1. VHF Weak Signal Operation
by Marc C. Tarplee, Ph.D.
N4UFP

2. 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

3. 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

4. VHF Weak Signal Propagation Modes

5. 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

6. Tropospheric Scatter
Tropospheric Ducting

7. Sporadic E (ES) Sporadic E (ES)
Clouds of high density ionization form without warning in the ionosphere’s 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.

8. Sporadic E Hop Geometry

9. 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 6:00 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.

10. Meteor Scatter

11. 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 earth’s atmosphere, and the earth’s 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

12. Auroral Zone

13. F-Layer Propagation F2 Layer Propagation Transequatorial F
Communications over long distances (> 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 ionosphere’s 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

14. F-Layer Path Geometries
Transequatorial F
F2 Propagation

15. 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 (> 250dB)

16. VHF Weak Signal Operating Modes

17. 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:
– MHz
– MHz
– MHz
Frequencies near , , and MHz are used as calling frequencies.
If the band is active, stations move off the calling frequency and spread down the band.

18. 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 QSO’s are made via sked using defined protocols
HSCW activity occurs primarily in the following band segments:
– MHz
– MHz
– MHz

19. 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 – MHz (USA) MHz (DX)
2m – MHz
1.25m – MHz
Operation begins on or near these frequencies and spreads up the band conditions improve

20. FSK-441 FSK441 Data rate = 147 characters per second (3 tones/char)
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 . , / ? # $ <sp>)
Single tone characters used for shorthand messages:
882 Hz - R Hz - RRR
1323 Hz – R Hz - 73
Data rate = 147 characters per second (3 tones/char)
Used for meteor scatter communications
Most activity takes place near MHz

21. 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.

22. JT6M 44 tone MFSK Requires a stable receiver, good timing
43 tones for characters – Hz (Df ~ Hz)
1 tone for synchronization ( Hz)
Each tone sent for msec
Effective data rate = 14.4 characters/second
Requires a stable receiver, good timing
Can copy signals when SNR < 0
Ideal for troposcatter, meteor scatter and moonbounce on the 6 meter band.

23. 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. 73’s ( )
Signal report OOO means signal is readable – it is the only valid JT65 signal report
QSO’s require at least several minutes to complete.
Most activity takes place near MHz, generally by sked.

24. JT65 65 tone MFSK with Reed-Solomon coding ~5.4 Hz operating bandwidth
100% copy is possible at SNR < -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. 73’s ( )
Signal report OOO means signal is readable – it is the only valid JT65 signal report

25. Getting on the Air: Station Requirements and Operating Notes

26. 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 > 10 dB
Horizontally polarized beam antenna, gain > 10 dB
Moonbounce.
RF output at least 500W
beam antenna, gain > 15 dB with az-el rotator

27. Operating Notes – Troposcatter and Tropospheric ducting
Listen near calling frequencies for activity before scanning the band.
Troposcatter signals are generally weak (typically < 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

28. Operating Notes – Sporadic E
Listen near calling frequencies for activity before scanning the band.
Sporadic E signals can be very strong (> 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 ( – MHz) for domestic contacts during a band opening

29. 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 PC’s 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 6:00 AM)

30. Operating Notes – Meteor Scatter
Links to MS QSO scheduling sites

31. Useful Tools General Meteor Scatter Info Aurora Info
Aurora Info
Tropospheric propagation prediction tools:
VHF Propagation from APRS data
William Hepburn’s Tropo Forecasts

32. 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.

33. 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
Driver
3 element Yagi (Aluminum tubing Z ~ 42 ohms gain ~ 5 dBd)
Element Half Length (in) (0.75 dia dia) Position (in)
Reflector
Driver
Director
5 element Yagi (Aluminum tubing Feed Z ~ 35 ohms Gain ~ 8 dBd)
Element Half Length (in) (0.75 dia dia) Position (in)
Reflector
Driver
Director
Director
Director

34. 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
Driver
Director
Director
Director
8 element Yagi (0.188 dia Al rod Feed Z ~ 40 ohms Gain ~ 11 dBd)
Reflector
Driver
Director
Director
Director
Director
Director
Director

35. 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
Driver
Director
Director
Director
Director
Director
Director

36. 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)

37. 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 earth’s 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.

38. World Grid Fields

39. Grid Square Map of the USA