LAPLACE INSTRUMENTS RF200 User manual

RF200
Antenna
User Manual
Version3.0
January2004
LAPLACEINSTRUMENTS LTD
3B, Middlebrook Way
CROMER
Norfolk NR27 9JR
UK
Tel: [phone]
Fax:[phone]

RF200/500 user manual
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Index
1.0 IntroductionPage3
Background
2.0 RF200 broadband antennaPage4
Assembly Page4
InusePage6
Antenna Factor Page7
3.0 Ground PlanePage8
4.0 Testsite calibrationPage9

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1.0 Introduction
1.1 Antenna Background
Formeasurementoffield strength (far field emission level)an antennais required
which will actas atransducer, convertingfield strength (mV/m)to mVsignals output
down acoaxcable.
Antennas to cover thewidefrequencyranges required bythelegislation are notsimple
devices!Thestandards call fortheuseofa‘tuned dipole’. Whilstthis is simpleto
manufacture and produces aeasilydefinableoutput, it will onlywork atone
frequency, thetuned frequency. Dipoles are tuned byadjustingthelength oftheir
elements. Forserious emissions measurementwork, theconstantretuningofthe
antennaforeach peak ofinterestis timeconsuming, hence theintroduction of‘broad
band’antennas thatcover awidespectrumwithouttheneed foranyretuning. These
includelogperiodic, bi-conical, bi-logand other specialisttypes. All suffer from
variation ofsensitivitywith frequencyand need acorrection chartso thatthe
appropriateadjustmentcan bemadeto thespectrum. This correction chartis called
theantennafactor.
TheLaplace RF200 broadband antennahas arelativelyripplefree antennafactor
characteristic, closeto theoptimum.
Iftheantennais used with theLaplace EMCanalysers and theEMCEngineer
software, selection oftheRF200 itemin theinputmenu automaticallyapplies the
RF200 antennafactorcorrection to thespectrum.
1.2 Dipoleorbroadband antenna, whichto use?
EN50022 specifies thatatuned dipolebeused as theantennaforradiated emissions
testing. Thedipoleis abasicstandard that, atits tuned frequency, has an easily
definableoutputvs field strength characteristic. Dipoles are tuned byadjustingthe
length ofeach elementto be¼wavelength long. Ifmeasuringtheemissions froma
productover awidefrequencyrange, this is tedious, timeconsumingand is asource
oferror. Broadband antennas haveaknown responseover awiderangeof
frequencies and need no adjustment. Theresponseis notflat, and all broadband
antennas should besupplied with an ‘antennafactor’ curve. This is aplotof
sensitivityvs frequencyover thefull workingrangeoftheantenna. TheRF200 has a
workingrangeof30MHzto 1GHzand thus matches therequirements oftheEN
standards.
TheRF200 maybeused with anyanalyser orreceiver buttheSA1020 pre-amplifier
should beused to ensure thatthecharacteristics oftheantennamatch thepublished
data.
Basically, you need theBroadband antennaifit is necessaryto measure absolutefield
strength with areasonablelevelofconfidence and havean effectivetestsite, free of
reflections.
NotethattheantennafactorfortheRF200 is included in theSA1000 software.
This antennameans thatyou can cover thewholeradiated emissions frequencyrange
in onesweep. No need to adjustdipoles to each frequencyofinterest, no need to
switch between logperiodicand biconicaltypes halfwaythrough thetesting.

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2.0 RF200 Broadband antenna
Thebroadband antennawill allowtheuser to detectand measure radiation over the
frequencyrange30MHzto 1GHz.
This is shipped in a‘knocked-down’formto easepackagingand to minimisethe
potentialfordamagein transit. Assemblyis straightforward butmustbedonewith
care.
Thebasicdesign oftheantennaconsists ofacentralmain beam, itselfcomprisingtwo
parallelaluminiumsections spaced apartbyinsulators. Equallength pairs ofstainless
steelrods formtheantennaelements, thesemounted on themain beamin order of
length, theshortestattheend fromwhich theoutputlead is attached. An insulating
mountingblock provides attachmentforthestand with facilities forhorizontaland
verticalmounting. Thenon-metallicstand allows adjustmentofantennaheightand
direction.
2.1 Antenna assembly (see fig 1)
1. Thealuminiumalloyelements are secured to thecentralbeamusingtheM4 bolts
and washers provided. An M4 hexdriver is also included to facilitateassembly. These
elements are mounted in equallength pairs with theshortestattheend ofthecentral
beamwhere theoutputcableis attached. There are two copper crinklewashers with
each bolt. Ensure thatonewasher is under thebolt head and theother is under the
antennaelement. Tighten thebolts until thecrinklewasher is flat. Do notovertighten
as this maydistortthebeam. Alternatetheelementdirection as shown in thediagram
so thatforeach sideofthecentralbeam, theelements alternateup, down, up,
down...etc. until element9 whichis out-of-sequence and is mountedsamesideas
element8. Element10 is alternateto element9 as shown in Fig1.
3. Thenumber ofelements (10 pairs)should match thenumber ofholepairs alongthe
centralbeam.
4. Apre-drilled plasticblock is supplied to formacentralmountingblock and pre-
amp support. This is screwed to both beams usingthesupplied nylon screws.

