > Photonics > Superconductivity > Digital beamforming

> Genetic algorithms > Fractal antennas > Wide band (shared apertures) > Frequency selective devices and surfaces > New and exotic materials

Note: Most of these terms are not precisely defined and they are not mutually exclusive. An antenna canfall into multiple categories.

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Antenna Installation Options Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

EXTERNALLY MOUNTED ANTENNA

CONFORMAL ANTENNA MOUNTEDON THE SURFACE

FREQUENCY SELECTIVE SURFACE

SHIELDED ANTENNA

AIRCRAFT SKIN

• The choice may limit operation of the system or degrade its performance• Externally mounted

> structural/environmental stress> if non-retractable, always in view> if retracted, system unusable

• Conformal surface mounted> aerodynamic (low profile)> curvature complicates design and manufacture

• Radome enclosures> controlled environment> inefficient use of volume> radome loss> wider field of view (FOV)> includes “pods”

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2~ τG

f

of HfLf

minG

Motivation for Wide Bandwidth Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

• Bandwidth is the range of frequencies over which the antenna has “acceptable” performance• Trend is toward wide band wave forms > low probability of intercept

> frequency hopping> multiple channels (i.e., orthogonal frequency division multiplexing)> high resolution and data rates

• Shared aperture (multi-mission) antenna: a single antenna used for all EM sensors (radar, EW, comms, etc.)

Bandwidth,

Center frequency, ( ) 2/LHo fff +=LH ffB −=

• Definitions (not standardized) > narrow band: < 2% > wide band: 2-10%

> ultra wide band: > 10%

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FREQ

UEN

CY

MU

LTIP

LEX

ERBROADBANDINPUT SIGNAL

1

2

N

∆f1

∆f2

∆fN

∆f1 ∆f2 ∆fNf

τ

BANDWIDTH OF AN INDIVIDUAL ANTENNA

TOTAL SYSTEMBANDWIDTH

Wide Bandwidth Approaches Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

• Single radiating structure that operates over the entire frequency band

• Collection of nested or integrated narrow band antennas

FEED POINT

WIRES

dmin

dmaxdmin

dmax

minmax 2dd >> λ

SPIRALBI-CONE

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Frequency Selective Surfaces (FSS) Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

f

FSS 1FSS 21

REF

LEC

TON

C

OEF

FIC

IEN

T

• Example of a FSS element (tripoles)

• Band-stop frequency characteristic

• Applications: > stealth -- shield antennas at high out of band frequencies> antennas -- reflector antennas; array ground planes (below)

FSS 1

FSS 2

≅ λ/4 AT HIGH FREQUENCIES

≅ λ/4 AT LOW FREQUENCIES

DIPOLE LENGTH AT HIGH FREQUENCIES

DIPOLE LENGTH AT LOW FREQUENCIES

DIPOLE ARM

HIGH FREQUENCYFEED POINTS

LOW FREQUENCY FEED POINT

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Multiple Beams Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

• Multiple beams share the same aperture (they exist simultaneously)• Cover large spatial volumes quickly• Receiver on each beam (increases the system bandwidth)• Beam coupling losses• Increased complexity

BLASS

BUTLER

LENS

REFLECTOR

-30 -20 -10 0 10 20 30-40

-35

-30

-25

-20

-15

-10

-5

0

PATTERN ANGLE, DEGREES

REL

ATIV

E PO

WER

, dB

25 dB TaylorN = 30

d = 0.4λ∆θs = 2.3

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APERTURE

LNA

RECEIVER SIGNAL PROCESSOR

TO DISPLAY

BEAM FORMER

LNA

TO DISPLAY

CONVENTIONAL (LNA PER BEAM)

ACTIVE (LNA PER RADIATING ELEMENT)

1

2

3

N M

N M

1

2

3

N

1

2

3

1

2

3

M BEAMSN RADIATING

ELEMENTS

Active vs. Passive Antenna Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

• Receive architecture

• Can be applied to transmit antennas using power amplifiers• Transmit and receive channels are packaged together to form T/R modules

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RADIATING ELEMENT

SYNCHRONOUS DETECTOR

I QTWO CHANNEL

ANALOG-TO- DIGITAL CONVERTER

SIGNAL PROCESSOR (COMPUTER)

I QI Q

OUTPUT, y( t)

1 2 3 N

s1(t ) s2 (t) sN (t)

