Embed Size (px)
Citation preview
������������������ ������������������ ����������������������������
�������
�����������������
���
����
�������
�����������������
���
����
���������������
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 user´s guide1. Radar2. Title621.3´848
ISBN 0-9538920-0-X
Cover by LinkDesign, Schramberg.Printed in Great Britain at VIP print, Heathfield, Sussex.
written byPeter Devine
additional informationKarl Grießbaum
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
The function of an antenna in a radarlevel transmitter is to direct the maxi-mum amount of microwave energytowards the level being measured andto capture the maximum amount ofenergy from the return echoes foranalysis within the electronics.
Antennas for level measurementcome in five basic forms:
Horn antennas and dielectric rodantennas are already commonly usedwithin process level measurement. Wewill be discussing how these designshave been developed for increasinglyarduous process conditions and howantenna efficiencies have beenimproved. The horn antenna and ver-sions of the dielectric rod antenna arealso used in measuring tube applica-tions within the process industry.
Parabolic antennas and planar arrayantennas have been applied to fiscalmeasurement radar systems rather thanfor level measurement within processvessels. We will discuss the design ofthese antennas although at present theiruse in process vessels is limited.
Antenna basicsAn important aspect of an antenna isdirectivity. Directivity is the ability ofthe antenna to direct the maximumamount of radiated microwave energytowards the liquid or solid we wish tomeasure.
No matter how well the antenna isdesigned, there will be somemicrowave energy being radiated inevery direction. The goal is to max-imise the directivity.
Fig 5.1 shows the pattern of radiatedenergy from a typical horn antenna.This is a 250 mm (10") horn antennaoperating at a frequency of 5.8 GHz.
The measurements are made somedistance from the antenna in what iscalled the far field zone. It is clear thatmost of the energy is contained withinthe main lobe, but also there is a rea-sonable amount of energy containedwithin the various side lobes.
Technical information and sales lit-erature on radar level transmittersquote beam angles for different anten-nas. Clearly there is not a tight beam.The convention is to measure the angleat which the microwave energy hasreduced to 50 percent of the value atthe central axis of the beam. This is quoted in decibels:- the - 3dB point.
77
5. Radar antennas
· Horn (cone) antenna· Dielectric rod antenna· Measuring tube antenna
(stand pipes/ bypass tubes etc.)· Parabolic reflector antenna· Planar array antenna
78
Extent of measured microwave energy showingmain lobe and side lobes
The - 3 dB point is the beam angle i.e. the energyhas reduced to 50%
Side lobe energy
Fig 5.1 Typical radiation pattern from a radar level transmitter
0
30
60
90
120
150
30
60
90
120
150
180
main lobe directionangular width (3dB)side lobe suppression
Max.:
Farfield E_Abs (Theta); Phi=90,0 deg.
:::
0 10 20 30
0,0 deg.14,9 deg.21,6 dB
20,4 dB
Radiation patterns of different antennas and radar frequencies are compared at theend of this chapter.
A measure of how well the antennais directing the microwave energy iscalled the ‘antenna gain’.
Antenna gain is a ratio between thepower per unit of solid angle radiated
by the antenna in a specific direction tothe power per unit of solid angle if thetotal power was radiated isotropically,that is to say, equally in all directions.
5. Radar antennas
79
Fig 5.2 Illustration of antenna gain
Isotropic equivalent with total powerradiating equally in all directions
Directional power from antenna
Antenna gain ‘G’ can be calculated as follows:
The aperture efficiencies of radarlevel antennas are typically betweenη = 0.6 and η = 0.8.
It is clear from equation 5.1 thatthe directivity improves in proportionto the antenna area. At a given fre-quency, a larger antenna has a narrow-er beam angle
Where η = aperture efficiency
D = antenna diameter.*
A = antenna area.*
λ = microwave wavelength *
* must be same units
isotropic power
directional power
G = =2
2η x η xπ x D 4π x Aλ λ
[Eq. 5.1]
( )
Also, we can see that the antennagain and hence directivity is inverselyproportional to the square of the wave-length.
For a given size of antenna the beamangle will become narrower at higherfrequencies (shorter wavelengths). Forexample the beam angle of a 5.8 GHzradar with a 200 mm (8") horn antennais almost equivalent to a 26 GHz radarwith a 50 mm (2") horn antenna. This
means that a 26 GHz antenna is lighterand easier to install for the same beamangle. However, as discussed inChapter 4, this is not the whole storywhen choosing the right transmitter foran application.
For a standard horn antenna thebeam angle φ, that is the angle to theminus 3 dB position, can be calculatedusing equation 5.2.
80
Fig 5.3 Graph showing relation between horn antenna diameter and beam angle for5.8 GHz, 10Ghz and 26GHz radar
The following graph shows horn anten-na diameter versus beam angle for the
most common radar frequencies,5.8 GHz, 10 GHz and 26 GHz.
