1. Basic (VHF) Radio Communications
Chapter 3
Basic (VHF) Radio Communications
PowerPoint slides by
Stf/C Harl Porter, SN
Marine Electronics
Rear Commander for Electro-Mechanical Systems is
R/C Gene Danko, SN
Left is 36” 3dB gain sailboat VHF antenna
Left center is Standard Horizon fixed-mount VHF radio
Right center is ICOM IC-M34 handheld VHF radio
Right is 8’ 6 dB gain power boat VHF antenna

2. Overview Basic VHF Transmitters Basic VHF Receivers VHF Antennas
Coaxial Cables
VHF Transceiver
Specifications
Summary
Major sections in this chapter
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3. Basic VHF Transmitter Transmitter RF Audio Very High Frequency Heat DC
Fig 3-1
MF/HF Transmitter Black Box showing Inputs and Outputs.
Audio is bandpass filtered inside transmitter to 300 to 3,000 Hz.
RF 156 to 162 is Marine VHF Band
Power output is either 1 or 25 watts (fixed mount)
1 or 5 watts for most handhelds
Heat is also produced by all electronics
Audio – 300 to 3,000 Hz DC – 12 to 14 5A
RF – MHz
@ 1 or 25 watts
Heat – a few watts
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4. (Frequency Modulation)
FM Transmitter
(Frequency Modulation)
(Antenna)
Variable
Oscillator FM
Modulator
Frequency
Multiplier RF
Amplifier
MHz
6.533 MHz
x 2 x 3 x 2 x 2
(Channel 16)
(Frequency Selection)
300 – 3,000 Hz (microphone)
Fig 3-2
Explain what each block does (see text)
FM modulator – changes frequency of oscillator
Variable (frequency) oscillator – MHz for Channel 16
Frequency multiplier – increases frequency to MHz
RF Amplifier – increases power to desired limit
Volume from mike is limited as volume relates to amount frequency swing and is limited to +/- 5 KHz at the output frequency. Rate of frequency swing is determined by frequencies into microphone (mike). It is easier to visualize using a single frequency into mike. Amplitude of output signal is held constant.
Note that PTT switch (control) and High/Low power switches are NOT shown here.
Modulator varies frequency of the MHz fundamental
Not shown
- PTT switch
- High/Low power switch
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5. 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)
How FM works
Given a single frequency modulating signal (1 KHz), the output is 150 MHz +/- 5 KHz. The volume of the modulating signal equates to a frequency swing from 150 MHz. The loudest input from the mike will give +/- 5 kHz swing; a soft input to the mike may only give less than 1 KHz swing.
Rate of swing, across the center frequency, is derived from the modulating signal frequency. In this case, rate of swing across center frequency, is 1 kHz.
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6. Frequency Modulation Note: Top two waveforms are voltage vs time
Bottom waveform is frequency vs time
Art on left shows
Carrier (top); 150 MHz in previous example
Modulating signal (center); 1 kHz
FM modulated signal (bottom)
Y-axis is amplitude; X-axis is time
We can see the change in frequency, but not the rate of change
On the right is an oscilloscope picture showing the modulating signal and resulting FM modulated signal.
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7. Basic VHF Receiver Receiver Audio Heat RF DC Very High Frequency
Fig 3-3
MF/HF Receiver Black Box showing Inputs and Outputs
RF 156 to 162 MHz
Wider than VHF Marine Band
156 to 158 MHz is Marine VHF band
162 to 163 MHz are the NOAA weather channels
DC current is only 0.5A (much less than transmitter power)
Audio is 300 to 3,000 Hz (determined by 3 watts
Heat is also produced by all electronics
RF – MHz DC – 12 to A
Audio – 300 to 3,000 Hz
@ 3 watts
Heat – a few watts
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8. FM Receiver (Antenna) (Speaker) RF Amplifier IF Amp De- modulator
Audio
Amplifier
Mixer
Limiter
MHz
10.7 MHz
300–3,000 Hz
(Channel 16)
(Squelch Control)
(Volume Control)
Local Oscillator
Fig 3-4
Explain what each block does (see text)
RF Amplifier
Local Oscillator
Mixer
IF Amplifier
Limiter
Demodulator
Audio Amplifier
Three controls – frequency, squelch and volume
Frequencies shown are for Channel 16
146.1 MHz
(Frequency selection)
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9. 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
Unique features of FM
Capture Effect - If there are two signals on the same frequency, of different signal strengths, only the stronger one will be detected and heard in the speaker. This is unlike AM, where both signals will be heard.
