VHF Weak Signal Operation

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VHF Weak Signal Operation

• by Marc C. Tarplee, Ph.D.
• N4UFP

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What is weak signal operation?

• Exactly what its name implies operation at low
  signal-to-noise ratios
• It does not include the following modes/types of
  operation
• FM, ATV
• Packet, RTTY, PACTOR
• To understand VHF weak signal operation, one
  needs to understand two things
• VHF propagation
• VHF operating modes

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VHF Weak Signal Operation

• VHF weak signal operation uses a variety of
  propagation techniques
• Tropospheric scatter
• Tropospheric ducting
• Sporadic E
• Meteor Scatter
• Auroral backscatter
• F-Layer propagation
• Moonbounce
• The following modes are used in VHF weak signal
  operation
• CW, HSCW
• SSB
• FSK-441, JT6M
• JT65
• VHF weak signal communications take place
  primarily on the 6m (50 MHz), 2m (144 MHz), and
  1.25m (222 MHz) bands

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VHF Weak Signal Propagation Modes
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Tropospheric Propagation Modes

• Tropospheric Scatter
• Variations in the humidity of the troposphere
  cause RF to be scattered over the horizon. This
  is known as tropospheric scatter
• Tropospheric Ducting
• Temperature inversions (warm dry air located
  above cool moist air) refract RF in the VHF range
  back towards the earth. Temperature inversions
  occur daily in the middle latitudes at sunrise
  and sunset. Communications are possible over a
  ranges up to 600 miles
• Over the oceans, stable temperature inversions
  can create a duct, through which VHF can travel
  without significant loss up to 2500 miles
• Ducting is most effective for signals in the high
  VHF and UHF region

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Tropospheric Scatter
Tropospheric Ducting
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Sporadic E (ES)

• Sporadic E (ES)
• Clouds of high density ionization form without
  warning in the ionospheres E layer
• ES is not dependent on solar activity. It may
  occur any time, but is most frequent between May
  and August, with a smaller peak of activity in
  December
• Single hop ES has a range of 1400 mi
• Double hop ES has a range of 2500 mi
• Sporadic E is most common on 6m (about 1 day in 3
  during the summer), rare on 2m and almost unknown
  on 1.25m
• Cause of Sporadic E is not known high altitude
  wind shear may be responsible.

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Sporadic E Hop Geometry
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Meteor Scatter

• As meteors are vaporized in the upper atmosphere,
  they leave behind ionized trails at heights of 60
  70 miles that are sufficiently dense to reflect
  VHF
• A long trail lasts only 15 seconds most trails
  are less than 1 second long
• Scattering of higher frequencies requires denser
  trails.
• Large meteors that produce denser trails are
  relatively rare
• The trail density decreases with time
• Meteor scatter is much less effective on 2m and
  1.25m than 6m
• Best time for meteor scatter is between midnight
  and 600 AM or during a meteor storm.
• Because the direction of arrival of meteors is
  not random, best scattering results generally
  occur when the antennas are pointed slightly to
  one side of the true bearing.

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Meteor Scatter
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Auroral Backscatter (Au)

• In the upper atmosphere above both poles, there
  are doughnut shaped regions of charged particles
  that cause the aurora borealis (and aurora
  australis)
• This region results from the interaction of the
  solar wind, the earths atmosphere, and the
  earths magnetic field.
• A solar flare can cause the auroral zone to
  increase in density and expand, making it capable
  of scattering VHF signals without significant
  absorption.
• RF interacts strongly with the aurora, resulting
  in significant distortion of the signal. Only
  narrow band modes such as CW are used during Au
  openings

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Auroral Zone
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F-Layer Propagation

• F2 Layer Propagation
• Communications over long distances (gt 2000 miles)
  are possible on 6 m via the F2 layer of the
  ionosphere during periods of high solar activity
  (solar flux above 220)
• Openings generally occur in spring and fall
  during daylight hours (similar to 10 m)
• F2 propagation does not occur on the higher VHF
  bands.
• Transequatorial F
• The ionospheres F layer is most intense in the
  region of the geomagnetic equator.
• Stations within about 2500 miles of the
  geomagnetic equator can launch VHF signals into
  these regions. The RF is refracted and travels
  across the equator and into the other hemisphere
  without scattering from the ground
• Stations using TE must be at approximately equal
  distances from the geomagnetic equator
• Like F2 propagation, Transequatorial F occurs
  only on 6m

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F-Layer Path Geometries
Transequatorial F
F2 Propagation
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Moonbounce

• The moon is used as a reflector for VHF signals
• Communications are possible between any two
  stations which can see the moon simultaneously
• Path losses are very high (gt 250dB)

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VHF Weak Signal Operating Modes
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CW

• Used on all VHF bands
• Used specifically for the following modes
• Tropospheric scatter
• Auroral backscatter
• CW activity occurs primarily in the following
  band segments
• 50.080 50.100 MHz
• 144.150 144.200 MHz
• 222.000 222.200 MHz
• Frequencies near 50.100, 144.200, and 222.100 MHz
  are used as calling frequencies.
• If the band is active, stations move off the
  calling frequency and spread down the band.

