Radar Book Chapter1-5

Radar level measurement- The users guide

Peter Devine

VEGA Controls / P Devine / 2000All rights reseved. No part of this book may reproduced in any way, or by any means, without priorpermissio in writing from the publisher:VEGA Controls Ltd, Kendal House, Victoria Way, Burgess Hill, West Sussex, RH 15 9NF England.

British Library Cataloguing in Publication Data

Devine, PeterRadar level measurement - The users guide1. Radar2. Title621.3848

ISBN 0-9538920-0-X

Cover by LinkDesign, Schramberg.Printed in Great Britain at VIP print, Heathfield, Sussex.

written byPeter Devine

additional informationKarl Griebaum

type setting and layoutLiz Moakes

final drawings and diagramsEvi Brucker

Foreword ixAcknowledgement xiIntroduction xiii

Part I1. History of radar 12. Physics of radar 133. Types of radar 33

1. CW-radar 332. FM - CW 363. Pulse radar 39

Part II4. Radar level measurement 47

1. FM - CW 482. PULSE radar 543. Choice of frequency 624. Accuracy 685. Power 74

5. Radar antennas 771. Horn antennas 812. Dielectric rod antennas 923. Measuring tube antennas 1014. Parabolic dish antennas 1065. Planar array antennas 108Antenna energy patterns 110

6. Installation 115A. Mechanical installation 115

1. Horn antenna (liquids) 1152. Rod antenna (liquids) 1173. General consideration (liquids) 1204. Stand pipes & measuring tubes 1275. Platic tank tops and windows 1346. Horn antenna (solids) 139

B. Radar level installation cont. 1411. safe area applications 1412. Hazardous area applications 144

Contents

ix

To suggest that any one type of leveltransmitter technology could be regar-ded as 'universal' would be unrealisticand potentially irresponsible due to thevariation and complexity of availableapplications when liquids, powders andsolids are all considered. However, therate at which radar based level trans-mitters have established themselvesover the last couple of years wouldtend to suggest that this technology iscloser to that definition that any princi-ple has ever been.

I have personally been involved inthe development, applications, salesand marketing of level transmitters,controllers and indicators of most typesover the last twenty years. In that timenothing has, in my opinion, come closeto matching the significance of radar interms of its overall suitability, for notjust conventional but extreme processconditions applications for the vastmajority of substances in vessels of vir-tually any size or complexity.

This unique principle combined withcurrent reflections processing software,materials of construction, simplicity ofinstallation and transmitter digital com-munications allows this to be conside-red as a day to day 'first consideration'for level, whereas only a very shorttime ago it was regarded as expensiveand specialised - this is no longer thecase.

The purpose of this publication isquite specific, and that is to explainsome of the principles involved, and toshow that by applying some simpleguidelines, what is obviously a sophi-sti-cated technology can be simple andreliably used in an enormously widerange of industrial and process applica-tions.

We make no apology for including achapter on Vega specific products, andhope this guide stimulates a radar user,or some greater depth of knowledge ifyou have some experience, we lookforward to hearing from you.

Mel HenryManaging DirectorVega Controls Ltd.

Foreword

xi

In writing and compiling this book Ihad the invaluable assistance of severalcolleagues from VEGA in Schiltachboth in the developing department andwithin the product management.

Particular thanks must go to KarlGriessbaum for his lucid explanationsof the 'secrets' of pulse radar; his insi-ght into the workings of FM - CWradar and the drawings to accompanythe explanations. Thanks also toJuergen Skowaisa and Juergen Motzerfor their technical contributions to thebook.

The publication of 'radar level mea-surement - the users guide' is a reflec-tion of the wealth of product knowled-ge of radar level application experiencein the VEGA group of companies andour agents and distributors world wide.

This experience has acceleratedsince the advent of the VEGAPULS 50series two wire, loop powered radar.

I would like to thank all those whocontributed to the section on radarapplications. This in-cludes DougAnderson, Dave Blenkiron, ChrisBrennan, Graeme Cross and JohnHulme in the UK, Paal Kvam ofHyptech in Norway, Dough Groh andhis colleagues at Ohmart VEGA in theUSA, and Juergen Skowaisa and RogerRamsden from VEGA Germany. Thankalso to the VEGA marketing depart-ment in Germany and the UK for their

assistance in producing and collatingpictures and photographs.

Thank to all the other unnamed con-tributors.

Finally, the most important contribu-tors to this book are all VEGA radarusers world wide without whom ourhigh level of expertise in process radarmeasurement applications would not bepossible.

Peter DevineTechnical managerVega Controls Ltd.

Acknowledgements

xiii

The technical benefits of radar as alevel measurement technique are clear.

Radar provides a non-contact sensorthat is virtually unaffected by changesin process temperature, pressure or thegas and vapour composition within avessel.

In addition, the measurement accu-racy is unaffected by changes in densi-ty, conductivity and dielectric constantof the product being measured or by airmovement above the product.

These benefits have become moresignificant to the process industry sincethe advent of low costs, high perfor-mance, two wire loop powered radarlevel transmitters.

This breakthrough, in the summer of1997, produced an unprecedentedboom in the use of non-contact micro-wave radar transmitters for liquid andsolids process level application.

'Radar level measurement - theusers guide' is offered as a referencebook for all those interested in the tech-nology, the application, and the prac-tical installation of radar level sen-sors.We cover many practical process levelapplications rather than the closedniche market of custody transfer mea-surement.

Radar history, physics and techni-ques are presented as well as descripti-ons of types of ra-dar antenna andmechanical and electrical installations.

Now radar is an affordable optionfor process level measurement. Wecompare it closely with all of the otherprocess level techniques and give manyexamples of the myriad applications ofradar across all industries.

Radar level measurement has comeof age. We hope that this book will beinvaluable in helping you to see thepotential of this latest and almost uni-versal level measurement technology.

More than anything, we hope thatyou enjoy delving into the pages of thisbook.

Peter DevineTechnical managerVega Controls Ltd

Introduction

1James Clerk Maxwell predicted theexistence of radio waves in his theoryof electromagnetism as long ago as1864. He showed mathematically thatall electromagnetic waves travel at thesame velocity in free space,independent of their wavelength. Thisvelocity is of the order of 300,000 kilo-metres per second, the speed of light.

Heinrich Rudolf Hertz, verifiedMaxwells theory by experiments car-ried out in 1886-87 at KarlsruhePolytechnic. He used a spark gap trans-mitter producing bursts of high fre-quency electromagnetic waves at about455 MHz, or a wavelength of 0.66metres.

Hertz confirmed that these electro-magnetic radio waves had the samevelocity as light and could be reflectedby metallic and dielectric bodies. Inaddition to their reflective properties,Hertz demonstrated that radio wavesexhibit refraction, diffraction, polariza-tion and interference in the same wayas light. These early experiments inreflecting radio waves off metal plateswere the first manifestations of radar aswe know it today.

The first practical form of radar wasproduced by a German engineer,Christian Hlsmeyer. Patented in vari-ous countries in 1904 as theTelemobiloscope, Hlsmeyers appa-ratus was described as A Hertzianwave projecting and receiving appara-tus adapted to indicate or givewarning of the presence of a metallicbody, such as a ship or a train, in theline of projection of such waves.

An addition to the patent in the sameyear described Improvements inHertzian wave projecting and receiving

1. History of radar

James Clerk Maxwell - predicted the existence of radio waves in

his theory of electromagnetism (Pic. 1.1 - J.C.M.F)

Heinrich Hertz - Hertz confirmed by experiment that elec-tromagnetic radio waves have the samevelocity as light and can be reflected by

metallic and dielectric bodies(Pic. 1.2 - I.N.T)

2apparatus for locating the position ofdistant metal objects.

A successful demonstration of thetelemobiloscope was made at theInternational Shipping Congress inRotterdam in 1904, and also to theGerman navy. However, the telemo-biloscope was considered to be limitedand was not a commercial success.

Guglielmo Marconi, is famous forpioneering trans-Atlantic radio commu-nications. In 1922 Marconi had alsorecognised the potential of using shortwave radio for the detection of metallicobjects. Marconi envisaged the use ofradio for ship to ship detection at nightor in fog. However, he did not appearto receive the support or have theresources to carry these ideas further atthe time.

Prior to World War II, radar wasbeing developed independently in anumber of different countries, includ-ing Britain, Germany, the UnitedStates, Italy, France and the SovietUnion.

In 1934, following a series of exper-iments at the Naval ResearchLaboratory in the United States, apatent was granted to Taylor, Youngand Hyland for a System for detectingobjects by radio.

In February 1935, British scientist,Robert Watson-Watt presented a paperon The detection and location of air-craft by radio methods to the TizardCommittee for the Scientific Survey ofAir Defence.

Christian Hlsmeyer produced the first practical radar

patented in 1904 (Pic. 1.3 - D.M.M)

Guglielmo Marconi recognised the potential of using shortwave radio for the detection of metallic

objects in 1922(Pic. 1.4 - GEC Marconi)

1. History of radar

3

Subsequently, a practical demonstra-tion was carried out using a BBC radiotransmitter at Daventry. About five anda half miles (9 km) away, a separateradio receiver connected to an oscillo-scope was used to detect the presenceof a Handley Page Heyford aircraft asit flew between the transmitter andreceiver.