Squelch Control - It eliminates speaker noise (hiss) when no signal is present. If set too high, weak signals will not be heard.
Another advantage of FM is the absence of AM noise spikes (interference). The limiter clips (eliminates) all noise spike from the received FM signal, and makes most received FM signals the same strength (this way you don’t have to readjust the volume control for different stations).
Demodulation
Volume for amount of frequency shift from center frequency
Frequency from rate of change across center frequency
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10. VHF Antennas 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 ( 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)
Note that the formula for wavelength contains a constant that gives the value in feet. This constant is 300 if you want length in meters.
Note that the second formula is for half-wavelength. The basic dipole antenna is half a wavelength long. The speed of RF in wire or antenna is assumed to be 0.95 the speed of light and is factored into the formula.
Boat antennas are based on quarter-wavelength verticals.
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MComm – Ch3 - 10 10

11. Antenna Length Wave Length λ = Speed of Propagation / Frequency (Hz) λ
λ (in feet) = 982 / f in MHz λ
Speed of Propagation assumed 0.95
Dipole antenna λ / 2 (in feet) = 468 / f in MHz 2 λ 4 λ
Vertical Ground Plane Antenna
- Ground acts like a virtual λ/4 antenna
- Omni-directional
Top illustrates one cycle of RF
Second from top illustrates a Dipole Antenna
Third from top illustrates a quarter-wavelength antenna. Note that the ground acts like a virtual ¼ wavelength antenna, making it electrically equivalent to a half-wavelength dipole antenna oriented vertically.
Bottom illustration show the use of a loading coil to make a short physical length antenna electrically longer.
Illustration on the right is a radiation pattern from a vertical antenna. Only half of pattern is shown. Think of it as a donut over the antenna.
Loading coils
- Antenna is electrically λ/4 in length
- Shorter physically
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12. Antenna Radiation Patterns
Zero dB gain (compared to dipole)
Z = 30 Ω, MHz = 18”
Omnidirectional in horizontal 4 λ
30 Ω
0 dB
3 dB gain (compared to λ/4 ground plane)
MHz = 38”
Vertical coverage closer to ground (apparent gain) 8
λ 5
3 dB
3 dB gain (compared to λ/4 ground plane)
Z = 50 Ω
Loading coil in bottom
- Matches impedance to 50 Ω
50 Ω
3 dB
All vertical antennas radiate horizontally in all directions as depicted in right illustration.
As gain increases, the coverage is reduced in the vertical direction. Think of a squashed donut with its volume held constant.
Top illustration is quarter-wavelength vertical. By definition is has a zero dB gain (as does a dipole antenna); this antenna has a gain of one and is sometimes called a unity gain antenna.
Next is a 5/8-wavelength vertical antenna with an apparent gain of 3 dB. Note that it achieves this horizontal gain by decrease in vertical gain; the donut is flatter.
Third from top illustrates tapping a loading coil at the 50 ohm point. This is how antennas are matched to the radio.
The bottom illustration shows two 3 dB antennas stacked vertically and phased to produce a 6 dB gain powerboat antenna.
6 dB
6 dB gain (compared to λ/4 ground plane)
Z = 50 Ω, length approximately eight feet
Multi elements, phasing, and impedance matching components inside
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13. 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
Basic Vertical Antenna
Gain Vertical Antennas (volume of “donut” is constant)
On left is Fig 3-5
On right is Fig 3-6
By definition the gain of a dipole (half-wavelength long) or quarter-wavelength vertical is zero dB.
Most sailboat VHF antennas are 5/8-wavelength verticals with a 3 dB gain. It radiates more energy straight out from the antenna, and less energy up and down. Think of it a a flattened donut with its volume held constant, where the volume of the donut is the total power radiated.
The “can” at the bottom of the sailboat VHF vertical antenna contains a loading coil. The loading coil is “tapped” at its 50-ohm point to match the antenna with the radio.