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Fig1 RF200 Assembly
RF200 Stand assembly

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Fig2 RF200 stand
This is supplied as acentralverticalsupportfitted with alegattachmentmouldingand
three legs.
Thelegs are apush fit into thelegattachmentmoulding.
Theverticalsupporthas afriction slidefit in thelegattachmentmouldingso thatthe
antennacan beadjusted in heightover thefull length oftheverticalsupport.
Theantennais located on thestand bylocatingthecentralsupportblock on thetop of
theverticalsupportin either thehorizontalorverticalpolarisation position. Nylon
bolts are provided so thattheantennacan beclamped in position
Ifused outdoors in strongwind conditions, thestabilityoftheantennacan be
considerablyincreased byfillingabagwith sand, soil orstones and supportingit by
stringtied round thelegattachmentmoulding.
2.2 RF200 inuse
ConnecttheSA1020 pre-amplifier directlyto theantennaoutputlead and secure the
pre-amp to thecentralsupportblock with thevelcro strips. Ensure thattheconnection
to theantennais madeto theinputofthepre-amplifier. Itis easyto gettheamplifier
wrongwayround!!
Pointtheantenna, sharp end forward, attheUUT. Notethattheantennais directional
butfull sensitivityis maintained over awideangleeither sideof‘dead ahead’.
Thereference pointforthemeasurementofEUT – Antennadistance is thecentral
mountingpoint, where theverticalpolemeets thehorizontalmain antennabeams.
Theheightoftheantennacan bechanged byslidingtheverticalsupportup ordown
within thelegattachmentmoulding. Ifthis friction fit is too slack ortight, slightly
adjustthenylon bolt to suit. Notethatantennaheightmaybeacriticalfactorin
obtainingvalid results. See section 4 on Ground Plane.
Themain feature ofthepolar plotoftheantenna(i.e. its directionalproperties)is a
sharp null at90º on either side. This can beused to null outanystrongbackground
emission forinstance froman FM radio transmitter IFusingatrueopen field site.
Notethatthenull is verysharp and care has to betaken to find therightangle.
Attenuation ofup to 20dBis possibleunder therightconditions. In thepresence of
buildings etc., theseemissions will beaffected byreflections and will notbe
unidirectional, makingthemimpossibleto null out.

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2.3 Antenna Factor
Thesensitivityofanyantennawill varywith frequency. i.e. it will bemore sensitiveat
somefrequencies and less sensitiveatothers. Aplotofsensitivityvs frequencyis
called theAntennaFactor.
TheSA1000 Windows software has theantennafactorfortheRF200 broadband
antennareadyinstalled. Selectingthis itemin theINPUT menu automaticallyapplies
theappropriateconversion to read outin absolutefield strength.
WARNING:Although theconversion is valid, thefield strength measured bythe
antennais subjectto yourtestsiteconditions and configuration and maybesubjectto
gross errors. Reception ofemissions radiated fromtheUUT depend on thetest
conditions, thetestsite, reflections, ground plane, background radiation, UUT to
antennadistance etc..etc.. Beverywaryaboutrelatingfield strengths to limit lines
unless you havesomeknown testresults to actas areference.
Fig3(a) RF200 Antennafactor, linear frequencyscaling.
Note. Antennafactorincludes SA1020 Pre-amplifier and 5 metres co-axcable.
A.F. (dB)
-15
-12.5
-10
-7.5
-5
-2.5
0
2.5
5
7.5
10
12.5
15
0
100 200 300 400 500 600 700 800 900
1000
Frequency (MHz)
Antennagain(dB)
A.F. (dB)
Fig3(b)RF200 AntennaFactor, Logfrequencyscaling
Note. Antennafactorincludes SA1020 Pre-amplifier and 5 metres co-axcable.
A.F. (dB)
-15
-12.5
-10
-7.5
-5
-2.5
0
2.5
5
7.5
10
12.5
15
10
1000
Frequency (MHz)
Antennagain(dB)
A.F. (dB)