050 75 100 125 150 175 200 225 250
5.8 GHz
10 GHz
26 GHz
20
40
60
80
φ = 70° xλD
[Eq. 5.2]
Antenna beam angles (diameter / frequency)
beam
ang
le in
deg
rees
(-3
dB)
antenna diameter, mm
Beam angle
The metallic horn antenna or coneantenna is well proven for process levelapplications. The horn is mechanicallyrobust and in general it is virtuallyunaffected by condensation and prod-uct build up, especially at the lowerradar frequencies such as 5.8 GHz.
There are variations in the internaldesign of horn antennas. Themicrowaves that are generated withinthe microwave module are transmitteddown a high frequency cable for encou-pling into a waveguide. The metalwaveguide then directs the microwavestowards the horn of the antenna. A lowdielectric material such as PTFE,ceramic or glass is often used withinthe waveguide.
At the transition from the wave-guide to the horn of the antenna the lowdielectric material is machined to apointed cone. The angle of this conedepends on the dielectric constant ofthe material. For example, ceramic hasa sharper angle than PTFE.
The microwaves are emitted fromthis pointed cone in a controlled wayand are then focused towards the targetby the metal horn.
After reflection from the productsurface, the returning echoes arecollected within the horn antenna forprocessing within the electronics.
5. Radar antennas
81
Fig 5.4 The transition ofmicrowaves from the lowdielectric waveguide into themetallic horn where they arefocused towards the productbeing measured
1. Horn antennas
82
In this first design of horn antennathe HF cable signal coupling is into anair filled waveguide with a rectangularcross section. The microwaves aredirected towards the antenna. There is atransition from rectangular to circularcross section. At this point the wave-guide changes to PTFE with a ¼ wave-length step design. The waveguide isthen glass filled until it reaches theinside of the antenna horn where itchanges to a PTFE cone for the imped-ance matching into the vapour space inthe horn
This PFTE cone in combination withthe metallic horn focuses themicrowaves towards the target.
An antenna of this design is capableof withstanding process temperaturesup to 250° C and up to 300 Bar.
A potential problem with the designis the sealing between the PTFE andglass on the process side. The thermalexpansion of glass and PTFE are differ-ent and it is possible for condensationto get between the glass and PTFE andto affect the transmission and receipt ofthe microwave signals.
The explosion proof design requiresmetallic grid around the glass of thewaveguide at the joint between thehousing casting and the flange casting.
Horn antenna design 1Fig 5.5
1. HF Cable
2. Signal coupling
4. PTFE transition
5. Glass waveguide
6. Metallic grid
7. Seal between glassand PTFE
8. PTFE cone
9. Metal horn antenna
3. Waveguide (air filled)
Transition rectan-gular to circularcross section
1
234567
8
9
5. Radar antennas
83
With this antenna design, the HFcable is encoupled into the PTFE mate-rial inside the waveguide. The metalwaveguide is welded to the flange andthere are two process seals between themetal waveguide and the PTFE. Theseseals protect the signal coupler fromthe process. This seal material can beViton for stainless steel horn antennasor Kalrez for Hastelloy C horn anten-nas.
There is a continuous transition forthe microwaves within a single piece ofPTFE which is machined into a cone
form for the transition into the hornantenna. The PTFE cone and the metal-lic conical horn focus the microwavesand collect the return signals in theusual manner.
An antenna of this design is capableof withstanding a process temperatureof 200° C + and a process pressure of40 Bar.
This antenna design can also be usedon very high temperature, ambientpressure applications with air or nitro-gen gas cooling of the antenna.
Horn antenna design 2Fig 5.6
1. HF cable
2. Signal coupling
3. Waveguide(PTFE filled)
5. PTFE cone
4. Process seals Vitonor Kalrez
6. Metallic hornantenna
1
234
5
6
84
Horn antenna design 2aFig 5.7 Very high temperature, ambient pressure applications.
Air/nitrogen cooling through flange
This adaptation of the previousantenna allows the antenna to be cooledwith air or nitrogen gas.
This is achieved by drilling twoholes, 180° apart, laterally from theflange edge into the horn antenna nextto the PTFE cone. The flow of air ornitrogen prevents hot gases fromaffecting the PTFE and the viton sealand it effectively cools the entire flangeand horn area.
This technique has been used suc-cessfully with very high temperatures,including 1500° C + in the steel indus-try with applications such as blast
furnace burden level and molten ironladle levels. The microwaves are unaf-fected by the air movement within thehorn area.
In addition to cooling, this air purg-ing technique is also used for solidsapplications where very high levels ofconductive dust, such as carbon, heavi-ly coat the inside of the horn and causesignal attenuation.
Water purging has also been usedwhere heavy product build up isexpected.
1. HF cable
2. Signal coupling
3. Waveguide(PTFE filled)
5. Metallic hornantenna
4. Tappings forair/nitrogen keepsantenna area cool
1
2
3
4
Air / N2
5
5. Radar antennas
85
Horn antenna design 3
This antenna is also a developmentof the antenna design in Fig 5.6.
The waveguide, PTFE transitioncone and process flange are standard.The face of the flange is all PTFE.
The difference is in the applicationof a special enamel (glass) coated hornthat provides excellent process materi-als compatibility without resorting tomore expensive metals such asTantalum.