Both the American system andWatson-Watts Daventry experimentwere types of continuous wave (CW)radar. Called CW wave-interferenceradar or bistatic CW radar, a continu-ous single frequency was transmittedfrom one point and detected by areceiver at a separate location. Thereceiver also detects doppler shiftedechoes from the target object. Theinterference between the frequency ofthe direct signal and reflected signals at

a slightly different frequency indicatedthe presence of the target object.

If you are unfortunate enough to liveon an airport flight path, you may havewitnessed this effect on your televisionscreen. As an aircraft approaches, thepicture on the screen may flicker withregular horizontal bands scrolling verti-cally on the screen. These diminishwhen the aircraft is directly overheadand then continue as the aircraft movesaway.

Although it proved a point atDaventry, CW wave-interference radarwas not a practical device. It coulddetect the presence but not the positionof the target.

After Daventry, the British effortcontinued at Orford Ness and thennearby Bawdsey Manor on the Suffolkcoast. It was clear that pulse radarwould be needed to provide therequired distance and direction infor-mation essential for a defensive radiodetection system.

The British, under the direction ofWatson-Watt developed a defensivesystem of CH (Chain Home) radar sta-tions which eventually covered all ofthe coastal approaches to Britain. Thestandard chain home radars had a rela-tively low frequency of between 22 &30 MHz (wavelength 10 to 13.5metres). They had a power of 200 kilo-watts and a range of up to 190 kilo-metres.

However, the long range CH radartransmitters were blind to low flyingaircraft and therefore they were supple-mented by CHL (Chain Home Low)radar transmitters which had a shorter range and covered the lower altitudesthat were overlooked by the main CH

Sir Robert Watson - Watt was a senior figure in the development

of British radar in the 1930s & 40s(Pic. 1.5 - I.W.M)

4transmitters. They operated on a fre-quency of 200 MHz (wavelength 1.5metres).

It is well documented that the CHand CHL network of radar stationswere a crucial factor during the Battleof Britain in the summer of 1940. Itenabled the fighters of the Royal AirForce to be deployed when and wherethey were needed and rested when thethreat receded. The limited resources inmen and machines were not wasted inlong standing patrols.

German radar research was also con-ducted in secret in the late 1930s.Whereas the development effort inBritain was focused on air defence, inGermany separate radar developmentswere carried out for the Navy, Armyand Luftwaffe.

Companies involved in Germannaval research produced a range of ship

mounted sea search radar transmitterscalled Seetakt. These were delivered asearly as 1938 with a frequency of 366MHz (wavelength 82 cm) and wereinstalled on many vessels including thefamous battleships, Bismarck and GrafSpee.

German Naval developments alsoproduced the Freya range of searchradars operating on 125 MHz (wave-length 2.4 metres). These were found tobe effective for tracking aircraft at longrange, and were subsequently suppliedto the Luftwaffe for early warning.However, they could not provide alti-tude information.

Other German radars in wide usewere the parabolic antenna Wrzburgand Wrzburg Riese (Giant Wrzburg)transmitters. The standard Wrzburgswere generally used for directingsearchlights and flak batteries and theWrzburg Riese for tracking individualintruders and directing night fighters tointercept them.

In a similar fashion to the BritishChain Home system, the Germans builta defensive network of Himmelbettradar stations. The literal translation ofHimmelbett is four poster bed. Thefour posts of the bed consisted of aFreya early warning radar, a Wrzburgradar for tracking the intruding aircraft,a Wrzburg radar to guide the nightfighter to the intruder and a Seeburgplotting table (Seeburgtisch) to monitorthe interception.

This defensive radar system becameknown by the British as theKammhuber Line after the Germangeneral in charge of night fighters.

British Chain Home Radar aerials - Radar was instrumental in the defenceof Britain during the second world war

(Pic. 1.6 - I.W.M)

1. History of radar

Above - The famous aerial reconnaissancephotograph of a German Wrzburg radarantenna at Bruneval in northern France.

This image alerted the British to thepresence and advanced state of German

defensive radar which led to a commandoaction in which components from theradar were taken back to Britain for

analysis(Pic. 1.7 - I.W.M)

Right - The German Wrzberg radar wasused for directing searchlights and flakbatteries and for tracking individual tar-gets and directing interceptors to them

(Pic. 1.8 - P.D)

5

6Both Britain and Germany devel-oped airborne radar for fighter inter-ception by night. British airborne radartrials started in 1937 with the produc-tion AI Mark 1 taking to the air in May1939. The first practical BritishAirborne Interception radar was the AIMark IV which was first tested inAugust 1940.

In Germany the Lichtenstein air-borne radar was available in mid 1941.The characteristic external radar aerialarray of the Lichtenstein caused signifi-cant aerodynamic drag. This couldreduce the aircraft speed by as much as40 kilometres per hour. By 1943 therange had been extended to 6000metres.

It became clear to radar researchersthat a shorter centimetric wavelengthwould be more useful for a number ofapplications. This would enable a morefocused airborne radar that would notsuffer from the ground returns thatrestricted capabilities of the first air-borne radars. The higher frequencycould be used for a ground mappingradar unit to locate towns and othergeographic features.

The problem was how to find amethod of generating sufficient powerat the desired wavelength of 10centimetres.

German Airborne Radar Lichtensteinavailable in mid 1941 - the external aerialradar caused significant aerodynamic drag

(Pic. 1.10 - I.W.M)

British Airborne Radar - AI Mark IVdeveloped for fighter interception by night

in 1940(Pic. 1.9 - I.W.M)

1. History of radar

7

In late February 1940, an historicbreakthrough was made by JohnRandall and Harry Boot, researchers atthe University of Birmingham, whenthey tested their world changing inven-tion the Cavity Magnetron.

The heart of this cavity magnetronwas a simple solid copper block withsix cavities machined into it. In thecentre was the cathode. When a strongmagnetic field and high voltage wasapplied between the copper block andthe cathode, the stream of electrons res-onated in unison within the cavitiesinstead of passing directly to the copperblock anode. The frequency of oscilla-tion was calculated to be about 3 GHz(10 centimetre wavelength).

The theoretical calculations of theprototype cavity magnetron were cor-rect. The actual wavelength was foundto be 9.87 centimetres and the allimportant power of the prototype was400 Watts.

Production of cavity magnetrons fol-lowed very quickly and the power out-put was significantly increased. Britaindeveloped microwave airborne inter-ception AI radar sets for night fighterswhich had a vastly improved long andnear range. The British microwave air-borne interception radar was the AIMark VII which was introduced in mid1942. The improved AI Mark VIII wasmass produced and in wide use byearly 1943.

Cavity Magnetron -the world changing invention by John

Randall and Harry Boot invented in 1940(Pic. 1.11 - GEC)

The Cavity Magnetron was used in centrimetric microwave airborne radar and pro-duced a quantum leap in performance. The radar dish was protected inside a

plastic nose assembly(Pic. 1.12 & 1.13 - H.R.A)

8Britain also used the cavity mag-netron in the development of a groundmapping radar called H2S. This deviceenabled aircraft to be accurately navi-gated to their destinations without theaid of ground based beacons or beams.

Britain shared this secret microwavetechnology with the United Stateswhere additional development tookplace at the Radiation Laboratory with-in the Massachusetts Institute ofTechnology. From the work carried outat MIT, further airborne interceptionradars and gun laying radars were massproduced and delivered to the alliedforces. The American SCR-720 (knownas AI Mark X in Britain) was firstdelivered to the USAAF by late 1942.

This radar unit became a standarddevice long after the war had finished.

War time secrecy meant that radiodetection devices were given codednames. In Britain, the early chain homeradar was called RDF after the existingRadio Direction Finding systems in thehope that it would mislead their real

function.In the same way in Germany,

radar was disguised as DezimeterTelegraphie or De-Te, translated asdecimetric telegraphy

It was the Americans who intro-duced the now universally used palin-drome, RADAR or RAdio DetectionAnd Ranging.

The history of the development ofradar during the course of the SecondWorld War is a huge subject initself. Many devices were developed.Measures and counter measures weretaken in the radar war.

Since 1945, radar has been used foran increasing number of peaceful appli-cations. The giant Wrzburg parabolicradar transmitters of the Second WorldWar became post war radio telescopes.The basic designs were developed andenlarged and can be seen at the wellknown Jodrell Bank Observatory nearManchester which has a dish diameterof 75 metres.

Viewed from Earth, the planet Venus

Modern radar systems are exemplified by this AWAC airborne early warning aircraft.Multiple targets can be detected at extreme range

(Pic. 1.14 - P.D)

1. History of radar

is one of the brightest celestial bodies.However, the mysteries of our closeneighbour in the Solar System wereonly uncovered with the assistance ofradar. The surface of Venus is shroudedin dense clouds of vapour includingcarbon dioxide gas at pressures of 90bar and an average temperature of750 K.