I Q

• The complex signal (I and Q, or equivalently, amplitude and phase) are measured and fed to the computer • Element responses become array storage locations in the computer • The weights are added and the sums computed to find the array response • In principle any desired beam characteristic can be achieved, including multiple beams

∑==

N

nnn tswty

1)()(

Digital Beamforming (DBF) Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

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Digital Beamforming (DBF) Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

• Direct conversion to baseband is preferred, but high speed A/Ds are a problem• Receive channel: (down conversion using two mixing stages)

VIDEO AMPLPFA/D

LO 1

LO 2

LNA BPF IF AMP

ANTENNA ELEMENT

I Q

VIDEO AMPLPFA/D

Complex received signal to signal processor

• Transmit channel (up conversion using one mixing stage)

D/A

LO C

OM

PUTE

R

BPF POWERAMP

ANTENNA ELEMENT

D/A

I Q

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Conformal Antennas Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

AIRCRAFT BODY

ANTENNA APERTURE

INTERNAL PRINTED CIRCUIT BEAMFORMING

NETWORKOUTPUT/INPUT

• Conformal antenna apertures conform to the shape of the platform• Typically applied to composite surfaces; the antenna beamforming network and circuitry are interlaced with the platform structure and skin• Can be active antennas with processing embedded (i.e., adaptive or “smart”)• Self-calibrating and fault isolation (errors and failures detected and compensated for or corrected)• Can be re-configurable (portion of the aperture that is active can be changed)• Infrared (IR) and other sensors can be integrated into the antenna

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-80 -60 -40 -20 0 20 40 60 80-30

-25

-20

-15

-10

-5

0

5

10

THETA, DEG

REL

ATIV

E PO

WER

, dB

ELEMENT 1ELEMENT 2ELEMENT 3ELEMENT 4ELEMENT 5

-80 -60 -40 -20 0 20 40 60 80-40

-35

-30

-25

-20

-15

-10

-5

0

THETA, DEG

REL

ATIV

E PO

WER

, dB

INFINITE GROUND PLANEFINITE GROUND PLANE

• Elements in an array interact with each other (patterns of edge elements deviate from those in the center)• Example: 10 element array (element 1 is at edge; element 5 at center)

Individual dipole element H-plane patterns (infinite ground plane ) Infinite vs. finite ground plane

Mutual Coupling Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

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Conformal Shapes Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

FLAT GROUND PLANE

CURVED GROUND PLANE

-80 -60 -40 -20 0 20 40 60 80-25

-20

-15

-10

-5

0

Rel

ativ

e Po

wer

(dB)

Theta (degrees)

CURVED GP (12.7 dB)FLAT GP (14.4 dB)

• Curvature must be considered in the design process, or pattern distortion occurs• Example below: finite ground plane, mutual coupling included

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Patch Antennas Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

• Lend themselves to printed circuit fabrication techniques• Low profile - ideal for conformal antennas• Circular or linear polarization determined by feed configuration• Difficult to increase bandwidth beyond several percent• Substrates support surface waves• Lossy• Feeding methods:

TOP VIEW

PROXIMITY COUPLING

SURFACE LINE

FEED THROUGH

LINE

SUBSTRATE

GROUND PLANE

PATCH

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True Time Delay for Wide Band Scanning Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

• For wideband scanning the phase shifter must provide true time delay

...

......

d

θ

1 N

WAVE FRONT

N k d sinθ

...

......

...

θ

1d

N

WAVE FRONT

N k d sin θ

BEAM SCANNING USING CABLES TO PROVIDE "TRUE TIME DELAY"

BEAM SCANNING WITH PHASE SHIFTERS GIVES A PHASE THAT IS CONSTANT WITH FREQUENCY

TEM CABLES

PHASE SHIFTERS

cfk /2/2 πλπ ==

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Element

T/R Module

FiberOptios

Switch

Receiver Transmitter

Osc Limiter

Low NoiseAmplifier

DiodeLaser

PhotoDiode

PowerAmplifier

Circulator

Microwave

Light

Fiber Optic Beamforming Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

• Fiber optic beamforming architecture and T/R module• Conversion loss from microwaves to light > 20 dB (as of 1998)

T/R module

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SWITCH

TIME DELAY FIBERS

INPUT OUTPUT

INPUT OUTPUT2∆

3∆

4∆

∆

2∆3∆

4∆

∆

∆ IS A TIME DELAY BIT

Photonic Time Delay Phase Shifters Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

