Basic (VHF) Radio Communications

1
Chapter 3

• Basic (VHF) Radio Communications

2
Overview

• Basic VHF Transmitters
• Basic VHF Receivers
• VHF Antennas
• Coaxial Cables
• VHF Transceiver
• Specifications
• Summary

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Basic VHF Transmitter
Very High Frequency Transmitter
RF 156-162 MHz _at_ 1 or 25 watts

• Audio 300 to 3,000 Hz
• DC 12 to 14 VDC _at_ 5A

Heat a few watts
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FM Transmitter
(Frequency Modulation)
(Antenna)
Variable Oscillator
Frequency Multiplier
RF Amplifier
FM Modulator
156.800 MHz
x 2 x 3 x 2 x 2
6.533 MHz
(Channel 16)
(Frequency Selection)
300 3,000 Hz (microphone)
Modulator varies frequency of the 156.800 MHz
fundamental
Not shown - PTT switch - High/Low power switch
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Frequency Modulation

• Input
• 300 Hz - 3 kHz (audio)
• 150 MHz (RF)
• Output
• 150 MHz (/- 5 kHz)
• Carrier shift (/- 5 kHz) is proportional to
  audio volume
• Rate of change, above below carrier, same as
  modulating audio freq (300 Hz to 3 kHz)

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Frequency Modulation
Note Top two waveforms are voltage vs time
Bottom waveform is frequency vs time
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Basic VHF Receiver
Very High Frequency Receiver

• RF 156 - 162 MHz
• DC 12 to 14 VDC _at_ 0.5A

Audio 300 to 3,000 Hz _at_ 3 watts
Heat a few watts
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FM Receiver
(Antenna)
(Speaker)
Audio Amplifier
RF Amplifier
De- modulator
IF Amp
Limiter
Mixer
156.800 MHz
3003,000 Hz
10.7 MHz
(Volume Control)
(Squelch Control)
(Channel 16)
Local Oscillator
146.1 MHz
(Frequency selection)
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Frequency Modulation

• Unique features
• Capture effect
• Squelch
• AM noise eliminated
• Demodulation
• Volume is derived from the shift from the center
  frequency
• Frequency is derived from the rate of change
  across the center frequency

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VHF Antennas

• Speed of light radio waves in free space
• 300,000,000 meters/sec
  or 186,000 statute miles/sec
• Wavelength (?) 982 / f (in MHz) in feet
• VHF Channel 16 (156.800 MHz)
• 6.26 feet or just over 75 inches
• Speed of RF in a wire is slower than in free
  space
• 0.92 to 0.98 (most antennas assume 0.95)
• ?/2 468 / f (in MHz)

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MComm Ch3 - 10
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Antenna Length
Wave Length ? Speed of Propagation /
Frequency (Hz) ? (in feet) 982 / f in MHz
?
Speed of Propagation assumed 0.95 Dipole
antenna ? / 2 (in feet) 468 / f in MHz
Vertical Ground Plane Antenna - Ground acts
like a virtual ?/4 antenna - Omni-directional
Loading coils - Antenna is electrically ?/4
in length - Shorter physically
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Antenna Radiation Patterns
Zero dB gain (compared to dipole) Z 30 O,
156.800 MHz 18 Omnidirectional in horizontal
3 dB gain (compared to ?/4 ground plane) 156.800
MHz 38 Vertical coverage closer to ground
(apparent gain)
3 dB gain (compared to ?/4 ground plane) Z 50
O Loading coil in bottom - Matches impedance to
50 O
6 dB gain (compared to ?/4 ground plane) Z 50
O, length approximately eight feet Multi
elements, phasing, and impedance matching
components inside
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Quarter-wavelength Vertical

• Has zero dB gain (compared to dipole)
• 5/8 wavelength vertical has 3 dB gain
• Use tapped loading coil to get a 50 ? antenna

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Gain and HF Antennas

• 6 dB formed from two 3 dB antennas
• Vertically stacked
• Properly phased
• 9 dB from two 6 dB antennas
• VHF range is function of antenna height
• Not antenna gain
• D (in nm) 1.32 x vh (in feet)
• HF antennas are covered in Chapter 7

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VHF Antenna Problems

• VHF antennas should be installed vertically
• Limit gain on a sailboat antenna to 3 dB

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Coaxial Cable

• 50? coax is used between radio and antenna
• normally cut to length on boat
• terminated with PL-259 male connectors
• dont splice, use PL-258 bulkhead connectors

Soldering of PL-259 connectors is covered in the
Marine Electrical Systems Chapter 2 and in the
USPS Marine Radio Guide
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Coaxial Cable Information

• Cable Z Diameter 150 MHz
• (impedance)
  (attenuation per 100)
• RG-58U 50 .194 - 5.8 dB
• RG-8X 50 .242 - 3.2 dB
• RG-213 50 .405 - 3.2 dB
• 9913 50 .410 - 1.7 dB

• Need less than 3 dB total loss (Radio to antenna)
• Approx 0.2 to 0.4 dB per connector
• Waterproof connection at antenna
• Silicone grease inside connector
• Tape outside with waterproof (Mastic) tape

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VHF Transceivers

• All VHF radios are actually transceivers
• Not separate transmitter and receiver
• Transmitter and receiver are in same package
• Share some common components

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Specifications

• Sensitivity
• Selectivity
• Transmitter power
• Water resistance
• Reliability
• Battery life

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Sensitivity and Selectivity

• Sensitivity (smaller is better)
• Ability to capture weak signals
• Determined by RF amplifier
• For fixed-mount 0.22 to 0.32 µvolts
• For handheld 0.25 to 0.35 µvolts
• Selectivity (bigger is better)
• Ability to reject unwanted signals
• Determined by IF amplifier
• For fixed-mount 65 to 85 dB
• For handheld 60 to 71 dB

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TX Power Water Resistance

• Transmitter Power
• Limited by FCC to 25 watts
• Handheld 1, 3 or 5 watts
• Impacts battery life
• Water Resistance
• JIS 3 Rain Falling rain at 60º angle
• JIS 4 Splash Splashing water any direction
• JIS 6 Water Tight Jetting water any direction
• JIS 7 Immersion At 1 meter for 30 minutes
• JIS 8 Submersible At 1.5 meters

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Reliability and Battery Life

• Reliability
• MTBF number not available
• Probably over 10,000 hours
• Reputation of manufacturer
• Most are guaranteed for 3 years
• Battery life
• When comparing use same duty cycle, same type
  size battery and same transmitter power
• Duty cycle 5-5-90
• 5 of time transmitting
• 5 of time receiving
• 90 of time listening for a signal

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Summary

• VHF uses FM
• Capture effect
• Frequency deviation is related to voice amplitude
• Range is due to antenna height (not power)
• VHF uses vertical antennas
• Omnidirectional
• Dont want high gain on a sailboat
• 50? coax cut to length and dont splice
• Sensitivity RF amp detects weak signal
• Squelch setting
• Selectivity IF amp rejects unwanted signal

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