Earth bound pulse radar measure-ments over an extended period of timewere used to calculate the radius of theorbit of Venus. Doppler shift measure-ments from the surface were used tocalculate the rate of rotation of theshrouded planet. The Venus day wasfound to be 243 Earth days.

During the 1970s, radar mapping ofthe planets surface by space probeuncovered surface features such ascraters.

Jodrell Bank - the observatory nearManchester which has a 75 metre

dish diameter(Pic. 1.15 - P.D)

Detection by radar is not always desirable. Huge sums of money have been spentreducing the radar signature of the F117 stealth fighter

(Pic. 1.16 - P.D)9

10

Radar technology is part of oureveryday lives. The cavity magnetronis used in microwave ovens.Continuous wave (CW) radars are usedin automatic door detection and vehiclespeed measurement. Other well knowncivilian radar applications include airtraffic control, shipping and weatherradar.

Radar altimeters developed in the1930s use a form of radar calledFM - CW or Frequency ModulatedContinuous Wave radar.

In the 1970s, the same FM - CWmeasurement technique was used inthe production of the first radar leveltank gauge. Initially these radar leveltransmitters were used to measurepetroleum products in supertankers.Further developments of FM - CWlevel transmitters led to their use onshore based storage tanks in the mid1980s. Originally these were expen-sive, high accuracy systems for fiscalmeasurement of petroleum products.

Later, lower accuracy FM - CW radartransmitters became available for theprocess industry.

In the late 1980s, pulse radar leveltransmitters were developed for processmeasurement applications. The avail-ability of suitable crystals and solidstate components such as GaAs FEToscillators enabled cost effective radarlevel transmitters to enter the market.

In 1997 a significant improvementin the specification of radar level trans-mitters was achieved. VEGA producedthe worlds first two wire, loop pow-ered, intrinsically safe radar level trans-mitter. For the first time low cost, highspecification radar level transmittersbecame available.

It is likely that these advances willcontinue into the new millennium andthat radar level transmitters willbecome a commodity item in the sameway as differential pressure transmit-ters.

In the field of radar levelmeasurement, technologicaladvances have resulted intwo wire, intrinsically safetransmitters(Pic. 1.17 - Vega)

1. History of radar

11

A raw oscilloscope echo trace had to be interpreted by skilled operators using the Britishwar time Chain Home Low radar(Pic. 1.18 & 1.19 - I.W.M)

Comprehensive information is available on the PC echo trace of the latest two wire looppowered radar level transmitters(Pic. 1.20 - Vega Pic. 1.21 - Vega)

Comparing the old with the new










Inhalt

13

The velocity of light in free space is299,792,458 metres per second, butwho is timing? For the purposes of thecalculations in this book, we will call it300,000 kilometres per second or3 x 108 metres per second.

Maxwells theories of electro-magnetism were confirmed by theexperiments of Heinrich Hertz. Theseshow that all forms of electromagneticradiation travel at the speed of light infree space. This applies equally to longwave radio transmissions, microwaves,infrared, visible and ultraviolet lightplus X-rays and Gamma rays.

Maxwell showed that the velocity oflight in a vacuum in free space is givenby the expression :Examples :-

The original cavity magnetron hada wavelength of 9.87 centimetres.This corresponds to a frequency of3037.4 MHz (3.0374 GHz).

The frequency of a pulse radarlevel transmitter may be 26 GHzor 26 x 108 metres per second. The wavelength is 1.15 centimetres.

The electromagnetic waves have anelectrical vector E and a magnetic vec-tor B that are perpendicular to eachother and perpendicular to the directionof the wave. This will be discussed andillustrated further in the section onpolarization. The electrical vector hasthe major influence on radar applica-tions.

2. Physics of radar

Fig 2.1

am

plitu

de

direction of wave

Electromagnetic waves

c velocity of electromagneticwaves in metres / second

f frequency of wave in second -1 wavelength in metres

[Eq. 2.1]

[Eq. 2.2]

co

c

)1

=

=

(o x o

f x

The velocity of an electromagneticwave is the product of the frequencyand the wavelength.

co velocity of electromagntic wavein a vacuum in metres / second

o the permeability of free space (4 x 10 -7 henry / metre)o the permittivity of free space(8.854 x 10 -12 farad / metre)

14

108 107 106 105 104 103 102 101 100 10-1 10-2 10-3 10-4

101 102 103 10

100 MHz 1 GHz 10 GHz 100 GHz

3 m 0.3 m 3 cm 3 mm

4 105 106 107 108 109 1010 1011 1012

infraradio waveselectric waves

The microwave frequencies of the electromagnetic spectrum.Radar level transmitters range between 5.8 GHz (5.2cm) and 26 GHz (11.5mm)

The Electromagnetic spectrum

15

2. Physics of radar

10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 10-13 10-14 10-15 10-16 m

1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 Hz

gamma raysX raysultra violetred

Fig 2.2 Electromagnetic spectrum.All electromagnetic waves travel at the speed of light in free space. This spectrumshows the range of frequencies and wavelengths from electric waves togamma rays

PermittivityIn electrostatics, the force between

two charges depends upon the magni-tude and separation of the charges andthe composition of the mediumbetween the charges. Permittivity isthe property of the medium that effectsthe magnitude of the force. The higherthe value of the permittivity, the lowerthe force between the charges. Thevalue of the permittivity of freespace (in a vacuum) o, is calculatedindirectly and empirically to be:8.854 x 10-12 farad / metre.

Relative permittivity ordielectric constant r

The ratio of the permittivity of amedium to the permittivity of freespace is a dimensionless propertycalled relative permittivity or dielec-tric constant. For example, at 20 Cthe relative permittivity of air is closeto that of a vaccum and is only about1.0005 whereas the relative permittivi-ty of water at 20 C is about 80.(Dielectric constant is also widelyknown as DK.)

The value of the dielectric constantof the product being measured is veryimportant in the application of radar tolevel measurement. In non-conductiveproducts, some of the microwave ener-gy will pass through the product andthe rest will be reflected off the surface.

This feature of microwaves can beused to advantage or, in some circum-stances, it can create a measurementproblem.

Permeability and relativepermeability r

The magnetic vector, B, of an elec-tromagnetic wave also has an influenceon the velocity of electromagneticwaves. However, this influence is neg-ligible when considering the velocity ingases and vapours which are non-mag-netic. The relative permeability of theproduct being measured has no signifi-cant effect on the reflected signal whencompared with the effects of the rela-tive permittivity or dielectric constant.For the non-magnetic gases above theproduct being measured, the value ofthe relative permeability, r = 1.Frequency, velocity and wave-length

As we have already stated, the fre-quency (f), velocity (c) and wavelength() of the electromagnetic waves arerelated by the equation c = f x .

The frequency remains uninfluencedby changes in the propagation medium.However, the velocity and wavelengthcan change depending on the electricalproperties of the medium in which theyare travelling. The speed of propaga-tion can be calculated using equation2.3.

16

c velocity of electromagnetic wavein the medium in metres/second

co velocity of electromagneticwaves in free space

r the relative permeability( medium / o)r the relative permittivity

[Eq. 2.3]

c )co

= (r x r

2. Physics of radar

Changes in the wavelength andvelocity of microwaves are apparent incertain radar level applications.Changes in temperature, pressure andgas composition have a small effect onthe running time of microwavesbecause the dielectric constant of thepropagation medium is altered to agreater or lesser extent. This is dis-cussed in detail later.

Radar level transmitters can be usedto measure conductive liquids throughlow dielectric windows such as glass,polypropylene and PTFE. The opti-mum thickness of the low dielectricwindow is a half wavelength or multi-ple of half wavelength.

For example, polypropylene has adielectric constant r of 2.3 and thehalf wavelength at a frequency of 5.8GHz is 17 mm compared with a halfwavelength of about 26 mm in a vacu-um. It follows that the speed of

microwaves in polypropylene is abouttwo thirds of the speed in air.

As with low dielectric windows,non-conductive, low dielectric constantliquids may absorb more power thanthey reflect from the surface. Thevelocity of the microwaves within theliquid is slower than in the vapourspace above.

For example, if there is about 0.5metres of solvent in the bottom of ametallic vessel, a radar level transmittermay see a larger echo from the vesselbottom than from the product. Thislarge echo will appear to be furtheraway than it really is because the run-ning time within the solvent is slower.For this reason, special considerationsmust be made within the echo process-ing software to ensure that the radarfollows the solvent level and does notfollow the vessel bottom as it apparent-ly moves away!

Empty vessel: large echofrom metalbottom

As the vessel fills withsolvent two echoesare received. Theecho from the vesselbottom appearsfurther away becausethe running time ofthe microwaves insolvent is slower

Fig 2.3 - Effect of dielectric constant on the running time of a microwave radar

solvent echo

17

18

Effects on the propagationspeed of microwaves

Microwave radar level transmitterscan be applied almost universallybecause, as a measurement technique,they are virtually unaffected by processtemperature, temperature gradient, vac-uum and normal pressure variations,gas or vapour composition and move-ment of the propagation medium.

