Using The Counter - Optoelectronics M1 Owner's Manual

USING THE COUNTER
HandiCounters~ are unique
in
theirabilityto find RFtransmission frequencies
quickly.Immediateresponseto frequencies
that
are 10
to
16dB greater than the
background RF floor is possible. This is simplydone by moving the HandiCounter"
into
the
nearfieldofthe radiotransmitter . The nearfieldis
the
area closeto the
antenna where the fieldstrengthis high but fallingoffrapidlyas distanceincreases .
Thisis comparedto the far fieldwerethe fieldstrengthis lowbut remainsfairly
constantover
great
distances.HandiCounters<ll work wellat relativelyclose
distancesand can measurea transmission frequency rapidlywithout having to
tune through the RF spectrum .
Several factors may prevent
a stable frequencycounterreadingeven whenthere is
a bargraph response. Amongtheseare:
• The signalissimply too small.While the Mode l Ml
is
vel)'
sensitive
it
can
not
work with extremely weaksignals.
~
Amplitudemodulation(AM). Sincethe counteris measuring the zerocrossings
of the signal,
it
issensitive
to
the amplitude of the received signal.Amplitude
modulatedsignalssuch as1V videocarriers,aircraftradios,garagedoor openers,
etc. can be difficult to countsincetheir peak amplitudemay be highenoughto
causea largebargraphindication, but the valleyis
too
lowin amplitude for reliable
counting.AMsignalscan be
veryfrustratingin thisregard,looking
like
a signal
that should easilycount and yet nevergivinga stab le count.
• Multiple signals
~
TheMOOel Ml wide-bandfrequency countermay not be able
to separateadjacentcarriers.
The
fM broadcast band is a goodexampleof this,a
20MHzwide bandwith channelson 200kHzcenters .
In
anycity,numerousstations
will
exist withina givenarea.It is possiblethat none
of
the signalsare ID- I 5dB
higherthan the restand thismakesaccuratecountingdifficult.
The table belowgivesactualtestdata showingthedistancesthat various transmit-
ters can be pickedup withand without usingthe ModelMI, in a typicalmetropoli -
tan RFenvironmen
t. If
you live in a particularlyquietlocationyour results may be
better.
T\'PICALRFPICKUPDISTANCES
Transmitter Type Counter only CounteriAPSI04
Cordles s Phone 1
foot
120feet
CBRadio 25feet 500feet
VHFTwoWay Radio &J feet 1/4 mile
10
ANfENNA SELECTION
Theultimateperformance of thecounterwillbe compromised if the best possible
antennais not used.
If
onlya single
type
of antennais availa ble,it should beof the
telescoping whip UHF/VHF type. For maximumpick up distances.theantenna
used shouldbe tuned for the frequency bandof interest. For example,with cellular
frequencies, a cellulartypeof antennawith male BNC connectoris essen tial for best
results . Themodel
RD800
rubberduck or model
GP800
ground planeantenna will
givegood results.TheGP800 is moreexpensivebut will givethe maximum
possibleperformance(pick up distance ). Checkthe Optoelectronics Catalog for
availableantenna packag es.
OPERATIONALCHARACTERISTICS
Multiple Transmissions
If two
transmittersare operatingat the sametimewithin the pass bandand they
appear to have the samesigna l strength, then the bargraph willdisplaya strong
signalindicationbut the counterdisplay will not stabilize. Tocompensateyou must
movephysicallycloser
to
the transmitterof interestuntil the counter seesits signal
as 10 to 15dB greaterin strength.
Overdriving the Counter
The
ModelMl can beoverdriven
in
the presence
of
a strong RFsigna l. Inthis
situationthe bargraph will alsodisplay
a strongsignalindication. It may be
necessary to reducesigna1 levelby movingaway fromthe sourceor removing/
shortening the antenna beforethecounterbeginsto count properly.
Multipath cancellation
Thedistanceat whichthe signalcan be detectedmaybe much greaterthan the
distanceat whichit can be counted.At 850MHz the wavelengthis about35emand
multipathcancellations can repeatat verydose intervals. As you decreasethe
distancethe problemgoes away. If you are in a vehicle,best resultscanbeobtained
whenyou cometo a completestopand the transmitteralso stopsmotion, providing
a stable signalwithout rnultipathdropouts. Alwaysoperatethecount e r on the
fastestmeasurementintervalpossible. Use the display hold switchas necessa ry. It
is
helpful to know the available frequencies in the area to assist in determinin g
whenyou have an accuratereading.
11