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Photonics for Reconfigurable Arrays Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

Low conductivitysemiconductorσ ~ 10-2 S/m

LASER

Becomes highconductivity regionσ ~ 104 S/m

� � � � � LASER

OUTPUTOPTO-ELECTRONIC

SWITCH

ARRAY ELEMENTS

• High energy beams are used to produce conducting antenna-shaped regions (left)

• Laser excitation of the switch activates a particular portion of the aperture (below)

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• Monolilthic microwave integrated circuit (MIMIC): All active and passive circuit elements, components, and interconnections are formed into the bulk or onto the surface, of a semi-insulating substrate by some deposition method (epitaxy, ion implantation, sputtering, evaporation, or diffusion)• Technology developed in late 70s and 80s is now common manufacturing technique• Advantages: > Potential low cost

> Improved reliability and reproducibility> Compact and lightweight> Potentially broadband > Design flexibility and multiple functions on a chip

• Disadvantages: > Unfavorable device/chip area ratios> Circuit tuning not possible> Troubleshooting is a problem> Coupling/EMC problems

> Difficulty in integrating high power sources

MMIC Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

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• Antennas with built-in multi-function capabilities and processing are often called smart antennas• If they are conformal as well, they are known as smart skins• Functions include:

> Self calibrating: adjust for changes in the physical environment (i.e., temperature)

> Self-diagnostic (built-in test, BIT): sense when and where faults or failures have occurred

• Tests can be run continuously (time scheduled with other system functions) or run periodically• If problems are diagnosed, actions include:

> Limit operation or shutdown the system > Adapt to new conditions when processing, or reconfigure the antenna

Smart Antennas Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

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T/R Module Concept Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

• Transmit and receive channels for each element are side by side• Depth is a disadvantage, but module replacement easy

FEED LINE

MODULE (PHASE SHIFTER, LNA, ETC)

RADIATING ELEMENT

EDGE ACTS AS GROUND

PLANE

• F-15 radar

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RADIATOR

FEED LINE

GROUND PLANE

OTHER DEVICE LAYERS

T/R Tile Concept Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

From paper by Gouker, Delisle andDuffy, IEEE Trans on MTT, vol 44,no. 11, Nov. 1996

• Low profile• A point failure requires that the entire tile be replaced

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Radomes Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

TRANSMITTED RAYS

REFRACTED

SCANNED ANTENNA

GIMBAL MOUNT

LOW LOSS DIELECTRIC RADOME

AIRCRAFT BODY

REFLECTIONS

1. beam pointing error from refraction by the radome wall2. gain loss due to loss in the radome material and multiple reflections3. increased sidelobe level from multiple reflections

• Radome must be transparent in the operating band• Protects the antenna from the environment• The antenna pattern with a radome will always be different than that without a radome• Radome effects on the antenna pattern:

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Superconductivity Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

POWERDIVIDER

SUPERCONDUCTINGTRANSMISSION LINES

INPUT

PROXIMITY COUPLED ARRAY ELEMENTS

CRYO-COOLER

VACUUMENCLOSURE(RADOME)

• Reduces loss in feed lines (as much as 25 dB for a 16 element array operating at 60 GHz)

• Makes possible “super-directive” arrays > gain much higher than expected for the given array area> requires some feed lines to have very high current, and therefore I2R losses are prohibitive in conventional conductors

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Pr

SIDELOBE MAINBEAM

REFLECTEDEMITTED

SKY BACKGROUND

EARTH BACKGROUND

Antenna Temperature Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

• Antenna noise temperature is specified in degrees Kelvin• Indication of the noise power out of the antenna when no signal is present• Depends on background radiation• Especially important when very low signal power is expected

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Example: Mini- and Micro-SAR Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

MiniSAR installed

http://www.imicrosensors.com/

• MicroSAR

• MiniSAR

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Vertical Takeoff UAV Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

• USN VTUAV has multiple missions• Use EM simulation codes to study

> antenna placement> effect of nearby structure on patterns> interference with other systems

VTUAV mesh model Pitch, roll, and yaw patterns

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ELEVATION

AZIMUTH

POD WITH AFTLOOKING ANTENNA

Gain specification(dashed)

HPBW contour ofcaptive antenna(solid)

JSOW Captive Carry Naval Postgraduate School Department of Electrical & Computer Engineering Monterey, California

• Problems similar to a UAV

> blockage> radome losses