However, changes in these processconditions do cause slight variations inthe propagation speed because thedielectric constant of the propagationmedium is altered.

Calculating the propagationspeed of microwaves

The temperature, pressure and thegas composition of the vapour space allhave an effect on the dielectric constantof the propagation medium throughwhich the microwaves must travel.This in turn affects the propagationspeed or running time of the instru-ment.

The dielectric constant or relativepermittivity can be calculated asfollows :

The same effect can be experienced when looking at interface detection usingguided microwave level transmitters to detect oil and water or solvent and aqueousbased liquids.

reference echo(water without oil)

water echooil echo

r calculated dielectric constant(relative permittivity)rN dielectric constant of gas/vapour

under normal conditions (temperature 273 K, pressure 1 barabsolute)

N temperature under normalconditions, 273 Kelvin

PN pressure under normalconditions, 1 bar absolute

process temperature in KelvinP process pressure in bar absolute

Fig 2.4 Oil/water interfacedetection using aguided microwavelevel transmitter. Notethat the water echohas a reduced ampli-tude and appears to befurther away. Therunning time ofmicrowaves in oil isslower than in air

r = + x1N x P x PN

(rN - 1)[Eq. 2.4]

2. Physics of radar

From equation 2.4 and equation 2.3,we can calculate the percentage errorcaused by variations in the dielectricconstant of different gases and vapoursand the relative effects of changes inprocess temperature and pressure.

Gases and vapoursBy definition, the dielectric constant

in a vacuum is equal to 1.0. The dielec-tric constants of the gases and vapoursthat may be present above the product

differ but they have only a very smalleffect on the accuracy of radar.

Radar level transmitters are usuallycalibrated in air. For this reason, thefollowing tables show

1. Dielectric constant of different gasesat normal temperature and pressure(273K, 1 Bar A)

2. Percent error in the running time inthe gases compared with air

Gas / VapourrN (dielectric

constant at normalconditions)

% Error from air (atnormal temperature

and pressure)Vacuum 1.0000 + 0.0316

Air 1.000633 0.0Argon 1.000551 + 0.0041

Ammonia / NH 3 1.006976 + 0.3154Hydrogen Bromide HBr 1.002994 - 0.1178Hydrogen Chloride HCl 1.004078 - 0.1717Carbon Monoxide / CO 1.000692 - 0.00295Carbon Dioxide / C0 2 1.000985 - 0.0176

Ethane / C 2H6 1.001503 - 0.0434Ethylene / C 2H4 1.001449 - 0.0407

Helium 1.000072 + 0.0280Hydrogen / H 2 1.000275 + 0.0179Methane / CH 4 1.000878 - 0.0122Nitrogen / N 2 1.000576 + 0.00285Oxygen / O 2 1.000530 + 0.0052

Table 2.1 The dielectric constants under normal conditions, rN and the error caused bythe dielectric constant of typical process gases under normal conditions

19

20

Temperature

Fig 2.5 Temperature effect on radar measurement of air at a constant pressure of 1 BarA

High temperature or large temperature gradients have very little effect on thetransit time of microwaves within an air or vapour space. At a temperature of2000 C the variation is only 0.026% from the measurement value at 0 C. Radarlevel transmitters with air or nitrogen gas cooling are used on molten iron and steelapplications.

Temperature in C

0

0.005

0.0

0.01

0.015

0.02

0.025

0.03

250 500 750 1000 1250 1500 1750 2000

% e

rror

21

2. Physics of radar

Fig 2.6 The influence of pressure on radar measurement in air at a constant temperatureof 273 K

Pressure does have a small but more significant influence on the velocity ofelectromagnetic waves. At a pressure of 30 Bar, the error is only 0.84%. Howeverthis becomes more significant and at a pressure of 100 Bar there is a velocitychange of 2.8%. If the pressure is varying constantly between atmospheric pressureand 100 Bar, the velocity variations can be compensated using a pressure transmit-ter.

Pressure

00

2

4

6

8

10

50 100 150 200 250 300 350 400

Pressure in Bar (absolute)

% e

rror

22

In the preceding equations, we haveassumed that the microwaves aretravelling in free space in a vacuum.However, in practice the proximityof metallic vessel walls and otherstructures will have an influence onthe propagation velocity of themicrowaves. This is particularly truewhen microwave radar level transmit-ters are fitted inside bypass tubes orstilling tubes or when a horn antenna isfitted with a waveguide extension.

When microwaves are propagating

within a metallic tube the running timeappears to slow down because themicrowaves travel further bouncingoff the inside wall of the tube andcurrents are set up on the inside surfaceof the tube.

This effect is discussed in moredetail in the chapters on antennas andmechanical installations. The wave-guide effect can be compensated duringcalibration and the use of stilling tubesand bypass tubes can be beneficial insome level applications.

Conductive productsUsing a spark gap transmitter,

Heinrich Hertz demonstrated that elec-tromagnetic waves could be reflectedoff metallic objects and objects with arelatively high dielectric constant.

In the same way, radar can easilymeasure conductive aqueous liquidssuch as acids and caustic and otherconductive products ranging frommolten metal to saturated spent grain inthe brewing process.

When microwaves from a radar hit aconductive surface the electrical field Eis short circuited. The resultant currentin the conductive product causes themicrowaves to be re-transmitted orreflected from the surface.

Radar level transmitters have noproblem in measuring conductive liq-uids and solids because the microwaveswith frequencies between 5.8 GHz and26GHz are readily reflected off a con-ductive surface producing relativelylarge echoes.

Non-conductive productsIf a liquid or solid is non-conductive,

the value of the dielectric constant (rela-tive permittivity r) becomes moreimportant. The theoretical amount ofreflection at a dielectric layer can be cal-culated using equation 2.5

Waveguides, stilling tubes & bypass tubes

Reflection of electromagnetic waves

Electromagnetic waves exhibit the same properties as light.

Reflection Refraction Polarization Interference Diffraction

23

2. Physics of radar

TolueneSolvent with a low dielectric constant,r = 2.4

AcetoneSolvent with a dielectric constant,r = 20

Fig 2.7 Reflected radar power depends upon the dielectric constant of the productbeing measured

Transmitted power: W1Reflected power: W2Dielectric constant: rThen the percentage of reflectedpower at the dielectric layer,

x

100

% p

ower

refle

cted

00

20

40

60

80

100

10 20 30 40 50 60 70 80

[Eq. 2.5]

=

=

1-

W2W1

4 x r

1 + r( )2

Typical examples are as follows:

4.46% power is reflected 40 % power is reflected

= 1- 4 x 4 x2.4

1 + (2.4)((

))

2 = 1-

20

1 + (20)((

))

2

Dielectric constant, r

24

In radar level measurement the reflected energy from a product surface becomesmore critical at a dielectric constant (r) of less than 5. The following graph showsthis important region.

Most electrically conductive products or products with a dielectric constant ofmore than 1.5 can be measured using microwave radar level transmitters. Stillingtubes can be used to concentrate the microwaves for lower dielectric constantproducts.

Fig 2.8 Reflected radar power depends upon the dielectric constant of the product beingmeasured. This graph shows the critical region where care must be taken overchoice of radar antenna

x

100

% p

ower

refle

cted

Loss

L, d

B

1.0

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

0

5

10

15

20

1.5 2.0 3.0 3.5 4.0 4.5 5.02.5

Fig 2.9 Reflection loss in dB: loss L = 10 log

- 60

- 40

- 20

- 10

0



2. Physics of radar

Electromagnetic waves have anelectrical vector E and magnetic vectorB that are in phase but perpendicular toeach other. The direction of propaga-tion of the waves is perpendicular tothe electrical and magnetic vectors asshown in the diagram below.

Polarization defines the orientationof the electromagnetic waves and refersto the direction of the electrical vectorE. Most process radar level transmittersexhibit linear polarization as in the dia-

gram. The direction of the linear polar-ization is set by the orientation of thesignal coupler from the microwavemodule. The properties of the polariza-tion of microwaves can be important inthe application of radar to level mea-surement.

In television and microwave com-munications, linear polarization is alsoreferred to as horizontal or verticalpolarization depending on the relativeorientation of the aerials or antennas.

Fig 2.10 Diagram showing linear polarization and the relative orientation of the electricvector E, the magnetic vector B and the direction of propagation of themicrowaves

direction of wave

E

B

Polarization

25

26

Fig 2.11 Circular polarization involves rotation of the electrical and magnetic vectorsthrough 360 within a wavelength

Another form of polarization iselliptical polarization. A specific formof elliptical polarization is circularpolarization where the electrical vectorE and magnetic vector B rotate through360 within the space of a single wave-length, when a linear or circular polar-ized signal is reflected the direction ofpolarization is reversed. With circularpolarization it is possible to use thereversal of polarization to distinguishbetween a direct echo and an echo thathas made two reflections.

Circular polarization can also beused in search radars to separate thereflections from aircraft or ships frominterference echoes from rain. Thealmost spherical shape of the rain dropscauses a definite reversal of polariza-tion which can be easily rejected by thereceiving antenna. However, the scat-tered reflections from the ship or air-craft provide roughly equal amounts ofreversed and un-reversed energy thatenables detection.

