Chapter 3 Basic (VHF) Radio Communications MComm – Ch3 - 1.
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Transcript of Chapter 3 Basic (VHF) Radio Communications MComm – Ch3 - 1.
Chapter 3Chapter 3
Basic (VHF) Radio Communications
MComm – Ch3 - 1
• Basic VHF Transmitters• Basic VHF Receivers• VHF Antennas• Coaxial Cables• VHF Transceiver• Specifications• Summary
OverviewOverview
MComm – Ch3 - 2>>
Audio – 300 to 3,000 HzDC – 12 to 14 VDC @ 5A
Basic VHF TransmitterBasic VHF
Transmitter
MComm – Ch3 - 3
Very High Frequency
Transmitter
Audio
DC
RF
Heat
RF – 156-162 MHz
@ 1 or 25 watts
Heat – a few watts
>>
FM TransmitterFM Transmitter
MComm – Ch3 - 4
(Frequency Modulation)
Variable
Oscillator
Frequency
Multiplier
RF
Amplifier
300 – 3,000 Hz (microphone)
Modulator varies frequency of the 156.800 MHz fundamental
(Frequency Selection)
Not shown
- PTT switch
- High/Low power switch
FM
Modulator
x 2 x 3 x 2 x 2
(Antenna)
6.533 MHz 156.800 MHz(Channel 16)
>>
• 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)
Frequency ModulationFrequency Modulation
MComm – Ch3 - 5>>
Frequency ModulationFrequency Modulation
MComm – Ch3 - 6
Note:
Top two waveforms are voltage vs time
Bottom waveform is frequency vs time
>>
RF – 156 - 162 MHzDC – 12 to 14 VDC @ 0.5A
Basic VHF ReceiverBasic VHF Receiver
MComm – Ch3 - 7
Very High Frequency
Receiver
Audio
HeatRF
DC
Audio – 300 to 3,000 Hz
@ 3 watts
Heat – a few watts
>>
FM ReceiverFM Receiver
MComm – Ch3 - 8
RF Amplifier Mixer IF
AmpAudio
AmplifierDe- modulator
(Antenna) (Speaker)
146.1 MHz
300–3,000 Hz
Limiter
Local Oscillator
(Squelch Control)
(Channel 16)10.7 MHz156.800
MHz (Volume Control)
(Frequency selection)
>>
• 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
Frequency ModulationFrequency Modulation
MComm – Ch3 - 9>>
• Speed of light & radio waves in free space 300,000,000 meters/sec
or 186,000 statute miles/sec
• Wavelength (λ) = 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) λ/2 = 468 / f (in MHz)
VHF AntennasVHF Antennas
MComm – Ch3 - 10>>
Antenna LengthAntenna Length
MComm – Ch3 - 11
Wave Length λ = Speed of Propagation / Frequency (Hz) λ (in feet) = 982 / f in MHz
Speed of Propagation assumed 0.95 Dipole antenna λ / 2 (in feet) = 468 / f in MHz
Vertical Ground Plane Antenna - Ground acts like a virtual λ/4 antenna - Omni-directional
Loading coils - Antenna is electrically λ/4 in length - Shorter physically
2λ
λ
4λ
>>
Antenna Radiation Patterns
Antenna Radiation Patterns
MComm – Ch3 - 12
Zero dB gain (compared to dipole)Z = 30 Ω, 156.800 MHz = 18”Omnidirectional in horizontal
3 dB gain (compared to λ/4 ground plane)156.800 MHz = 38”Vertical coverage closer to ground (apparent gain)
3 dB gain (compared to λ/4 ground plane)Z = 50 ΩLoading coil in bottom - Matches impedance to 50 Ω
6 dB gain (compared to λ/4 ground plane)Z = 50 Ω, length approximately eight feet
Multi elements, phasing, and impedance matching components inside
8λ 5 3 dB
4λ
30 Ω
0 dB
6 dB
50 Ω
3 dB
>>
Quarter-wavelength Vertical
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 Ω
antenna
MComm – Ch3 - 13
Basic Vertical Antenna
Gain Vertical Antennas (volume of “donut” is constant)
>>
Gain and HF Antennas
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 √h (in feet)
• HF antennas are covered in Chapter 7MComm – Ch3 - 14>>
VHF Antenna ProblemsVHF Antenna Problems
• VHF antennas should be installed vertically• Limit gain on a sailboat antenna to 3 dB
MComm – Ch3 - 15
Disadvantage of high gain (over 3 dB) antenna on a sailboatPower boat antennas
should be vertical>>
Coaxial CableCoaxial Cable• 50Ω coax is used between radio and antenna
normally cut to length on boat terminated with PL-259 male connectors don’t splice, use PL-258 bulkhead connectors
MComm – Ch3 - 16
Soldering of PL-259 connectors is covered in the Marine Electrical Systems Chapter 2 and in the USPS Marine Radio Guide
RG-213 PL-259 PL-258
>>
Cable Z Diameter 150 MHz (impedance) (attenuation per 100’)
RG-58U 50 .194 - 5.8 dBRG-8X 50 .242 - 3.2 dBRG-213 50 .405 - 3.2 dB 9913 50 .410 - 1.7 dB
Coaxial Cable Information
Coaxial Cable Information
MComm – Ch3 - 17
• 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
>>
VHF TransceiversVHF Transceivers
• All VHF radios are actually transceivers Not separate transmitter and receiver Transmitter and receiver are in same package Share some common components
MComm – Ch3 - 18>>
• Sensitivity• Selectivity• Transmitter power• Water resistance• Reliability• Battery life
SpecificationsSpecifications
MComm – Ch3 - 19>>
• 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
Sensitivity and Selectivity
Sensitivity and Selectivity
MComm – Ch3 - 20>>
• 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
TX Power & Water Resistance
TX Power & Water Resistance
MComm – Ch3 - 21>>
• 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
Reliability and Battery Life
Reliability and Battery Life
MComm – Ch3 - 22>>
• 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 Don’t want high gain on a sailboat
• 50Ω coax – cut to length and don’t splice• Sensitivity – RF amp detects weak signal
Squelch setting
• Selectivity – IF amp rejects unwanted signal
SummarySummary
MComm – Ch3 - 23>>