The external dimensions of theantenna represent a simple cylinder.The internal dimensions of the antennaare identical to a standard horn antenna(150 mm (6")) is illustrated. At the bot-tom of the antenna there is a gradual lip
between the external cylinder and theinternal horn.
The top of the cylinder has a flangefor sealing between the PTFE transitioncone and the process flange and alsobetween the glassed antenna and thevessel nozzle. External studs hold theenamel antenna to the process flangeand PTFE seals are used to provideinternal sealing.
The antenna is manufactured fromcarbon steel with blue enamel coatingwhich is identical to the enamel foundin glass lined vessels. It provides theefficiency benefits of a horn antennawith first class materials compatibility.
Fig 5.8 Special enamel coated antenna
2. PTFE waveguide
1. Signal coupling
3. PTFE flange face
5. Lapped flange
4. PTFE seal
6. Steel internals ofhorn antenna
7. Enamelled coating
12
345
6
7
The above antenna has beendesigned with both high temperatureand high pressure in mind. Themechanical strength and sealing abilityof PTFE degrades at elevated tempera-ture and is therefore limited to about200° C.
This special design of radar hasa chemically and thermally stableceramic (Al2O3) waveguide within astainless steel or Hastelloy C hornantenna and flange. The ceramicwaveguide is fused to a ‘vacon’ steelbush using a special brazing technique.‘Vacon’ is used because it has acoefficient of thermal expansion that issimilar to ceramic, whereas normal
stainless steel expands more than twiceas much as ceramic. A double graphiteseal is fitted on the process side of the‘vacon’ bush. The entire waveguideassembly is laser welded to ensure thatthe transmitter is gas tight and thatdifferential thermal expansion isnegligible.
In order to withstand constant pro-cess temperatures of 400° C, the elec-tronics housing of the radar is mechani-cally isolated from the high processtemperature by a temperature extensiontube. The microwave module is con-nected via the HF cable and an aircoaxial tube to the signal coupler in theceramic waveguide.
86
Horn antenna design 4Fig 5.9 High temperature / high pressure antenna with ceramic waveguide
1. Connection to HFcable frommicrowave module
2. Coaxial tube tosignal coupling
3. Signal coupling inceramic waveguide
4.Vacon/ceramicbrazing seal
5. Graphite seal
6. Ceramic waveguidecone
1
2
3
4
5
6
5. Radar antennas
87
1. HF cable (coaxial)
2. Signal coupling
3. Ceramic waveguide
4. Brazing of ceramicto vacon
5. Vacon bush
6. Graphite seal
7. Metallic hornantenna
Fig 5.11 This antenna design is capableof with standing 160 Bar at400° C with dual graphite seals.Graphite seals have proved to besuperior to tantalum seals
Ceramic signal coupling
Vacon/ceramic brazing
Graphite / Tantalum seal
Fig 5.10 Close up of ceramic waveguide assembly
1
23
4
5
6
7
Another possible variation of a hornantenna radar is measurement througha low dielectric window. We have dis-cussed Hastelloy, Tantalum and thespecial enamel coated horn antenna.However, if a liquid is being measured and it is conductive or has a dielectric
constant of more that εr = 10, then it ispossible to measure through a lowdielectric window or lens.
Some antennas are manufacturedwith a PTFE window as part of theconstruction.
88
Fig 5.12 Horn antenna radar is constructed with a metal housing around the antennaand a PTFE process ‘window’
Fig 5.13 Variations of this design include the use of cone shaped windows. The cone canpoint towards the horn or towards the process
Adapting horn antenna radars
a. Measurement through a PTFE window
Antenna housing
Horn antenna
Process flange
PTFE window
b. Horn antenna -waveguide extensionIn the first section of Chapter 6,
Radar level installations, we discusshow horn antenna radars should beinstalled. It is recommended that theend of the antenna is a minimum of10 mm inside the vessel. A 150 mm(6") horn antenna is 205 mm (8") long.
If the nozzle is longer than 200 mm,we should consider a waveguide exten-sion piece between the radar flange andthe horn antenna. Waveguide exten-sions should only be used with highlyreflective products.
c. Horn antenna -bent waveguide extensionsAs well as simple waveguide exten-
sions it is possible to bend waveguideextensions in order to avoid obstruc-tions or to utilise side entry flanges.
A simple 90° bend or an ‘S’ shapedextension tube are possible.
The waveguide extensions should befree from any internal welds and theminimum radius of curvature should be200 mm.
5. Radar antennas
89
Fig 5.15 Waveguide extensionswith bends. The directionof the polarization isimportant
Waveguide extension with 90° bend
Waveguideextension with ‘S’bend
Fig 5.14 Extended waveguide hornantenna to enable measurementin long nozzles or through aconcrete tank or sump roof
The majority of antennas in thischapter are designed for microwavefrequencies of between 5.8 GHz and10 GHz. Later in this chapter, we dis-cuss the use of radar in measuringtubes where there is a minimum criticaldiameter for each frequency. A measur-ing tube is a waveguide. The minimumtheoretical tube diameter for a 5.8 GHzradar is 31 mm.