27

2. Physics of radar

The linear polarization that is com-mon with process radar level transmit-ters can be used to minimise the effectsof false echo returns from the internalstructure of a process vessel. Thesefalse echoes could be reflected fromprobes, welds, agitators and baffles.

In some applications, the effect offalse echoes within a vessel can be sig-nificantly reduced by rotating the radarin the connection flange or boss. Theprinciple is illustrated below anddetailed in the section on mechanicalinstallations in Chapter 6.

Fig 2.12 If a metallic or high dielectric object is orientated in the same plane as theelectrical vector of the polarized microwaves, the radar level transmitter willreceive a large amplitude echo

Fig 2.13 If the same object is orientated at right angles to the plane of the electrical vector,the received echo will have a smaller amplitude

Large echo

Small echo

Direction of wave

E

B

Polarization can be used to reduce the amplitude of false echoes

Direction of wave

B

E

Beam angle is often discussed inrelation to radar transmitters. This cangive the impression that the radarantenna can direct a finely focusedbeam towards the target. Unfortunatelythis is not the case.

In practice, although they aredesigned to produce a directed beam, aradar antenna radiates some energy inall directions. As well as the main lobe

which accounts for most of the radiatedpower, there are also weaker side lobesof energy. This phenomenon is caused,in part, by diffraction. In addition tothis, destructive interference causes thenull points or notches that form thecharacteristic side lobes.

Chapter 5 provides a detailed expla-nation of beam angles, side lobes andtypes of antennas.

Fig 2.14 The lobe structure of antenna beams is caused by diffraction and destructiveinterference

Fig 2.15 Refraction & reflection

RefractionIn the same way as light is refracted

at an air/glass or air/water interface,microwaves are refracted when theyencounter a change in dielectric. This could be a low dielectric window(PTFE/glass/polypropylene) or a non-conductive low dielectric liquid such asa solvent.

The angle of refraction depends onthe angle of the incident wave and alsoon the ratio of the dielectric constantsat the interface.

It is possible to utilise the refractiveproperties of electromagnetic waves toconstruct a dielectric lens that willfocus microwaves.

Diffraction

main lobeside lobes

antenna

a a

B

microwave

interface

refracted energy

dielectric window / product

reflected energy

28

29

2. Physics of radar

Problematic interference effects are caused primarily by the inadvertent mixingof signals that are out of phase. The microwave signals have a sinusoidal wave-form.

Fig 2.16 In this illustration both of the sine waves have an identical frequency andamplitude but the second wave has a 45 phase lag

Interference - Phase

Phase angle

45

Interference can be constructive where in-phase signals produce a signal with ahigher amplitude or it can be destructive where signals that are 180 out of phaseeffectively cancel each other out.

signals in-phase

180 out of phase

constructive interference

destructive interference

Fig 2.17 Illustration of constructive and destructive interference

30

Microwaves can manifest interfer-ence effects in exactly the same way aslight. Potentially this can cause mea-surement problems. The causes ofinterference should be understood andavoided by design and installation con-siderations.

The wrong choice of antenna, instal-lation of an antenna up a nozzle, posi-tioning transmitters too close to vesselwalls or other obstructions can all lead

to interference of the signal. The chap-ter on mechanical installation shouldhelp a radar level user to avoid thispotential problem.

However, we use destructive inter-ference to our advantage when weapply pulse radar level measurementthrough a low dielectric window tomeasure conductive or high dielectricliquids.

Interference

Fig 2.18 Interference caused by positioning an antenna too close to the vessel wall. If aradar level transmitter is installed too close to the vessel wall it is possible thatinterference will occur. With indirect reflection A B B C, the phase may bealtered by 180 when compared with the direct reflection A B C. For this reasonthe microwaves may partially cancel out due to destructive interference

+ =

C

A

B

B B

31

2. Physics of radar

The thickness of the dielectric win-dow must be a half wavelength of thewindow material. When the half wave-length is used, there is destructive inter-ference between the reflection off thetop surface of the window and thereflection off the internal second surfaceof the window.

There is a 180 phase shift betweenthese reflections and they cancel each

other out. This type of installationis explained more fully in Chapter 6on the mechanical installations ofradar level transmitters together witha table showing the optimum thicknessof most important plastics and glasseswhich are suitable for penetration withradar sensors.

Fig 2.19 Destructive interference is a benefit when using pulse radar to measure througha low dielectric window. The reflection from the top surface and the reflectionfrom the internal second surface cancel each other if the thickness is a halfwavelength

emitted wavereflection withphase shift from topsurface

plastic vessel ceiling

reflection withoutphase shift frominternal surface

D

emitted wave

reflection with phase shift offtop surface of window

reflection without phase shiftoff internal face of window










Contents

In continuous wave or CW Radar, acontinuous unmodulated frequency istransmitted and echoes are receivedfrom the target object. If the targetobject is stationary, the frequency ofthe return echoes will be the same asthe transmitted frequency. The range ofthe object cannot be measured.

However, the frequency of the returnsignal from a moving object is changeddepending on the speed and directionof the object. This is the well knowndoppler effect. The doppler effect isapparent when the siren note of anemergency vehicle changes as it speedspast a pedestrian. The pitch of the siren

note is higher as it approaches the lis-tener and lower as it recedes. Thedoppler effect is also used byastronomers to monitor the expansionof the Universe. By measuring the redshift of the spectrum of distant starsand galaxies the rate of expansion canbe measured and the age of distantobjects can be estimated.

In the same way, when an object thathas been illuminated by a CW Radarapproaches the transmitter, the frequen-cy of the return signal will be higherthan the transmitted frequency. Theecho frequency will be lower if theobject is moving away.

33

3. Types of radar

Fig 3.1 CW radar uses doppler shift to derive speed measurement

target velocity v

received freque

ncy ft + fdp

transmitted fre

quency ft, wa

velength

1a. CW, continuous wave radar

34

1b. CW wave-interference radar or bistatic CW radar

1c. Multiple frequency CW radar

In Fig 3.1, the aircraft is travellingtowards the CW radar. Therefore thereceived frequency is higher thanthe transmitted frequency and the signof fdp is positive. If the aircraft wastravelling away from the radar at the

same speed, the received frequencywould be ft - fdp.

The velocity of the target in thedirection of the radar is calculated byequation 3.1

c is the velocity of microwavesv is the target velocityft is the frequency of the

transmitted signal fdp is the doppler beat frequency

which is proportional to velocityft+fdp is received frequency. The sign

of fdp depends upon whether thetarget is closing or receding

Standard continuous wave radar isused for speed measurement and, asalready explained, the distance to a sta-tionary object can not be calculated.However, there will be a phase shiftbetween the transmitted signal and thereturn signal.

If the starting position of the objectis known, CW radar could be used todetect a change in position of up to halfwavelength (/2) of the transmittedwave by measuring the phase shift ofthe echo signal. Although furthermovement could be detected, the range

would be ambiguous. With microwavefrequencies this means that the usefulmeasuring range would be very limited.

If the phase shifts of two slightlydifferent CW frequencies are measuredthe unambiguous range is equal to thehalf wavelength (/2) of the differencefrequency. This provides a usable dis-tance measurement device.

However, this technique is limited tomeasurement of a single target.Applications include surveying andautomobile obstacle detection.

We have already mentioned that CWradar was used in early radar detectionexperiments such as the famousDaventry experiment carried out byRobert Watson - Watt and his col-leagues. In this case, the transmitterand receiver were separated by a con-siderable distance. A moving objectwas detected by the receiver becausethere was interference between the fre-

quency received directly from thetransmitter and the doppler shifted fre-quency reflected off the target object.Although the presence of the object isdetected, the position and speed cannotbe calculated.

In essence, this is what happenswhen a low flying aircraft interfereswith the picture on a television screen.See Fig 3.2.

v2= =

x fdp c x fdp2 x ft

[Eq. 3.1]

3. Types of radar

targ

et

trans

mitt

erte

levi

sion

inte

rfere

nce

refle

cted

sig

nal

(dopp

ler sh

ift)tra

nsm

itted

sign

al in

dire

ct

trans

mitt

ed s

igna

l dire

ct

Fig

3.2

The

effec

t of lo

w fly

ing ai

rcraft

on

tele

visio

n re

cept

ion

is sim

ilar t

o th

e met

hod

ofde

tect

ion

by C

Ww

ave

-inte

rferen

ce rad

ar

35

Single frequency CW radar cannotbe used for distance measurementbecause there is no time reference markto gauge the delay in the return echofrom the target. A time reference markcan be achieved by modulating the fre-quency in a known manner.

If we consider the frequency of thetransmitted signal ramping up in alinear fashion, the difference betweenthe transmitting frequency and thefrequency of the returned signal will beproportional to the distance to thetarget.

If the distance to the target is R,and c is the speed of light, then thetime taken for the return journey is:-

We can see from Fig. 3.3 that ifwe know the linear rate of change ofthe transmitted signal and measure thedifference between the transmitted andreceived frequency fd, then we cancalculate the time t and hence derivethe distance R.

