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>>