At a higher frequency the minimumdiameter of a waveguide is smaller.
At this minimum diameter, themicrowaves are established within thewaveguide with a single mode andhence a single velocity.
As the waveguide diameter increas-es in size, more modes become estab-lished for the given frequency.
Measurement problems will beencountered if there are multiple modeswithin an antenna waveguide. This isbecause with different modes themicrowaves travel at different veloci-ties in the waveguide and therefore asingle target will reflect more than onereturn echo. Measurement will becomeinaccurate or impossible.
For this reason, the encoupling of ahigh frequency radar must be made intoa small waveguide. The small wave-guide assemblies of high frequencyradar are susceptible to contaminationby condensation and build up whencompared with lower frequencies suchas 5.8 GHz.
A special patented high frequencyantenna design from VEGA minimisesthe potential problems associated withsmall waveguide assemblies.
The encoupling is made within asmall PTFE waveguide to establish asingle mode. As the microwaves traveltowards the horn antenna, there is acarefully designed transition thatincreases the diameter of the PTFEwaveguide while maintaining the singlemode.
The increased diameter of the PTFEwaveguide reduces the adverse effectsof condensation and build up where thetapered cone of the waveguide entersthe metallic horn of the antenna.
Compare this design with hornantenna design 2, Fig 5.6. The 5.8 GHzradar does not need a transition in thewaveguide diameter and the angle ofthe metallic horn is not as sharp as forthe high frequency radar.
Viton or Kalrez process seals are fit-ted between the PTFE and stainlesssteel body of the waveguide.
Extended versions of the highfrequency antenna design involvelengthening the HF cable within astainless steel extension tube and weld-ing the waveguide assembly to the endof the extension tube.
90
High frequency radar antennas
5. Radar antennas
91
Fig 5.16 High frequency (26GHz) horn antenna design
1
2
3
4
5
6
1. HF cable frommicrowave module
2. Signal coupling intosmaller diameter PTFEwaveguide assembly
4. Viton or Kalrez processseals between PTFE andstainless steel of thewaveguide
5. Cone shape of PTFEwaveguide for thetransition into themetallic horn of theantenna
6. Metallic horn antennaof high frequency radar.It has a sharper anglethan the lower frequencyradars
3. Carefully designedtransition from smalldiameter to largerdiameter withoutaffecting the waveguidemode
92
The dielectric rod antenna is anextremely useful option when applyingradar level technology to modernprocess vessels. Dielectric rods can beused in vessel nozzles as small as40 mm (1½") and they are manufac-tured from PP, PTFE or ceramic wettedparts.
This means that, normally, radarlevel transmitters can be retro-fittedinto existing tank nozzles and theyhave low cost materials compatibilitywith most aggressive liquids includingacids, alkalis and solvents.
The design of dielectric rod antennashas been refined in recent years.Essentially the microwaves are fedfrom the microwave module through anHF cable to a signal coupler in thewaveguide. As with the horn antennathe waveguide can be air filled or filledwith a low dielectric material such asPTFE .
The waveguide feeds themicrowaves to the antenna. Themicrowaves pass down the parallelsection of the rod until they reach thetapered section of the rod. The taperedsection of the rod acts like a lens and itfocuses the microwaves towards theproduct being measured. The size andshape of the dielectric rod depends onthe frequency of the microwaves beingtransmitted.
The reflected echoes are captured ina similar fashion for processing by theradar electronics.
Rod antennas should only be usedon liquids and slurries and not on pow-ders and granular products.
There are some important considera-tions when applying rod antennaradars.
First of all, the tapered section of therod must be entirely within the vessel.
If the tapered section is in a nozzle,it will cause ‘ringing’ noise that willeffectively blind the radar. This isexplained more fully in Chapter 6.
Also, it can be seen from Fig 5.17that the microwaves rely on the rodantenna being clean. If a rod antenna iscoated in viscous, conductive and adhe-sive products, the antenna efficiencywill deteriorate very quickly.
With the horn antenna product buildup is not a particular problem.However, product build up worksagainst the reliable functioning of a rodantenna radar.
2. Dielectric rod antennas
5. Radar antennas
93
The microwaves travel down the inactiveparallel section of the rod towards thetapered section .
The tapered section of the rod focuses themicrowaves toward the liquid beingmeasured .
It is very important that all of the taperedsection of the rod must be inside the vessel
It is not good practice to allow a rodantenna to be immersed in the product
If a rod antenna is coated in viscous,conductive and adhesive product, theantenna efficiency will deteriorate
Fig 5.17 Dielectric rod antenna
94
This rod antenna is a simple and lowcost design that provides a radar leveltransmitter with good materials com-patibility. It is ideal for vented and lowpressure vessels such as acid and alkalitanks. It is designed for use in short1½" BSP / NPT process nozzles. Thenozzle height should not exceed 60 mm(2½").
The process connection is a 1½"PVDF boss and the antenna ispolypropylene (PP) or PTFE.