36

time

fd

transm

itted fr

equenc

y

receiv

ed freq

uency

frequ

ency

Fig 3.3 The principle of FM - CW radar

2. FM-CW, frequency modulated continuous wave radar

t = 2 x Rc

t =

t

2 x Rc

[Eq. 3.2]

37

3. Types of radar

FM - CW wave forms transmitted frequencyreceived frequency

Fig 3.6 Saw tooth waveMost commonly usedon most FM - CWprocess radar leveltransmitters

Fig 3.5 Triangular waveUsed on FM - CWradar transmitters

frequency

4.4GHz

4.2GHz

10 GHz

9 GHz

frequency

time

time

time

frequency

Fig 3.4 Sine waveCommonly used on air-craft radio altimetersbetween 4.2 and4.4 GHz

In practice, the FM - CW signal hasto be cyclic between two different fre-quencies. Radio altimeters modulatebetween 4.2 GHz and 4.4 GHz. Radarlevel transmitters typically modulatebetween about 9 GHz and 10 GHz or

24 GHz and 26 GHz.The cyclic modulation of FM - CW

radar transmitter takes different forms.These are sinusoidal, saw tooth ortriangular wave forms.

38

If we look at a triangular waveform we can see that there is an inter-ruption in the output of the differencefrequency , fd. In practice, the receivedsignal is heterodyned with part of thetransmitted frequency to produce thedifference frequency which has a posi-

tive value independent of whether themodulation is increasing or decreasing.

The diagram below makes theassumption that the target distance isnot changing. If the target is moving,there will be a doppler shift in the dif-ference frequency.

frequency

time

time

differencefrequency

fd

Fig 3.7 & 3.8 The change in direction between the ramping up and down of the frequencycreates a short break in the measured value of the difference frequency.This has to be filtered out. The transmitted frequency is represented by thered line and the received frequency is represented by the dark blue line.The difference frequency is shown in light blue on the bottom graph

Pulse radar is and has been usedwidely for distance measurement sincethe very beginnings of radar technolo-gy. The basic form of pulse radar is apure time of flight measurement. Shortpulses, typically of millisecond ornansecond duration, are transmittedand the transit time to and from the tar-get is measured.

The pulses of a pulse radar are notdiscrete monopulses with a single peak

of electromagnetic energy, but are infact a short wave packet. The numberof waves and length of the pulsedepends upon the pulse duration andthe carrier frequency that is used.

These regularly repeating pulseshave a relatively long time delaybetween them to allow the return echoto be received before the next pulse istransmitted.

39

3. Types of radar

The inter pulse period (the timebetween successive pulses) t is theinverse of the pulse repetitionfrequency fr or PRF. The pulse durationor pulse width, , is a fraction of theinter pulse period.

The inter pulse period t effectivelydefines the maximum range of theradar. ExampleThe pulse repetition frequency(PRF) is defined as

If the pulse period t is 500 microsec-onds, then the pulse repetition frequen-cy is two thousand pulses per second.In 500 microseconds, the radar pulseswill travel 150 kilometres. Consideringthe return journey of an echo reflectedoff a target, this gives a maximum the-oretical range of 75 kilometres.

If the time taken for the returnjourney is T, and c is the speed of light,then the distance to the target is

3. Pulse radar

Fig 3.9 Basic pulse radar

t

3rd pulseTransmitted pulses

2nd pulse 1st pulse

1t

T x c2

fr = R =[Eq. 3.3]

a. Basic pulse radar

The pulses transmitted by a standardpulse radar can be considered as a veryshort burst of continuous wave radar.There is a single frequency with nomodulation on the signal for the dura-tion of the pulse.

If the frequency of the waves of thetransmitted pulse is ft and the target ismoving towards the radar with velocityv, then, as with the CW radar alreadydescribed, the frequency of the returnpulse will be ft + fdp , where fdp is thedoppler beat frequency. Similarly, thereceived frequency will be ft - fdp if thetarget is moving away from the radar.

Therefore, a pulse doppler radar canbe used to measure speed, distance anddirection.

The ability of the pulse dopplerradar to measure speed allows the sys-tem to ignore stationary targets. This isalso commonly called moving targetindication or MTI radar.

In general, an MTI radar has accu-rate range measurement but imprecisespeed measurement, whereas a pulsedoppler radar has accurate speed mea-surement and imprecise distance mea-surement.

The velocity of the target in thedirection of the radar is calculated inequation 3.4:

This is the same calculation as forCW radar. The distance to the target iscalculated by the transit time of thepulse, equation 3.3.

As well as being used to monitorcivil and military aircraft movements,pulse doppler radar is used in weatherforecasting. A doppler shift is measuredwithin storm clouds which can be dis-tinguished from general ground clut-ter. It is also used to measure theextreme wind velocities within a torna-do or twister.

40

b. Pulse doppler radar

c

R

2

2

=

=

=

x fdp c x fdp2 x ft

T x c

[Eq. 3.4]

[Eq. 3.3]

3. Types of radar

Fig

3.10

Pu

lse d

oppl

er ra

dar p

rovi

des t

arg

et sp

eed,

dist

an

ce a

nd

dire

ctio

n

f t+

f dp

f t

R

Pul

se d

opp

ler

rada

r

41

42

With pulse radar, a shorter pulseduration enables better target resolutionand therefore higher accuracy.However, a shorter pulse needs a sig-nificantly higher peak power if therange performance has to be main-tained. If there is a limit to the maxi-mum power available, a short pulsewill inevitably result in a reducedrange.

With limited peak power, a longerpulse duration, , will provide more

radiated energy and therefore range but(with a standard pulse radar) at theexpense of resolution and accuracy.

Pulse compression within a Chirpradar is a method of achieving theaccuracy benefits of a short pulse radartogether with the power benefits ofusing a longer pulse. Essentially, Chirpradar is a cross between a pulse radarand an FM - CW radar.

Fig 3.11 Chirp radar wave form. Chirp is a cross between pulse and FM - CW radar

c. Pulse compression and Chirp radar

f1

f2

t1 t2

time

time

frequ

ency

am

plitu

de

43

3. Types of radar

Each pulse of a Chirp radar has lin-ear frequency modulation and a con-stant amplitude.

The echo pulse is processed througha filter that compresses the echo bycreating a time lag that is inversely

proportional to the frequency.Therefore, the low frequency that

arrives first is slowed down the mostand the subsequent higher frequenciescatch up producing a sharper echo sig-nal and improved echo resolution.

Another method of echo compres-sion uses binary phase modulationwhere the transmitted signal is special-ly encoded with segments of the pulseeither in phase or 180 out of phase.The return echoes are decoded by a fil-ter that produces a higher amplitudeand compressed signal.

The name Chirp radar comes fromthe short rapid change in frequency ofthe pulse which is analogous to thechirping of a bird song.

The above methods of radar detec-tion are used widely in long range dis-tance or speed measurement. In thenext chapter we look at which of thesemethods can be applied to the uniqueproblems involved in measuring liquidor solid levels within process vesselsand silos.

Pulse compression of chirp radar echo signal

Long frequency modulated echo pulse

Tim

e la

g

Frequency

Filter

Compressed signal

Fig 3.12 Pulse compression of chirp radar echo signal

45

Part III

Radar level measurementRadar antennas

Radar level installations










Contents

The benefits of radar as a level mea-surement technique are clear.

Radar provides a non-contact sensorthat is virtually unaffected by changesin process temperature, pressure or thegas and vapour composition within avessel.

In addition, the measurement accu-racy is unaffected by changes in densi-ty, conductivity and dielectric constantof the product being measured or by airmovement above the product.

The practical use of microwaveradar for tank gauging and process ves-sel level measurement introduces aninteresting set of technical challengesthat have to be mastered.

If we consider that the speed of lightis approximately 300,000 kilometresper second. Then the time taken for

a radar signal to travel one metreand back takes 6.7 nanoseconds or0.000 000 006 7 seconds.

How is it possible to measure thistransit time and produce accurate ves-sel contents information?

Currently there are two measure-ment techniques in common use forprocess vessel contents measurement.They are frequency modulated continu-ous wave (FM - CW) radar and PULSEradar

In this chapter we explain FM - CWand PULSE radar level measurementand compare the two techniques. Wediscuss accuracy and frequency consid-erations and explore the technicaladvances that have taken place inrecent years and in particular two wire,loop powered transmitters.

47

4. Radar level measurement

radar_applied_to_level_rb.qxd 15.01.2007 18:46 Seite 47

The FM - CW radar measurementtechnique has been in use since the1930's in military and civil aircraftradio altimeters. In the early 1970's thismethod was developed for marine usemeasuring levels of crude oil in super-tankers. Subsequently, the same tech-nique was used for custody transferlevel measurement of large land basedstorage vessels. More recently, FM -CW transmitters have been adapted forprocess vessel applications.