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HSCW

• CW sent at high speeds (200 wpm or more)
• Used primarily for meteor scatter
• Is being phased out in favor of new digital modes
• Most QSOs are made via sked using defined
  protocols
• HSCW activity occurs primarily in the following
  band segments
• 50.250 50.300 MHz
• 144.100 144.150 MHz
• 222.000 222.200 MHz

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SSB

• Widely used for
• sporadic E communications
• tropospheric ducting communications
• Can be used for auroral backscatter
  communications on 6m.
• Each band has a defined SSB calling frequency
• 6m 50.125 MHz (USA) 50.110 MHz (DX)
• 2m 144.200 MHz
• 1.25m 222.100 MHz
• Operation begins on or near these frequencies and
  spreads up the band conditions improve

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FSK-441

• FSK441
• Uses triplets of 4 tones to transmit data
• 882, 1323, 1764, 2205 Hz
• Each character is sent as a 3 tone sequence
• 43 Character alphabet (letters, numbers . , / ?
  ltspgt)
• Single tone characters used for shorthand
  messages
• 882 Hz - R26 1764 Hz - RRR
• 1323 Hz R27 2205 Hz - 73
• Data rate 147 characters per second (3
  tones/char)
• Used for meteor scatter communications
• Most activity takes place near 50.270 MHz

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FSK-441 Protocols

• Messages sent in 30 sec intervals westernmost
  station transmits during first 30 seconds of each
  minute.
• A complete FSK441 QSO consists of exhange of call
  signs, signal reports and rogers by both stations
• Message sequence
• 1. both call signs (ex N0ABC N4UFP)
• 2. call signs plus signal report (ex N0ABC 26
  N4UFP 2626)
• 3. roger report (ex R27 R27)
• 4. final roger (RRR RRR)
• Both stations begin by sending both call signs.
• First station to copy both call signs sends calls
  plus signal report.
• When other station receives calls signal
  report, he sends roger his report
• When first station receives roger report, he
  sends his roger.
• Final exchange of 73 is optional
• Time to complete a QSO ranges from 2 minutes to
  over an hour.

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JT6M

• 44 tone MFSK
• 43 tones for characters 1099.19 2002.59 Hz (Df
  21.53 Hz)
• 1 tone for synchronization (1077.66 Hz)
• Each tone sent for 46.44 msec
• Effective data rate 14.4 characters/second
• Requires a stable receiver, good timing
• Can copy signals when SNR lt 0
• Ideal for troposcatter, meteor scatter and
  moonbounce on the 6 meter band.

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JT6M Protocols

• Messages sent in 60 sec intervals westernmost
  station transmits during even-numbered minutes.
• Message sequence
• 1. both call signs (ex N0ABC N4UFP)
• 2. call signs plus signal report (ex N0ABC N4UFP
  OOO)
• 3. roger report (ex RO RO)
• 4. final roger (RRR RRR)
• 5. 73s (73 73 73)
• Signal report OOO means signal is readable it
  is the only valid JT65 signal report
• QSOs require at least several minutes to
  complete.
• Most activity takes place near 50.270 MHz,
  generally by sked.

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JT65

• 65 tone MFSK with Reed-Solomon coding
• 5.4 Hz operating bandwidth
• 100 copy is possible at SNR lt -10dB
• Message sequence
• 1. both call signs (ex N0ABC N4UFP)
• 2. call signs plus signal report (ex N0ABC N4UFP
  OOO)
• 3. roger report (ex RO RO)
• 4. final roger (RRR RRR)
• 5. 73s (73 73 73)
• Signal report OOO means signal is readable it
  is the only valid JT65 signal report

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Getting on the AirStation Requirements
andOperating Notes
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VHF Station Requirements

• Terrestrial Modes
• RF output at least 100 W
• Good VHF transceiver or transverter with MDS of
  -136 dBm (3 KHz BW)
• Mast-mounted pre-amp, gain gt 10 dB
• Horizontally polarized beam antenna, gain gt 10 dB
• Moonbounce.
• RF output at least 500W
• Good VHF transceiver or transverter with MDS of
  -136 dBm (3 KHz BW)
• Mast-mounted pre-amp, gain gt 10 dB
• beam antenna, gain gt 15 dB with az-el rotator

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Operating Notes Troposcatter and Tropospheric
ducting

• Listen near calling frequencies for activity
  before scanning the band.
• Troposcatter signals are generally weak
  (typically lt 2 s-units) and subject to slow fades
  over 30 sec or so.
• QSO information should be sent quickly while
  signal levels are up.
• Tropo ducting signals are moderately strong and
  relatively steady, but quickly disappear when the
  duct deteriorates

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Operating Notes Sporadic E

• Listen near calling frequencies for activity
  before scanning the band.
• Sporadic E signals can be very strong (gt s9), but
  may suddenly disappear
• QSO information should be sent quickly while
  signal levels are up.
• As the opening develops, general operating
  practice is for SSB stations to spread up from
  the calling frequency and for CW stations to
  spread down.
• Do not use the 6m DX window (50.100 50.125 MHz)
  for domestic contacts during a band opening