The HF cable from the microwavemodule is coupled into PTFE/PP insidea metallic tube that acts as a wave-guide. This metallic tube is totallyenclosed within the PTFE/PP parallelsection of the antenna. The microwavespass down the metallic waveguidedirectly to the tapered section of theantenna where they are focusedtowards the product being measured.
Rod antenna design 1
1. HF cable
Fig 5.18 Rod antenna for short process nozzles
2. Process connectionPVDF boss
3. Signal couplingwithin PTFE/PPfilled waveguide
4. Inactive sectionwith metallic wave-guide, PTFE/PPinner and outerparts
5. Solid PTFE/PPactive taperedsection of antennafocuses themicrowaves towardsthe product surface
123
4
5
5. Radar antennas
95
With this design of rod antenna thesignal coupling is into an air filledwaveguide. The microwaves are direct-ed towards the antenna. There is a tran-sition to PTFE via a cone shaped ele-ment. The microwaves continuethrough the PTFE waveguide to thesolid PTFE dielectric rod. The taperedsection of the rod focuses themicrowaves towards the product beingmeasured.
If this type of antenna is to be usedin a long nozzle, the parallel section ofthe solid rod is extended to ensure thatthe tapered section is entirely withinthe vessel.
An extended, solid PTFE rod anten-na can suffer from ‘ringing’ noisecaused by microwave leakage from theparallel section resonating within thenozzle. See Fig 5.20.
Rod antenna design 2Fig 5.19 Rod antenna with solid PTFE extendible rod
1. HF cable
2. Signal coupling
3. Air waveguide
4. PTFE cone
5. Process connection
7. Solid PTFE taperedsection
6. Solid PTFE parallelsection length canbe extended
1
234
5
6
7
96
In theory, the microwaves shouldtravel within the parallel section for theentire length until it reaches the taperedsection. However, in practice, some ofthe microwave energy escapes from theparallel sides.
Some solid PTFE rod antennasare supplied with screw - on extendibleantennas.
In addition to the ‘ringing’ noiseproblem described, this design can suf-fer from condensation forming betweenthe rod sections causing signalattenuation.
Also the PTFE expands at elevatedtemperatures and under certain processconditions it is possible for the rod sec-tions to detach.
The potential problems of solidPTFE rod antennas have been solvedby the latest designs. It is important tohave a completely inactive parallel sec-tion within a vessel nozzle. This isachieved by special screening or signalcoupling beyond the nozzle.
Fig 5.20 Extended rod antenna in solid PTFE. This design can suffer from ‘ringing’noise caused by leakage of microwave energy from the parallel section of thesolid PTFE rod resonating in the vessel nozzle
5. Radar antennas
97
This antenna is designed for use innozzles of either 100 mm length or250 mm length. All wetted parts of theantenna are PTFE. The parallel sectionthat is designed to be within the nozzlehas a PTFE coating on a cast metaltube.
Below this parallel section is theactive, solid PTFE, tapered antenna.
The HF cable from the microwavemodule is fed through the metal castingand the signal coupling is made justabove the tapered rod. The parallel and
tapered sections are sealed together andare designed to withstand a processtemperature of 150° C .
This antenna design is used with1½" BSP (M) stainless steel bosses orwith PTFE faced flanged transmitters.
The flanged version is designed formaximum chemical resistance to acids,alkalis and solvents. The flange face isPTFE with a tight seal between theflange PTFE and the top of the PTFEcovered inactive section.
Rod antenna design 3Fig 5.21 Extended rod antenna with inactive section and signal coupling below nozzle
level
1. HF cable
2. Rod extensioncasting(metal within PTFE)
3. Signal coupling atthe bottom of therod extension
4. Inactive section
5. Solid PTFE tapered‘active’ section ofrod antenna
4
5
1
2
3
98
For less arduous applications a stainless steel extension tube is used instead of thePTFE covered tube. The tapered section of the antenna is made of polyphenylenesulphide (PPS).
Fig 5.22 Extended rod antenna with inactive section and signal coupling below nozzlelevel. All wetted parts are PTFE on the flanged version of this antenna
Extended rod antennafor 250 mm nozzle
Extended rod antennafor 100 mm nozzle
Fig 5.23 Extended rod antenna with stainless steel inactive section and PPS rod antenna.This is for less chemically arduous process conditions
5. Radar antennas
99
This design of dielectric rod antennais for use with flanged process connec-tions.
The HF cable is connected into aPTFE filled waveguide which directsthe microwave energy towards the rodantenna. There is a PTFE male screwedfitting at the end of the waveguidewithin the process flange. The fabricat-ed, one piece, rod antenna screws on tothis connection.
The antenna flange facing and theparallel section of the antenna have car-bon impregnated PTFE wetted parts.
Inside the parallel section of the rodthere is a tubular metallic grid that acts
as an extension to the waveguide.Inside the grid the waveguide is virginPTFE, outside the grid the PTFE is car-bon impregnated.
At the end of the parallel section,there is a transition into a solid PTFEtapered rod which provides the imped-ance matching and focusing of themicrowaves towards the product beingmeasured.
This antenna has the option for100 mm or 250 mm nozzle lengths. Asalready discussed, the tapered sectionmust be entirely within the vessel.