FM - CW, or frequency modulatedcontinuous wave, radar is an indirectmethod of distance measurement. Thetransmitted frequency is modulatedbetween two known values, f1 and f2,and the difference between the trans-mitted signal and the return echosignal, fd, is measured. This differencefrequency is directly proportional to the

transit time and hence the distance.(Examples of FM - CW radar leveltransmitters modulation frequencies are8.5 to 9.9 GHz, 9.7 to 10.3 GHz and 24to 26 GHz).

The theory of FM - CW radar issimple. However, there are many prac-tical problems that need to beaddressed in process level applications.

An FM - CW radar level transmitterrequires a voltage controlled oscillator,VCO, to ramp the signal between thetwo transmitted frequencies, f1 and f2.It is critical that the frequency sweep iscontrolled and must be as linear as pos-sible. A linear frequency modulation isachieved either by accurate frequencymeasurement circuitry with closed loopregulation of the output or by carefullinearisation of the VCO output includ-ing temperature compensation.

48

FM-CW, frequency modulated continuous wave fre

quen

cy

f2

f1t1

t

fd

time

Transmitted signal

Receivedsignal

Fig 4.1 The FM - CW radar technique is an indirect method of level measurement.fd is proportional to t which is proportional to distance


49


!

"

!

!

#!

!

#!

Fig

4.2

Typ

ical

blo

ck d

iagr

am o

f FM

- CW

rada

r. A

very

acc

urat

e lin

ear s

weep

is re

quire

d

FM - CW block diagram (Fig 4.2)The essential component of a fre-

quency modulated continuous waveradar is the linear sweep control cir-cuitry. A linear ramp generator feeds avoltage controller which in turn rampsup the frequency of the VoltageControlled Oscillator. A very accuratelinear sweep is required. The outputfrequency is measured as part of theclosed loop control.

The frequency modulated signal isdirected to the radar antenna and

hence towards the product in the ves-sel. The received echo frequencies aremixed with a part of the transmissionfrequency signal. These difference fre-quencies are filtered and amplifiedbefore Fast Fourier Transform (FFT)analysis is carried out. The FFTanalysis produces a frequency spec-trum on which the echo processing andecho decisions are made.

Simple storage applications usuallyhave a large surface area with very lit-tle agitation, no significant false echoesfrom the internal structure of the tankand relatively slow product movement.These are the ideal conditions forwhich FM - CW radar was originallydeveloped.

However, in process vessels there ismore going on and the problemsbecome more challenging.

Low amplitude signals and falseechoes are common in chemical reac-tors where there is agitation and lowdielectric liquids.

Solids applications can be trouble-some because of the internal structureof the silos and undulating product sur-faces which creates multiple echoes.

An FM - CW radar level sensortransmits and receives signals simulta-neously.

50

Pic 2 Typical glass linedagitated processvessel. A radarmust be able tocope with variousfalse echos fromagitatior bladesand baffles


51


In an active process vessel, the vari-ous echoes are received as frequencydifferences compared with the frequen-cy of the transmitting signal. These fre-quency difference signals are receivedby the antenna at the same time. Theamplitude of the real echo signals aresmall compared with the transmittedsignal. A false echo from the end of theantenna may have a significantly high-er amplitude than the real level echo.The system needs to separate and iden-tify these simultaneous signals beforeprocessing the echoes and making anecho decision.

The separation of the variousreceived echo frequencies is achievedusing Fast Fourier Transform (FFT)analysis. This is a mathematical proce-

dure which converts the jumbled arrayof difference frequencies in the timedomain into a frequency spectrum inthe frequency domain.

The relative amplitude of each fre-quency component in the frequencyspectrum is proportional to the size ofthe echo and the difference frequencyitself is proportional to the distancefrom the transmitter.

The Fast Fourier Transform requiressubstantial processing power and is arelatively long procedure.

It is only when the FFT calculationsare complete that echo analysis can becarried out and an echo decision can bemade between the real level echo and anumber of possible false echoes.

Fig 4.3a FM - CW radar level transmitters in an active process vessel

Transmitted signal

f2

fd1, -f-fd2d2, -fd3, -fd4, -fd5

f1t1

Real echo signal

False echo signals


52

Fig 4.3b combined echo frequencies are received simultaneously

Fig 4.3c The individual frequencies must be separated fromthe simultaneously received jumble of frequencies

Sign

al a

mpl

itude

Sign

al a

mpl

itude

Mixture of frequencies received by FM - CW radar

Combination of mixed difference frequencies received by FM - CW radarIndividual difference frequencies fd1, ffd2d2, fd3, are shown

fd1, fd2, fd3, fd4, fd5 etc combined


Complex process vessels and solidsapplications can prove too difficult forsome FM - CW radar transmitters.Even a simple horizontal cylindricaltank can pose a serious problem. Thisis because a horizontal tank producesmany large multiple echoes that arecaused by the parabolic effect of thecylindrical tank roof. Sometimes theamplitudes of the multiple echoes are

higher than the real echo. The proces-sors that carry out the FFT analysis areswamped by different amplitude sig-nals across the dynamic range all at thesame time. As a result, the FM - CWradar cannot identify the correct echo.

As we shall see, this problem doesnot affect the alternative pulse radartechnique.

53


Fig 4.4 FM - CW frequency spectrum after Fast Fourier transform. The Fast Fouriertransform algorithm converts the signals from the time domain into the frequencydomain. The result is a frequency spectrum of the difference frequencies. Therelative amplitude of each frequency component in the spectrum is proportional tothe size of the echo and the difference frequency itself is proportional to thedistance from the transmitter. The echoes are not single frequencies but a spanof frequencies within an envelope curve

Frequency spectrum echoesEach echo is within an envelope curve of frequencies

ampl

itude

frequency


PULSE radar level transmitters

54

Pulse radar level transmitters pro-vide distance measurement based onthe direct measurement of the runningtime of microwave pulses transmittedto and reflected from the surface of theproduct being measured.

Pulse radar operates in the timedomain and therefore it does notrequire the Fast Fourier transform(FFT) analysis that characterizes FM -CW radar.

As already discussed, the runningtime for a distance of a few metres ismeasured in nanoseconds. For this rea-son, a special time transformation pro-

cedure is required to enable these shorttime periods to be measured accurately.The requirement is for a slow motionpicture of the transit time of themicrowave pulses with an expandedtime axis. By slow motion we meanmilliseconds instead of nanoseconds.

Pulse radar has a regular and period-ically repeating signal with a high pulserepetition frequency (PRF). Using amethod of sequential sampling, theextremely fast and regular transit timescan be readily transformed into anexpanded time signal.

Fig 4.5 Pulse radar operates purely within the time domain. Millions of pulses aretransmitted every second and a special sampling technique is used to produce atime expanded output signal


A common example of this principleis the use of a stroboscope to slowdown the fast periodic movements ofrotating or reciprocating machinery.

Fig 4.7 shows how the principle of

sequential sampling is applied topulse radar level measurement. Theexample shown is a VEGAPULS trans-mitter with a microwave frequency of5.8 GHz.

55


To illustrate this principle, considerthe sine wave signal in Fig 4.6. It is aregular repeating signal with a periodof T1. If the amplitude (voltage value)of the output of the sine wave is sam-pled into a memory at a time period T2

which is slightly longer than T1, then atime expanded version of the originalsine wave is produced as an output.The time scale of the expanded outputdepends on the difference between thetwo time periods T1 and T2.

Fig 4.6 The principle of sequential sampling with a sine wave as an example.The sampling period, T2, is very slightly longer than the signal period, T1. Theoutput is a time expanded image of the original signal

Fig 4.7 Sequential sampling of a pulse radar echo curve. Millions of pulses per secondproduce a periodically repeating signal. A sampling signal with a slightly longerperiodic time produces a time expanded image of the entire echo curve

Periodic Signal(sine wave)

Periodic Signal(radar echoes)

Samplingsignal

Samplingsignal

Expandedtime signal

T1

T2

T1

Emission pulse

Echo pulse

T2


ExampleThe 5.8 GHz, VEGAPULS radar level transmitter has the following pulse repeti-tion rates.

Transmit pulse 3.58 MHz T1 = 279.32961 nanosecondsReference pulse 3.58 MHz - 43.7 Hz T2 = 279.33302 nanoseconds

56

Therefore the time expansion factoris 81920 giving a time expanded pulserepetition period of 22.88 milliseconds.

There is a practical problem in sam-pling the emission / echo pulse signalsof a short (0.8 nanosecond) pulse at 5.8GHz. An electronic switch would needto open and close within a few picosec-onds if a sufficiently short value of the5.8 GHz sine wave is to be sampled.These would have to be very specialand expensive components.

The solution is to combine sequen-tial sampling with a cross correlationprocedure.

Instead of very rapid switch sam-pling, a sample signal of exactly thesame profile is generated but with aslightly longer time period between thepulses.

Fig 4.9 compares sequential sam-pling by rapid switching with sequen-tial sampling by cross correlation witha sample pulse.

This periodically repeating signalconsists of the regular emission pulseand one or more received echo pulses.These are the level surface and anyfalse echoes or multiple echoes. Thetransmitted pulses and therefore thereceived pulses have a sine wave formdepending upon the pulse duration. A5.8 GHz pulse of 0.8 nanosecond dura-tion is shown in Fig 4.8.

