NTIA REPORT 99-362

Evaluation of Marine VHF Radios:

Performance in the Savannah, Ga. 

and New Orleans, La. Port Areas

Robert L. Sole

Brent Bedford

U.S. DEPARTMENT OF COMMERCE

William Daley, Secretary

Larry Irving, Assistant Secretary

for Communications and Information

April 1999
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ii

EXECUTIVE SUMMARY

The maritime mobile frequency band supports maritime communications worldwide. Appendix 18

of the ITU Radio Regulations (RR) defines the channels of the maritime mobile service. These channels

support a variety of communication functions including: public correspondence, intership, ship-to-coast,

coast- to-ship, port operations, calling and various safety purposes. Safety functions include distress, search

and rescue, ship movement, navigation (bridge-to-bridge) communications, and maritime safety information

broadcasts.

Mariners in the United States and other countries are experiencing interference on channels

allocated to the above functions. The Radio Technical Commission for Maritime Services (RTCM)

established Special Committee 117 to investigate the interference and determine if the International

Electrotechnical (IEC) standard 1097-7 “Global Maritime Distress and Safety System (GMDSS)-Part

7: Shipborne VHF Radiotelephone Transmitter and Receiver-Operational and Performance

Requirements, Methods of Testing and Required Test Results” would be sufficient to protect marine

VHF radios from interference. In support of this effort, NTIA, in coordination with the Coast Guard and

RTCM SC-117, undertook a task to perform tests on commercial and recreational grade marine VHF

radios to the IEC standard and perform radiated tests in areas where severe cases of interference are

occurring. Laboratory testing of the radios to the IEC standard was performed in Boulder, Colorado. The

radiated tests were performed in Savannah, Georgia on the Savannah River and on the Mississippi River

in New Orleans, Louisiana Mariners in both locations have been reporting cases of severe interference in

the marine VHF band on the waterways for quite some time now. Some of the channels experiencing the

interference are key channels used for safety and bridge-to-bridge communications. The interference is very

disruptive to normal operations on the river and is distracting to the radio operators. 

The IEC tests and radiated tests were based on receiver SINAD measurements. In the IEC 1097-

7 test procedures, the SINAD of a receiver being tested was set to 20 dB by adjusting the desired signal

power and then injecting interference into the circuit to reduce the SINAD to 14 dB. The resulting

interference-to-signal ratio (I/S) was then calculated in dB and compared to the minimum IEC requirement.

In the IEC tests, the interference was simulated using signal generators. The IEC tests simulated adjacent

and co-channel interference, receiver saturation (blocking), and intermodulation interference. The radiated

tests used emitters that were present in the electromagnetic environment of the test area to degrade the

SINAD. The radiated testing revealed that emitters in the environment were causing receiver saturation and

intermodulation/cross modulation interference in the radio receivers.

Spectrum recordings were taken during the radiated tests when the SINAD measurements were

14 dB or less. These recordings were used to identify the sources of interference in Savannah and New

Orleans. The recordings seem to indicate that the interference is due to NOAA weather broadcasts and

land mobile transmitters ( 1997 Code of Federal Regulations, Parts 90 and 22 Title 47) operating within

and around the marine VHF band. The antennas of the weather broadcast and land mobile transmitters

are located very close to the river’s edge in both locations and transmit at higher power levels than the

mobile marine VHF radios. Both of these factors contribute to the severity of the interference. However,

it should be noted that the land mobile transmitters are operating in compliance with respect to the FCC

rules and regulations for output power, frequency tolerance, and spurious emissions. The NOAA VHF

weather broadcasts also seem to be operating properly. 
iii

Because they are operating in compliance with the FCC rules and regulations, it would be difficult

to impose any operating restrictions on land mobile systems operators. Therefore practical solutions to

solve this problem would be to continue the development of receiver standards through RTCM SC-117,

encourage mariners to use radios that are more resistant to interference, and develop antenna siting

guidelines for future deployment of weather broadcast and land mobile transmitters to reduce interference.