Rod antenna design 4Fig 5.24 Extended rod antenna with metallic grid waveguide extension within carbon
impregnated PTFE inactive rod. Tapered active section of virgin PTFE
1. HF cable
2. Signal coupling
3. PTFE waveguide
4. Screwed connection
5. Carbon impregnatedPTFE antenna parallelsection and flange face
6. Internal metal grid actsas extended waveguideand prevents microwaveleakage from theparallel section of theantenna
7. PTFE waveguide
8. Virgin PTFE taperedantenna
1
2345
6
7
8
100
Rod antenna design 5Fig 5.25 This is a high temperature ceramic rod antenna design. There is temperature
separation between the electronics and the signal coupling (similar to the hightemperature horn antenna Fig 5.10). The ceramic rod has a sharper taper thanthe equivalent PTFE rod
Rod antennas are available with thedielectric rod manufactured fromceramic (Al2O3).
Ceramic has good chemical andthermal resistance. However, care must
be taken when installing ceramic rodsbecause they are brittle and prone toaccidental damage.
1
2
3
4
1. Signal coupling
2. Ceramic waveguide
3. Process seal (graphite ortantalum)
4. Active tapered ceramicrod
5. Radar antennas
101
As discussed, conical horn antennasand dielectric rod antennas are usedwidely within the process industry.
In general horn antennas aremechanically more robust and do notsuffer as much from build up or heavycondensation.
On the other hand, dielectric rodsare smaller, weigh less and can be con-structed from low cost but chemically
resistant plastics such as PTFE andpolypropylene.
However, there are applicationswithin the process industry where theinstallation of an antenna directly with-in a vessel is not suitable for reasons ofvessel design or radar functionality. Inthese cases a measuring tube (bypasstube or a stand pipe within the vessel)may be an alternative.
· Highly agitated liquid surfaces -a stilling tube ensures that theradar sees a calm surface withno scattering of the echo signal
· Low dielectric liquids such asliquefied petroleum gas (LPG) -a stand pipe concentrates andguides the microwaves to theproduct surface giving themaximum signal strength fromliquids with low levels ofreflected energy
· Toxic and dangerous chemicals -a stand pipe installation makes asmall antenna size possible.This can be used to look througha full bore ball valve into thestand pipe.The instrument can be isolatedfrom the process formaintenance
· Small vessels - stand pipes orbypass tubes can be used formeasurement in very smallprocess vessels such as vacuumreceivers. There may not beenough head space for a rodantenna or a suitable connectionfor a horn antenna. A small boretube can be used with a radar
· Foam - a stilling tube can oftenprevent foam affecting themeasurement
· Replacing existing floats anddisplacers - radar can beinstalled directly into existingbypass tubes
3. Measuring tube antennas
Bypass tube and stand pipes are used for the following reasons:
102
Horn antenna radars are most com-monly used in measuring tube levelapplications. Stilling tube internaldiameters can be 40 mm (1 ½"), 50 mm(2"), 80 mm (3"), 100 mm (4") and 150mm (6"). Larger tubes are possible.
Normally, the 40 mm and 50 mmtubes do not require a horn. The PTFEor ceramic waveguide impedancematching cone can be installed directlyinto the tube.
For 80 mm and above, the appropri-ate horn antenna is attached and this isdesigned to fit inside the tube.
As discussed in Chapter 2, Physicsof radar and Chapter 6, Radar levelinstallations, the linear polarization ofthe radar must be directed towards thetube breather hole or mixing slots, ortowards the process connections in thecase of a bypass tube.
Measuring tube radar 1 - horn antennas Fig 5.26 Installation of horn antenna radars into stand pipes or bypass tube
DN50
∅ 50 ∅ 80 ∅ 100 ∅ 150
DN80 DN100 DN150
5. Radar antennas
103
The standard length dielectric rodantennas should not be installed withinmeasuring tubes. There is a high levelof ‘ringing’ noise which severelyreduces the efficiency of the antenna.
However, a special design of short,offset rod antenna can be used on smalldiameter tubes (50 mm and 80 mm).
This design is similar in constructionto rod antenna design 3. All wettedparts are in PTFE and the short antennais off centre. This asymmetric designproduces improved signal to noiseratios within a measuring tube.
Measuring tube radar 2 - offset rod antennasFig 5.27 Offset rod antenna for use on 50 mm and 80 mm measuring tubes
1. HF cable
2. Signal coupling
3. PTFE faced flange
4. Offset short solid PTFErod antenna
1
23
4
The speed of microwaves within ameasuring tube is apparently slowerwhen compared to the velocity in freespace. The degree to which the runningtime slows down depends on the diam-eter of the tube and the wavelength ofthe signal.
The microwaves bounce off thesides of the tube and small currents areinduced in the walls of the tube. For acircular tube, or waveguide, thevelocity change is calculated by thefollowing equation :
Fig 5.28 The transit time of microwavesis slower within a stilling tube.This effect must be compensatedwithin the software of the radarlevel transmitter
cwg is the speed of microwaves inthe measuring tube / waveguide
co is the speed of light in freespace
λ is the wavelength of themicrowaves
d is the diameter of the measur-ing tube
Microwave velocity within measuring tube
cwg 1 -=2
2co xλ
1.71d( }{ )[Eq. 5.3]
104
5. Radar antennas
105
There are different modes of propa-gation of microwaves within a wave-guide. However, an important value isthe minimum diameter of pipe that willallow microwave propagation.