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Antenna Factortabulardata
Freq(MHz)A.F. (dB/m)
30 0
40 -1
50 -2
60 -3
70 -5
80 -7
100 -9
120 -9
140 -8
160 -7
180 -6
200 -5
Freq(MHz)A.F. (dB/m)
220 -3
240 -2
260 -4
280 -5
300 -3
320 -2
340 -1
360 0
380 1
400 2
420 3
440 3
Freq(MHz)A.F. (dB/m)
460 4
480 5
500 5
550 6
600 7
650 9
700 10
750 11
800 12
850 13
900 13.5
950 14
1000 14.5
3.0 Ground plane
In general, anyUUT will emit radiation in all directions. Someofthis will impingeon
theground which will partiallyreflectthis radiation.
When measuringemissions in thefar field, thesignalreceived bytheantennawill
compriseadirectsignaland asignalwhich has been reflected fromtheground.
(Assumingthatthetestsitehas been chosen so thatno other reflections are present).
Theamountofthis reflected signaldepends on ground conditions and mayvaryvery
considerablyin amplitude. On ‘soft’ground such as earth (soil)thereflection will
varyfromdayto dayas conditions change. This means thattheintegrityand
consistencyoftheresults will bevariable. To overcomethis problem, thestandards
require atestsiteto haveametalground planeconsistingofacontinuous metalsheet
(orequivalent)under theUUT and between theUUT and theantenna. This gives a
consistent100%reflection. This is in onesense‘worstcase’ becausetheeffectofthe
reflection will bemaximised, butatleastit will beconsistent.
Theeffectofthereflection will depend on frequencyand thedifference in path length
between thedirectpath and thereflected path. Ifthis difference is equalto halfa
wavelength atthefrequencyofinterest, thetwo signals will be180°outofphaseand
will cancel, producingup to 20dBreduction in signalstrength.
Fig4 Ground planereflection
UUT Direct path
Reflected
path

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1.00 1.10 1.20 1.30 1.40 1.50 1.60 1.70 1.80 1.90 2.00 2.10 2.20 2.30 2.40 2.50 2.60 2.70 2.80 2.90 3.00 3.20 3.40 3.60 3.80 4.00 4.20 4.40 4.60
50
100
150
200
250
300
350
400
450
500
550
600
650
700
750
800
850
900
950
1000
antennaheight
MHz
3-6
0-3
-3-0
-6--3
-9--6
-12--9
-15--12
-18--15
To overcomethis effect, thestandards call fortheantennato bemounted on amastso
thatit can bevaried in heightover arangeof1 to 4 metres. Foreach frequencythere
will beaheightatwhich thetwo signals are in phaseand additive. This is theheight
atwhich thatfrequencyis measured.
Fig5 shows therelationship between frequency, antennaheightand signal
gain/attenuation. Thewhiteareas correspond to gain and thedark areas represent
attenuation.
Ifusingan open field siteon drysoil, thereflection will besmall and the
gain/attenuation effectwill beminimal. However, ifanymetalsurface is in the
vicinity(filingcabinetforinstance) this chartgives someidea oftheconsequences!
Notethatthechartis correctfortheconditions listed only. Ifforinstance the
polarisation is changed to verticaltheplotwill completelychange.
Fig5 Ground planeeffect
Conditions: Productheight: 0.8m
Antennadistance:3m
Polarisation:Horizontal
Ground plane:Metalsheet(ideal)
4.0 TestSite Calibration
Anyarea ortestcell used forfar field radiation testingshould becalibrated. Purpose
madecells such as aG-TEM cells are supplied readywith acalibration sheetwhich
defines thecharacteristicrelationship between UUT emissions and cell outputvs
frequency.

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Ifusingaan open field testsite, this should also becalibrated. Theground plane
reflection alonecan haveasignificanteffecton thetestresults. To calibrateasite, a
known source should beused. Laplace can supplyacalibrated source, completewith
calibration curves, which is specificallydesigned forthecalibration ofsites and
antennas. ContactLaplace fordetails ofthis ERS (Emission Reference Source). An
alternativeforachievingarough calibration is to useaproductwith known emissions
(i.e. onewhich has alreadybeen tested atatesthouse) on yourtestsiteunder exactly
thesameconditions as applied duringthetesthousemeasurement. Bycorrelation of
yourresults with thetesthouseresults, an approximatecalibration ofthesitecan be
derived. Notethatifthesiteis outdoors, it will need acalibration check everytimeit
is used becauseweather conditions can affectthesitesignificantly.
Drawbacks to this techniqueare theproblems ofobtainingconsistentemissions from
aproductand thefactthatthecorrelation can onlybeapplied atthosefrequencies for
which emissions exist.

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LAPLACEINSTRUMENTSLTD
3B, Middlebrook Way
Cromer
Norfolk
NR27 9JR
Tel: [phone]
Fax:[phone]
E:[e-mail]
Web:www.laplaceinstruments.com