 
iv

TABLE of CONTENTS

Section 1. Introduction

 1.1 Background ....................................................... 1-1

 1.2 Test Objectives .............................................. 1-2

 1.3 Test Radios ....................................................... 1-2

Section 2. Test Results

 2.1 Savannah, Georgia Tests ............................................ 2-1

 2.2 New Orleans, Louisiana Tests ................................. 2-6

 

Section 3. Interference Coupling Mechanisms .......................... 3-1

 3.1 Blocking and Receiver Front-end Overload/Saturation ............. 3-1

 3.2 Intermodulation and/or Cross-modulation ............................. 3-2

 3.2 RTCM Receiver Standard ........................................... 3-3

Section 4 Evaluation of Radio Performance. .................................. 4-1

Section 5. Conclusions and Recommendations ..........................5 -1

Appendix A: Radiated Test Procedures ...................................... A-1

Appendix B: Savannah SINAD Histographs ............................. B-1

Appendix C: New Orleans SINAD Histographs ................................ C-1
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1-1

SECTION 1

INTRODUCTION

1.1 Background

NTIA published a report for the Coast Guard that studied the compatibility between 12.5 kHz and

25 kHz channelized marine VHF radios, NTIA TR-97-343 “Assessment of Compatibility between 25

and 12.5 kHz Channelized Marine Radios.” One of the conclusions of the report was that the adoption

of appropriate standards for marine VHF radio receivers would help alleviate problems due to congestion

in the marine electromagnetic environment. Currently, the United States does not have receiver standards

for marine VHF radios. An organization comprised of mariners, radio manufacturers, and frequency

regulators, the Radio Technical Commission for Maritime services (RTCM), established RTCM SC-117

to investigate receiver standards developed by other regulatory agencies or organizations. One organization,

the International Electrotechnical Commission (IEC), has recently published a document that contains

proposed standards for marine VHF radios. 

The IEC standards for marine radios are in a document titled “IEC 1097-7: Global Maritime

Distress and Safety System (GMDSS)-Part 7: Shipborne VHF Radiotelephone Transmitter and

Receiver-Operational and Performance Requirements, Methods of Testing and Required Test

Results”.

In support of this effort, NTIA, in coordination with the Coast Guard and RTCM SC-117,

undertook a task to perform tests on commercial and recreational grade marine VHF radios to the IEC

standard and perform radiated tests in areas where severe cases of interference are occurring. This task

was accomplished by testing marine VHF radios in a laboratory to the IEC 1097-7 standard and then

testing those same radios in marine environments where mariners have reported severe cases of

interference. Savannah, Georgia and New Orleans, Louisiana were chosen as locations for “live” testing

due to the numerous complaints from mariners that their communications are being disrupted on the

Savannah and Mississippi Rivers due to interference in the marine VHF band.

The IEC procedures and standards are based on receiver SINAD measurements and signal-tointerference (S/I) ratios. A receiver SINAD measurement is the ratio, expressed in dB, of the desired signal

power to interference, noise, and distortion. The methodology of the IEC laboratory tests was to measure

the receiver SINAD with interference injected into the radio along with the desired signal. Each test had

a minimum performance objective for the SINAD and signal-to-interference (S/I) ratio in the presence of

interfering signals. The laboratory tests used signal generators as interference sources to degrade the

receiver SINAD measurements. The tests performed in Savannah and New Orleans used emitters that

were present in the electromagnetic environment to degrade the receiver SINAD measurement. This report

documents the results of performing SINAD tests on marine VHF radios in the Savannah, Georgia and

New Orleans, Louisiana port areas and how the results relate to the laboratory tests. This report also

attempts to identify the interference coupling mechanisms and ways to mitigate their effects. The results of

the laboratory tests conducted to the IEC 1097 standard, which contains the recorded data for each radio

tested are published in NTIA Report 99-363.
1-2

1.2 Test Objectives

The objectives of these tests were to:

! Measure the SINAD of marine radios operating in a shipboard environment at Savannah,

Georgia and New Orleans, Louisiana.