The value of the critical diameter,dc , depends upon the wavelength λ ofthe microwaves: The higher the fre-quency of the microwaves, the smallerthe minimum diameter of measuringtube that can be used.
Equation 5.4 shows the relationshipbetween critical diameter and wave-length. For example, 5.8 GHz has awavelength λ of ~ 52 mm. The mini-mum theoretical tube diameter isdc = 31 mm
With a frequency of 26 GHz, awavelength of 11.5 mm, the minimumtube diameter is dc = 6.75 mm. In prac-tice the diameter should be higher. Thediameter for 5.8 GHz should be at least40 mm.
Fig 5.29 Graph showing the effect of measuring tube diameter on the propagation speedof microwaves
Higher frequencies such as 26 GHzwill be more focused within largerdiameter stilling tubes. This will min-imise false echoes from the stilling tubewall.
The installation requirements ofradar level transmitters in measuringtubes are covered in the next chapter.
% s
peed
of l
ight
, c
Tube diameter / wavelength, d / λ
0.60
20
40
60
80
100
0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0
dc =λ
1.71
[Eq. 5.4]
106
The subject of this book is radarlevel measurement in process vessels.
Although they are usually applied tocustody transfer applications and notprocess vessel applications, the subjectof antennas would not be completewithout discussion of parabolic anten-nas.
The parabolic antenna is well knownto all. The parabolic form is widelyseen from satellite television dishes andradio telescopes to car headlights andtorch beams.
The main structure of a parabolicantenna is the parabolic reflector dish.This is usually of stainless steel con-struction and is designed to focus themicrowaves as accurately as possible.
The microwaves are fed through thecentre of the dish to the primary anten-na that is in front of the dish at thefocus. The microwave energy is trans-mitted from the primary antenna backtowards the parabolic dish, the sec-ondary antenna, which reflects theenergy and focuses it towards the prod-uct being measured.
Fig 5.30 Typical parabolic antenna
4. Parabolic dish antennas
1
2
34
1. Feed from microwavemodule
2. Parabolic reflector -secondary antenna
3. Primary antenna
4. Focus of parabolicreflector
5. Radar antennas
107
The reflected energy is captured bythe dish and focused back to the prima-ry antenna for echo analysis.
Parabolic antennas are used widelyin custody transfer applications and arewell proven in large storage tanks.
The benefits of parabolic antennas inthese applications are clear. The goodfocusing of the paraboloid shapeensures high antenna gain or directivi-ty. Also this narrow beam angle resultsin higher sensitivity.
However, parabolic antennas arelarge, heavy, relatively complex andexpensive to manufacture. These fac-tors limit the use of parabolic antennasin most process level applications.
The central feed to the primaryantenna at the focus of the dish causesa blind area directly in front of the
antenna. This can reduce the antennaefficiency.
Parabolic antennas have beenapplied to bitumen storage tanks wherebuild up on the parabolic dish is said tocause minimum signal attenuation. Ifthe primary antenna was coated in vis-cous product, this would cause a majorproblem to the signal strength.
In conclusion, the parabolic antennahas a niche application in fiscal mea-surement of large, slow moving prod-uct tanks, but is not suitable for thearduous conditions that are prevalent inthe wide variety of vessels within theprocess industries.
Pic 1. Parabolic antennas have beenaround since the beginning ofradar
108
Planar array antennas were original-ly designed and built for aerospaceradar applications. When the nose coneof a modern jet fighter is removed, itreveals a flat circular disk faced withdielectric material and covered withsmall slots instead of the more ‘tradi-tional’ parabolic metal dish. This flatdisk is typical of the planar array anten-nas which have been developed for useon radar level transmitters.
Planar array antennas have theadvantage of being relatively small andlight in weight especially when com-pared with parabolic antennas.
The construction of a planar arrayantenna for a radar level transmitter isquite complex. The antenna is backedwith a round stainless steel disk thatprovides rigidity and strength to theassembly. The steel disk is faced with amicrowave absorbing material. Thismaterial ensures that the microwaveenergy is directed towards the processand that there is no ‘ringing’ noiseinterference from microwave energybouncing off the steel back plate.
Fig 5.31 Planar antenna - side view
5. Planar array antennas
1. Electronics housing
2. Process flange
3. Antenna feed
4. Stainless steel back
6. Microwave patches
7. PTFE process seal
5. Microwave absorbingmaterial
1
2
3
4
567
5. Radar antennas
109
The microwaves pass in a commonfeed from the microwave modulethrough the stainless steel and absorp-tion material to a feed network acrossthe area of the planar antenna. A patternof microwave patches are fed from thisnetwork.