Note use of the Ohm ”Ω” symbol in last bullet.
Different antenna gains are shown in the right illustration.
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14. 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 7
A 6 dB antenna consists of two vertically stacked and properly phased 3 dB antennas.
Similarly a 9 dB antenna consists of two 6 dB antennas.
A high gain VHF antenna may be a waste of money as range is primarily a function of antenna height, not antenna gain. To increase VHF range, install a higher antenna. If you are using a handheld, get as high as practical. This is the reason sailboats mount their VHF antennas at top of their masts. A sailboat has a longer VHF radio range that a powerboat with a lower antenna.
Formula for deterring VHF radio range is as shown on this slide
Antenna at 16 feet gives approx 5.3 nm
Antenna at 49 feet gives approx 8.6 nm
This is to a radio at sea level; to get real range also do range calculations for other radio and then add the two results.
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15. VHF Antenna Problems VHF antennas should be installed vertically
Limit gain on a sailboat antenna to 3 dB
Power boat antennas should be vertical
Disadvantage of high gain (over 3 dB) antenna on a sailboat
On right is Fig 3-7
VHF antennas should be vertical.
In the example on the left, the power boat has raked their VHF antenna aft to make the boat look fast. This is not good for VHF coverage; on-water coverage will be reduced (but good for talking with airplanes in front of the boat).
The gain of a sailboat antenna is normally limited to 3 dB, as when heeled over, their mast is tilted which causes antenna radiation pattern to be tilted from the horizontal. This is shown on the right illustration.
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16. Coaxial 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
PL-259
PL-258
RG-213
Recommend soldered PL-259 connectors (not solderless).
Badly made terminations may have high attenuation.
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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17. Coaxial Cable Information
Cable Z Diameter MHz
(impedance) (attenuation per 100’)
RG-58U dB
RG-8X dB
RG dB 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
Table 3-8
Table gives attenuation at 150 MHz
Note that smallest cable has highest attenuation
Cost goes up with low loss
Loss in connections is a soft number
Total attenuation is coax loss plus connector loss
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18. VHF Transceivers All VHF radios are actually transceivers
Not separate transmitter and receiver
Transmitter and receiver are in same package
Share some common components
Upper right ICOM M34, about $170
Lower right Standard Horizon Quest under $200. Typical medium grade fixed VHF transceiver.
Left ICOM 604 approx $600 shown with remote mike. High end radio. Remote mike can also be added to most Standard Horizon radios. This allows the radio to be operated (all controls) from either the radio (e.g., helm) or a remote location (e.g., a navigation station below deck).
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19. Specifications Sensitivity Selectivity Transmitter power
Water resistance
Reliability
Battery life
Topics in this section
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20. 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
Sensitivity is given in micro “μ” volts
Smaller (ability to detect a weak signal) is better
There are two Sensitivity standards:
SNR (Signal-to-Noise) ratio
SINAD (Signal-to-Noise and Distortion)
Both figures are given in μvolts with the standard normally not specified unless you read the product’s detailed specification.
Selectivity is given in dB, usually with a negative number implied
Bigger (greater rejection of adjacent channel signal) is better.
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21. 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
Transmit power limited by FCC Regulation to 25 watts maximum. On some channels, e.g. 09 and 13, limited to 1 watt. Today's radios automatically reduce power to one watt when required.
JIS 8 (Submersible) time in the water at 1.5 meters is given as continuous (over 30 minutes, but actual duration not specified).
Fixed-mount radios should be watertight (JIS 6) or better.
Handheld radios normally are Immersion (JIS 7) or Submersible (JIS 8).
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22. Reliability and Battery Life
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% of time transmitting
5% of time receiving
90% of time listening for a signal
There are 8,670 hours in a year. The Mean Time Between Failures (MTBF) number is generally not given, even in detailed product specifications, so you have to “measure” reliability by the duration of a manufacturer’s guarantee and reputation.
Most manufacturers give battery life; however, duty cycle is not given nor power setting. When comparing battery life, make sure is same size and type of battery at same power setting for same duty cycle. Good Luck: like MTBF, there is usually not enough information given to compare different radios by different manufacturers.
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23. Summary VHF uses FM VHF uses vertical antennas
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
No notes
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