! Determine the severity of the interference in relation to the IEC 1097 standard. 

! Attempt to identify the sources of interference and the interference mechanisms. 

! Investigate ways to mitigate and preclude the interference.

1.3 Test Radios

Commercially available analog 25 kHz channelized marine VHF FM radios were tested. These 25

kHz radios included four commercial grade radios representative of the type used by commercial boaters

and government agencies. 

Most recreational boaters use less expensive 25 kHz radios that retail for under 300 dollars. These

types of radios, which were found to be more susceptible to interference, were also tested. NTIA

purchased three fixed mount and two hand-held radios of these types from local retailers for testing.

The radios are identified by alphabetical code. Manufacturers’ names and model numbers are not

included in this report. The radios are categorized as either recreational or commercial grade radios and

as either fixed-mount or handheld below in Table 1-2.

Table 1-2

Radio Description

Radio Type Grade

A fixed-mount recreational

25 kHz

B fixed-mount commercial

25 kHz

E hand-held recreational

25 kHz

F fixed-mount commercial

25 kHz

G hand-held recreational

25 kHz

H fixed-mount recreational

25 kHz

I fixed-mount recreational

25 kHz
1-3

K fixed-mount recreational

25 kHz

L fixed-mount commercial

25 kHz

M fixed-mount commercial

25 kHz

Radio L had a local/distance switch for its receiver accessible on a front panel control which was

separate from the squelch control. Radio M had an adjustable attenuator for its receiver which was part

of the squelch control. These radios were designed with these features for operations in congested EM

environments. 
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2-1

SECTION 2

TEST RESULTS

2.1 Savannah, Georgia 

The test area for Savannah is shown below in Figure 2-1. The radios were tested using the

procedures outlined in Appendix A with the test set-up shown in Figure A-1. 

Figure 2-1

Savannah Test Area

The performance of the radios was evaluated on channel 18A (156.900 MHz) during normal

tugboat operations by monitoring the test radios SINAD measurements. This channel is used by a

local tugboat company as a primary communications link between their dispatch office and the

tugboat fleet. The interference on this channel is very disruptive to their business operations and

distracting to radio operators. Other marine channels are affected by the interference in this area as

well, including channels 16 and 13. The test set-up was installed in the wheelhouse of one of the

tugboats and data was collected as the tugboat traveled on the river or was stationary. The test area

is identified in Figure 2-1 as the stretch of the Savannah River between the points labeled the East

Dock and the West Dock. The tugboat dispatch office is also shown in Figure 2-1.

For a baseline configuration, the SINAD of the radio under was set to 20 dB by adjusting

the RF power of the signal generator with a load connected into the circuit in place of the antenna

(see Fig. A-1). When the load was removed and the tugboat antenna was connected into the circuit,

the receiver of the radio being tested immediately experienced de-sensitization and periodic audible

interference was heard in the radio receiver. The de-sensitization was observed by watching the

degradation of the receiver SINAD as measured by the test set. Depending on the radio under test,
2-2

the SINAD was immediately degraded from the 20 dB starting point to approximately 2-14 dB when

the tugboat antenna was connected into the circuit. The audible interference further degraded the

SINAD to 0-4 and was periodic in nature. 

The receiver de-sensitization was not audible in the receivers. Except for radio L operating

in local mode, the periodic audible interference occurred in all radios and rendered the channel

useless for communications during the period that it occurred. The duration of the audible

interference events was approximately 3.5-4 seconds. Multiple consecutive events compounded the

problem.

A SINAD histogram for each radio tested is shown in Appendix B. The histograms show the

percent of time versus SINAD value and includes all SINAD values, not just those below 14 dB.