There is a pattern of microwave ele-ments across the area of the antenna.Each element is built up of three ormore microwave patches with dielec-tric material between. This forms amultiple microwave array with manyindividual elements transmitting fromthe face of the planar antenna.
Finally, the microwave elements andthe bonding materials that form thestructure of the planar antenna are pro-tected by a PTFE process seal coveringthe face of the antenna. Additional anti-static material is used for hazardousarea applications.
Planar antennas can be designedwith good focusing of the microwavesand minimal side lobes. As well asapplications within vessels, they can beused for measuring tube applications.
Fig 5.32 Cut away of planar array antenna for radar level transmitter
1. Stainless steel back toantenna provides rigidity
5. PTFE process seal withanti-static elements
2. Microwave feed throughantenna back into feednetwork to microwavepatches
3. Microwave absorbingmaterial preventsringing from stainlesssteel back
4. Microwave patches withlow dielectric layersbetween them focus themicrowaves from eachelement of the array
12
3
4
5
At the beginning of this chapter westated that the definition of ‘beamangle’ is the angle at which themicrowave energy measured at the cen-tre line of the radar beam has reducedto 50% or minus 3 dB.
We discussed directivity and antennagain and stated that even the bestdesigned antennas have side lobes ofenergy. The aim is to maximize the
directivity and minimise the effect ofside lobes.
The metallic horn (or cone) antennaand the dielectric rod antenna are themost practical for process level mea-surement. The following pages showantenna radiation patterns for differentantenna types, frequencies and sizes.These can be summarised as follows :
1. Comparison of horn antenna beam angle with horndiameterThe following diagrams show the comparison of 100 mm, 150 mm and 250mm (4",6" & 10") horn antennas at 5.8 GHz
0
30
60
90
120
150
30
60
90
120
150
180
main lobe directionangular width (3dB)side lobe suppression
Max.:
Farfield E_Abs (Theta); Phi=90,0 deg.
:::
-10 0 10 20
0,0 deg.32,1 deg.16,9 dB
14,3 dB
Fig 5.33 Horn antenna100mm (4"),frequency 5.8GHz,beam angle 32°
· Larger horn antennas have more focused beam angles
· Dielectric rod antennas have more side lobes than hornantennas
· For a given size of horn antenna - the higher the frequencythe more focused the beam angle
Antenna energy patterns
110
5. Radar antennas
0
30
60
90
120
150
30
60
90
120
150
180
main lobe directionangular width (3dB)side lobe suppression
Max.:
Farfield E_Abs (Theta); Phi=90,0 deg.
:::
-10 0 10 20
0,0 deg.27,9 deg.20,9 dB
15,4 dB
0
30
60
90
120
150
30
60
90
120
150
180
main lobe directionangular width (3dB)side lobe suppression
Max.:
Farfield E_Abs (Theta); Phi=90,0 deg.
:::
0 10 20 30
0,0 deg.14,9 deg.21,6 dB
20,4 dB
Fig 5.34 Horn antenna150mm (6"),frequency 5.8GHz,Beam angle 27.9°
Fig 5.35 Horn antenna250mm (10"),frequency 5.8GHz,Beam angle 14.9°
111
112
The following show a 5.8 GHz hornantenna compared with a 5.8 GHz rodantenna.
Although the beam angles aresimilar, the rod has more significantside lobes.
2 Comparison of dielectric rod antenna with horn antenna
0
30
60
90
120
150
30
60
90
120
150
180
main lobe directionangular width (3dB)side lobe suppression
Max.:
Farfield E_Abs (Theta); Phi=90,0 deg.
:::
-10 0 10 20
0,0 deg.32,0 deg.14,6 dB
15,2 dB
0
30
60
90
120
150
30
60
90
120
150
180
main lobe directionangular width (3dB)side lobe suppression
Max.:
Farfield E_Abs (Theta); Phi=90,0 deg.
:::
20100-10
0,0 deg.27,9 deg.20,9 dB
15,4 dB
Fig 5.36 Dielectric rodantenna, 5.8 GHz.Beam angle 32°
Fig 5.37 150mm (6"), hornantenna, 5.8 GHz.Beam angle 27.9°
5. Radar antennas
113
The following diagrams show thebeam angle of 26 GHz radar with a40 mm (1½" ) and 80 mm (3") horn
antenna. These should be comparedwith the previous 5.8 GHz hornantenna patterns.
3 Frequency differences and beam angles
0
30
60
90
120
150
30
60
90
120
150
180
main lobe directionangular width (3dB)side lobe suppression
Max.:
Farfield E_Abs (Theta); Phi=90,0 deg.
:::
20100-10
0,0 deg.18,2 deg.17,2 dB
19,3 dB
0
30
60
90
120
150
30
60
90
120
150
180
main lobe directionangular width (3dB)side lobe suppression
Max.:
Farfield E_Abs (Theta); Phi=90,0 deg.
:::
0 10 20 30
0,0 deg.9,4 deg.
22,1 dB
24,3 dB
Fig 5.38 40 mm (1½") hornantenna, 26 GHz.Beam angle 18.2°
Fig 5.39 80 mm (3") hornantenna, 26 GHz.Beam angle 9.4°