The period of the pulse repetition isshown as T1 in Fig 4.7. Period T1 is

the same for the emission pulse repeti-tion as for any echo pulse repetition asshown.

However, the sampling signalrepeats at period of T2 which is slight-ly longer in duration than T1. This isthe same time expansion procedure bysequential sampling that has alreadybeen described for a sine wave. Thefactor of the time expansion is deter-mined by T1 / (T2-T1).

Fig 4.8 Emission pulse (packet).The wave form of the 5.8GHz pulse with a pulseduration of 0.8nanoseconds


Instead of taking a short voltagesample, cross correlation involves mul-tiplying a point on the emission or echosignal by the corresponding point onthe sample pulse. The multiplicationleads to a point on the resultant signal.All of these multiplication results, oneafter the other, lead to the formation ofthe complete multiplication signal.

Fig 4.10 shows a short sequence ofmultiplications between the receivedsignal (E) and the sampling pulsesignal (M). The resultant E x M curvesare shown on page 58.

Then the E x M curve is integratedand represented on the expanded curveas a dot. The sign and amplitude of the

signal on the time expanded curvedepends on the sum of the area of theE x M curve above and below the zeroline. The final integrated value corre-sponds directly to the time position ofthe received pulse E relative to thesample pulse M.

The received signal E and samplesignal M in Fig 4.10 are equivalent tothe periodic signal (sine wave) andsample signal in Fig 4.6. The result ofthe integration of E x M in Fig 4.10 isdirectly analogous to the expandedtime signal in Fig 4.6.

57


Fig 4.9 Comparison of switch sampling with cross correlation sampling. The pulseradar uses cross correlation with a sample pulse. This means that rapid picosec-ond switching is not required

Sampling with picosecond switching

Sample signal

Emission / Echo pulse

Sampling by cross correlation with asample pulse


The pulse radar sampling procedureis mathematically complicated but atechnically simple transformation toachieve. Generating a reference signalwith a slightly different periodic time,multiplying it by the echo signal andintegration of the resultant product areall operations that can be handled easi-ly within analogue circuits. Simple, butgood quality components such as diodemixers for multiplication and capaci-tors for integration are used.

This method transforms the highfrequency received signal into an accu-rate picture with a considerablyexpanded time axis. The raw valueoutput from the microwave module isan intermediate frequency that is simi-lar to an ultrasonic signal. For examplethe 5.8 GHz microwave pulse becomesan intermediate frequency of 70 kHz.The pulse repetition frequency (PRF)of 3.58 GHz becomes about 44 Hz.

58

Fig 4.10 Cross correlation of the received signal E and the sampling M.The product E x M is then integrated to produce the expanded time curve. Thetechnique builds a complete picture of the echo curve

E

IntegralE x M

max

min

0

M

E x M


59

4. Radar level measurementa

mpl

itude

transmit pulse

t1 tt22 t3 t4 t5

time

Pulse radar operates entirely withinthe time domain and does not need thefast and expensive processors thatenable the FM - CW radar to function.There are no Fast Fourier Transform(FFT) algorithms to calculate. All ofthe pulse radar processing is dedicatedto echo analysis only.

Part of the pulse radar transmissionpulse is used as a reference pulse thatprovides automatic temperature com-pensation within the microwave mod-ule circuits.

The echoes derived from a pulseradar are discrete and separated in time.This means that pulse radar is betterequipped to handle multiple echoes andfalse echoes that are common inprocess vessels and solids silos.

Pulse radar takes literally millions ofshots every second. The return echoesfrom the product surface are sampledusing the method described above. Thistechnique provides the pulse radar withexcellent averaging which is particular-ly important in difficult applicationswhere small amounts of energy arebeing received from low dielectric andagitated product surfaces.

The averaging of the pulse techniquereduces the noise curve to allow small-er echoes to be detected. If the pulseradar is manufactured with welldesigned circuits containing good qual-ity electronic components they candetect echoes over a wide dynamicrange of about 80 dB. This can makethe difference between reliable andunreliable measurement.

Fig 4.12 With a pulse radar, all echoes (real and false) are separated in time. This allowsmultiple echoes caused by reflections from a parabolic tank roof to be easilyseparated and analysed

Pulse echoes in a process vessel are separated in time


60

Fig

4.11

Blo

ck d

iagr

am o

f PUL

SE ra

dar m

icrow

ave m

odule


61


Pulse block diagram - (Fig 4.11)The raw pulse output signal (inter-

mediate frequency) from the pulseradar microwave module is similar, infrequency and repetition rate, to anultrasonic signal.

This pulse radar signal is derived inhardware. Unlike FM - CW radar,PULSE does not use FFT analysis.Therefore, pulse radar does not needexpensive and power consumingprocessors.

The pulse radar microwave modulegenerates two sets of identical pulseswith very slightly different periodictimes. A fixed oscillator and pulse for-mer generates pulses with a frequencyof 3.58 MHz. A second variable oscil-lator and pulse former is tuned to a

frequency of 3.58 MHz minus 43.7 Hzand hence a slightly longer periodictime. GaAs FET oscillators are used toproduce the microwave carrier fre-quency of the two sets of pulses.

The first set of pulses are directedto the antenna and the product beingmeasured. The second set of pulses arethe sample pulses as discussed in thepreceding text.

The echoes that return to the anten-na are amplified and mixed with thesample pulses to produce the raw, timeexpanded, intermediate frequency.

Part of the measurement pulse sig-nal is used as a reference pulse thatprovides automatic temperature com-pensation of the microwave moduleelectronics.

Pic 3 Two wire pulse radar level transmitter mounted in a process reactor vessel


Process radar level transmittersoperate at microwave frequenciesbetween 5.8 GHz and about 26 GHz.Manufacturers have chosen frequenciesfor different reasons ranging fromlicensing considerations, availability ofmicrowave components and perceivedtechnical advantages.

There are arguments extolling thevirtues of high frequency radar, low

frequency radar and every frequencyradar in between.

In reality, no single frequency isideally suited for every radar levelmeasurement application. If we com-pare 5.8 GHz radar with 26 GHz radar,we can see the relevant benefits of highfrequency and low frequency radar.

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Choice of frequency

Fig 4.14 Comparison of 5.8 GHz and 26 GHz radar antenna sizes. These instrumentshave almost identical beam angles. However this is not the full picture when itcomes to choosing radar frequencies

2.6 GHz

5.8 GHz


The higher the frequency of a radarlevel transmitter, the more focused thebeam angle for the equivalent sizeantenna.

With horn antennas, this allowssmaller nozzles to be used with a morefocused beam angle.

For example, a 1" (40 mm) hornantenna radar at 26 GHz has approxi-mately the same beam angle as a 6"(150 mm) horn antenna at 5.8 GHz.

However, this is not the completepicture. Antenna gain is dependent onthe square of the diameter of the anten-na as well as being inversely propor-tional to the square of the wavelength.

Antenna gain is proportional to:-

Antenna gain also depends on the aper-ture efficiency of the antenna.Therefore the beam angle of a smallantenna at a high frequency is notnecessarily as efficient as the equiva-lent beam angle of a larger, lower fre-quency radar. A 4" horn antenna radarat 6 GHz gives excellent beam focus-ing.

A full explanation of antenna gainand beam angles at different frequen-cies is given in Chapter 5 on radarantennas.

63


Focusing at different frequencies

5 GHz 10 GHz 15 GHz 20 GHz 25 GHz

Fig 4.13 For a given size of antenna, a higher frequency gives a more focused beam

Antenna size - beam angle

diameterwavelength

2

2


A 26 GHz beam angle is morefocused but, in some ways, it has to be.

The wavelength of a 26 GHz radar isonly 1.15 centimetres compared with awavelength of 5.2 centimetres for a5.8 GHz radar.

The short wavelength of the 26 GHzradar means that it will reflect off many

small objects that may be effectivelyignored by the 5.8 GHz radar. Withoutthe focusing of the beam, the high fre-quency radar would have to cope withmore false echoes than an equivalentlower frequency radar.

Antenna focusing and false echoes

Fig 4.15 a Low frequency radar has a wider beamangle and therefore, if the installationis not optimum, it will see more falseechoes. Low frequencies such as5.8 GHz or 6.3 GHz tend to be moreforgiving when it come to false echoesfrom the internal structure of a vesselor silo

Fig 4.15 b High frequency radar has a muchnarrower beam angle for a givenantenna size. The narrower beam angleis important because the shortwavelength of the higher frequencies,such as 26 GHz, reflect more readilyfrom the internal structures such aswelds, baffles, and agitators.The sharper focusing avoids thisproblem

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High frequency radar transmittersare susceptible to signal scatter fromagitated surfaces. This is due to the sig-nal wavelength in comparison to thesize of the surface disturbance.

The high frequency radar willreceive considerably less signal than anequivalent 5.8 GHz radar when the liq-

uid surface is agitated. The lowerfrequency transmitters are less affectedby agitated surfaces.

It is important that, whatever

Radar Book Chapter1-5

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