O
orn
Initialassessmentof complaintsof Interference
to amateurradio
stationsandshort waveenihusiasts
lntroduction
This proposalwas raisedat the Ofcom/RSGBForumheldat the Bedford
headquarters
of the RSGBon 7'nApril.
methodology
shouldbe employedby FieldEngineersfor
The followingmeasurement
period
of 6 months.The intentionis to quantifythe levelof interference
a trial
experiencedby amateursin the presenceof widebandinterferes.This is not an exact
of the interfererabove
measurement
of the noisefloorbut a relativemeasurement
the noisefloorwhenmeasuredat the receiverend of the amateurantennafeeder.
FieldStaffare reouestedto foMard resultsand commentsto theirSeniorField
Engineer(SFE).
Background
of the new EMCdirective'authorisedusersare entitledto
Sincethe introduction
protectionfrom harmfulinterference
from unauthorised
radioservices... (and)from
(see:
non-radiosources'
htto://www.rsqb.orq/emc,/odfs/leaflets/emc09inbound.pdO.
This hasgivenriseto complaintsmainlyassociatedwith PowerlineAdapters(PLAs)
and plasmascreentelevisions.lt is recognisedthat bothamateursand shortwave
listenersare workingcloseto theirequipmentnoisefloor andthat manyinstancesof
arewell belowany 'protectionratios'affordedto licensed
reportedinterference
(VHF
radioand terrestrialTV).
broadcastreception
Action
It is understoodthat the ambientnoiseflooron HF variesbothdiurnallyand
(possibly
seasonally.Hencecaremustbe exercisedto completemeasurements
acrossa numberof bands)in the presenceof the widebandinterfererto identifythe
'worsecase'signalto noisefigure.
antennasvstem
It requiresthat an Ofcomreceiveris connectedto the complainant's
and an assessmentmadeof the systemnoisefloor beforetuningintothe interferer.
Frequency
svstemnoisefloor
Interferersiqnal
Interferersiqnal
Interferersiqnal
svstemnoisefloor
Interferersional
signalamplitude
apove
thenoisefloo|.
3240MH2
3.750MH2'
3.666MH2'
3.562MH2'
7.050MH2'
7124MHX
1=17d8
2--11d8
Detector
9kHz
9kHz
9kHz
9kHz
9kHz
9kHz
peak
peak
peak
peak
peak
Peak
Level in
dBuV
Antenna
28
160m/80m
longwire
40m
1/2G5RV
20l15l1Om
2elemini
tri-bander
HFQI
45
37
45
16
26
wpe'
3=17d8
5=10dB
Table 1
See notesdverleaf...
1 Seevaluesin Table2.
2 If the resuftsshow that the measuredinterfereris 6 dB abovethe noisefloor
then the investigationcan be accepted.
3 The type of antennais vitatlyimpoftantand as suchshoutdbe describedwith
enoughdetail so that the measurementresultscan be fully understood.
The Dresenceof noisein the receiveroassbandresultsin an errorin the measured
amplitude
of the signal.lf the amplitude
of the signalis at least6 dB abovethe
receivernoisefloorthis erroris small.Belowthis valuethe errorincreases
significantly
and adverselyaffectsthe accuracyof the measurement.
lf the interfererliesbelowthis 6 dB thresholdthenthe caseshouldbe referredto the
SFE beforeproceedingwiththe investigation
further.
The defaultreceiversettingsshownin the tablebeloware a compromise;
butthey do
providea set of repeatablemeasurement
conditions.
Frequency
Range
150 kHz - 30 MHz
30 MHz- 1000MHz
Measurement
Bandwidth
9 kHz'
120 kHz
Detector
Meaaurement
unita
PeaK
dBuV
dBuV
Peak
potentialto a shortwavebroadcast
1. CISPRbandwidth
lo assesthe interference
transmission.
Correctfor modulated
andunmodulated
stablecarriers.
in lhe frequencyrange4.5- 21 MHz.
[al HomePlug1.0PLA: 84 equallyspacedsub-carriers
The 30 dB notchesin the amateurbandsreducethe numberof useablesub-caniers
to 76.
Characterised
by a 1 second'lick whenidling(no datathroughput).
'1536equallyspacedsub-carriers
in the frequencyband
lbl DS2PLA(includingComtrend):
2-28 MHz.Numberof sub-carriers
reducedby amateurbandnotching.Eachmodemis
pollingevery1 ms for networkactivitywhichproducesthe characteristic
1 kHztone.
interference
suchas thatproducedby plasmascreentelevisions
mayrequirea
[c] Broadband
morepragmatic
approachin assessingthe truenoisefloor.
Table 2
Pleaseensurethatthe resultsof thesemeasurements
are reportedin the Siebel
casenotes.
(Senior)FieldEnsineer
contactdetails
SiebelCasenumber.
CraiqCostford.0'12'14235236
Mr. Conwav.0116 2825884
1-137586581
Table3