This level is the minimum allowable SINAD value with interference present in the IEC 1097-7 test

procedures. Radios that had a higher percentage of time with a greater SINAD value had better

performance. This effect is shown in Figures B-1 and B-2. Figure B-1 shows the SINAD histogram

for radio L in the distance mode and Figure B-2 is the same radio operating in local mode. In the

distance mode, the SINAD values ranged from 1-13 dB with the major portion of the distribution

in the 7-10 dB range. In the local mode the SINAD values ranged from 7-19 dB with the major

portion of the distribution in the 10-15 dB range. The differences between the figures show that for

the overall percentages of time, the local mode of operation measured higher SINAD values than

distance mode and the radio had audibly better performance. However, for local mode operation the

radio requires a minimum desired signal power of about -107 dBm versus -117 dBm for distance

mode for a 20 dB SINAD.

Some of the less costly recreational grade radios were essentially “flatlined” by the

desensitization and audible interference. For example, the SINAD for radio E in Figure B-9 never

got above 3 dB. Other recreational grade radios never had a SINAD above 5 dB. These types of

radios are used for commercial and non-commercial marine communications. 

In an attempt to identify the sources of interference, a spectrum analyzer was used to sweep

the 150-174 MHz band and record the power of the emitters when the SINAD of the radio under

test fell below 14 dB. The receiver de-sensitization effect and the audible interference caused the

SINAD to be below the trigger point so that data was collected for each type of interference event.

With the exception of the hand held radios, data was collected for each radio during tugboat

operations. Data was collected for the hand held radios at the tugboat office located at the river’s

edge. 

Since the radios were tested in the same environment and experienced the same type of

interference, the spectrum record taken for an interference event for any one radio is typical for all

radios tested. However, in some cases of interference events, different emitters were observed to be

active at different times. 

The spectrum recordings shown below in Figures 2-2, 2-3, 2-4, and 2-5 were recorded while

testing radio L, tuned to channel 18A (156.9 MHz), on the tugboat. These figures only show emitters

that were transmitting at that instant that this particular spectrum snapshot was taken. Other

spectrum snapshots may show different emitters active at different frequencies other than those

shown in Figures 2-2 to 2-5. Figure 2-2 shows a spectrum record for the 150-164 MHz band.

Figures 2-3, 2-4, and 2-5 show portions of that band in greater detail to help identify individual

emitters.
2-3

-100 

-80 

-60 

-40 

-20 

0 

Received power (dBm)

151 153 155 157 159 161 163 

Frequency (MHz)

Radio L, Distance Mode

Ch 18A, 1.5 dB Sinad

-90 

-80 

-70 

-60 

-50 

-40 

-30 

Received power (dBm)

152 153 154 

Frequency (MHz)

Radio L, Distance Mode

Ch 18A, 1.5 dB Sinad

Figure 2-2

151-164 MHz Band Spectrum Record

This spectrum trace was recorded when the audible interference reduced the SINAD of receiver

L to 1.5 dB. The signals shown below in Figure 2-3 are transmitters operating at the following frequencies:

152.24,152.48, and 152.84 MHz. Their operations were periodic and peaked in the morning and

evening. 

. 

Figure 2-3

152-154 MHz Band Spectrum Record

. 

The signals shown below in Figure 2-4 are transmitters operating at 158.7 and 158.925

MHz. Their operations were periodic and peaked in the morning and evening. 
2-4

-90 

-80 

-70 

-60 

-50 

-40 

-30 

Received power (dBm)

158 159 160 

Frequency (MHz)

Radio L, Distance Mode

Ch 18A, 1.5 dB Sinad

-100 

-80 

-60 

-40 

-20 

0 

Received power (dBm)

162 163 164 

Frequency (MHz)

Radio L, Distance Mode

Ch 18A, 1.5 dB Sinad

Figure 2-4

158-160 MHz Band Spectrum Record

Figure 2-5

162-164 MHz Band Spectrum Record

The signal shown in Figure 2-5 is identified as a NOAA weather transmitter broadcasting at 162.4

MHz. This transmitter is also identified as WX2 in the VHF marine weather channel assignments. This

transmitter was “on” at all times and not periodic like some of the emitters in the previous figures. The figure

shows the received power to be approximately -10 dBm. This signal and its interference mechanism was

very similar to the IEC “blocking” test, which was shown to saturate the radio receivers at such a high

power level.