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Operating Notes Meteor Scatter

• Most meteor scatter operation is done using the
  FSK-441 or JT6M modes.
• Time synchronization is important be sure to
  set your PCs clock to WWV before beginning a MS
  contact.
• Random contacts can be made (especially on 6m) by
  calling CQ on the calling frequency, but most
  contacts are made via sked.
• When calling CQ
• Transmit during the first 30 sec period when
  beaming in an easterly direction
• Transmit during the second 30 second period when
  beaming in a westerly direction
• MS contacts may be made at any time, but the best
  time is in the early morning hours (before 600
  AM)

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Operating Notes Meteor Scatter

• Links to MS QSO scheduling sites
• http//www.pingjockey.net/
• http//www.dxworld.com/hsms.html

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Useful Tools

• General Meteor Scatter Info
• http//www.qsl.net/w8wn/hscw/hscw.html
• Aurora Info
• http//aurora.n1bug.net/
• Tropospheric propagation prediction tools
• VHF Propagation from APRS data
• William Hepburns Tropo Forecasts

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VHF Antennas

• VHF antennas are relatively small, light and
  easily rotatable.
• Best choices for a new operator
• Quad (at least 2 el on 6m, 5 or more elements on
  2m and 1.25m)
• Yagi (at least 3 el on 6m, 5 or more elements on
  2m and 1.25m)
• For weak signal work (CW/SSB) the antenna should
  be horizontally polarized
• For EME an array of 4 long yagis is typical
• Meteor scatter operation requires an antenna with
  good gain and broad beamwidth 5 to 9 element
  yagis work well.

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6 Meter Quad and Yagi Antennas

• 2 element Quad (square loops of 14 ins. wire Z
  60 ohms Gain 4 dBd)
• Element Loop Length (in) Position (in)
• Reflector 245.0 0
• Driver 235.5 29
• 3 element Yagi (Aluminum tubing Z 42 ohms gain
  5 dBd)
• Element Half Length (in) (0.75 dia 0.625
  dia) Position (in)
• Reflector 24 35.875 0
• Driver 24 31.875 50
• Director 24 26.375 87
• 5 element Yagi (Aluminum tubing Feed Z 35 ohms
  Gain 8 dBd)
• Element Half Length (in) (0.75 dia 0.625
  dia) Position (in)
• Reflector 24 35.875 0
• Driver 24 33.875 49
• Director 1 24 30.000 72
• Director 2 24 29.500 121
• Director 3 24 28.000 169

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2 Meter Yagi Antennas

• 5 element Yagi (0.188 dia Al rod Feed Z 40 ohms
  Gain 8 dBd)
• Element Half Length (in) Position (in)
• Reflector 20.625 0
• Driver 20 17
• Director 1 18.625 25
• Director 2 18.5 42
• Director 3 17.875 59
• 8 element Yagi (0.188 dia Al rod Feed Z 40 ohms
  Gain 11 dBd)
• Element Half Length (in) Position (in)
• Reflector 19.875 0
• Driver 19 16
• Director 1 18.25 26
• Director 2 18. 49
• Director 3 17.625 80
• Director 4 17.5 113
• Director 5 17.375 146
• Director 6 16.875 178

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1.25 Meter Yagi Antenna

• 8 element Yagi (0.125 dia Al rod Feed Z 40
  ohms Gain 11.5 dBd)
• Element Half Length (in) Position (in)
• Reflector 12.6875 0
• Driver 12.4375 10.5
• Director 1 11.9375 18
• Director 2 11.8125 32
• Director 3 11.5 51.5
• Director 4 11.375 73
• Director 5 11.3125 95
• Director 6 11.25 115

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VHF Operating Activities

• Contests
• ARRL January VHF Sweepstakes (3rd weekend in
  January)
• ARRL June VHF QSO Party (2nd weekend in June)
• SMIRK QSO Party (3rd weekend in June
• CQ WW VHF Contest (2nd weekend in July)
• Six Club 6 m Sprint (3rd weekend in July)
• ARRL September QSO Party (2nd weekend in
  September)
• Operating Awards
• VUCC contacts with 100 Grid Squares, not
  difficult
• WAS tough, but not impossible
• DXCC very tough from North America, but it has
  been done
• Grid Square Hunting
• There are over 500 grid squares in the
  continental US
• No one has worked them all yet (except perhaps
  for W5FF)

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What is a Grid Square?

• Almost all VHF operating awards and contests
  involve grid squares
• Grid Squares are 2º longitude x 1º latitude
  sections of the earths surface (there are 32,400
  in total)
• Each grid square has a 4 character designator
  containing 2 letters and 2 numbers.
• The two letters designate the field. There are
  324 fields lettered AA through RR
• Each field is divided into 100 squares numbered
  00 through 99
• The continental US includes grid squares in
  fields CM,CN, DL, DM, DN, EL,EM,EN, FM and FN
• Most of Rock Hill is in grid square EM94.

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World Grid Fields
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Grid Square Map of the USA