The land mobile transmitters identified in Figures 2-3 and 2-4 produced received power levels up

to -27 dBm at the radio receiver. These power levels were shown to produce intermodulation products

in some receivers in the IEC intermodulation bench test.
2-5

At the tugboat dispatch office, additional tests were performed by using RF step attenuators to

decrease the interferer powers till the audible periodic interference was no longer heard in the radio

receiver. This test was performed aurally and did not involve a SINAD measurement. The amount of

attenuation each radio needed to effectively eliminate the audible interference is shown below in Table 2-1.

Table 2-1

Receiver Attenuation Values

Radio Receiver Attenuation (dB)

B 10

F 10

G 20

H 10

I 30

K 20

As shown in Table 2-1, the range of attenuation needed to lower the power of the land mobile

transmitters and weather channel emissions into the radio receiver to reduce the audible interference so that

the 1 kHz test tone was audible ranged from 10 to 30 dB. The attenuation also reduced the sensitivity of

the radio. The resolution for these tests was 10 dB. It was difficult to “hear” the change in the level of the

audible interference using smaller steps of attenuation. 
2-6

2.2 New Orleans, Louisiana

The test area for New Orleans is shown below in Figure 2-6. The radios were tested using

the procedures outlined in Appendix A. 

Figure 2-6

New Orleans Test Area

The SINAD performance of the radios was evaluated on marine VHF channels 25 (public

correspondence receive, 161.850 MHz) and 67 (bridge-to-bridge, 156.375 MHz). The test set-up

was installed in a fire boat and data was collected as the boat traveled the river or was docked. The

radios were tested using channel 25 as the desired signal channel while docked at location 1 shown

in Figure 2-7. The radios were tested using channel 67 as the desired signal channel while docked

at location 2 shown in Figure 2-6. Channel 67 is the primary channel for bridge-to-bridge

communications on the lower Mississippi River. Mariners have complained about the interference

on this channel for quite some time. Interference on this channel can make communications difficult

between ships on the river. The interference on channel 67 seemed to be “worst” in the river main

channel. However, due to the large amount of traffic in the river channel it was not safe to loiter in

the channel itself and collect data while testing multiple radios. 

At both test locations, the radios experienced receiver de-sensitization and periodic audible

interference was heard in the radio receiver. The de-sensitization was observed by watching the

degradation of the receiver SINAD as measured by the communication test set. Depending on the

radio being tested, the SINAD was reduced from the 20 dB starting point to approximately 2-15 dB

when the ship’s antenna was connected into the circuit. The periodic audible interference further

reduced the SINAD to approximately 1-5 dB. 
2-7

The receiver de-sensitization was not audible on the receivers. Except for radio L operating

in local mode and radio M with the built in attenuator set at its maximum value, the periodic audible

interference occurred in all radios and rendered the channel useless for communications during that

period. The duration of the audible interference events was approximately 3.5-4 seconds. Multiple

consecutive events compounded the problem..

A SINAD histogram for each radio tested in New Orleans is shown in Appendix C. As

shown in Savannah, radios that had a higher percentage of time with greater SINAD values had

better performance. This effect is once again shown for radio L in Figures C-1 and C-2. Figure C-1

shows the SINAD histogram for radio L in distance mode and Figure C-2 is radio L in local mode

operation. In distance mode the SINAD values ranged from 1-8 dB with the major portion of the

distribution in the 2-4 dB range. In the local mode the SINAD values ranged from 14-19 dB with

the major portion of the distribution in the 14-17 dB range. The differences between the figures

show that for the overall percentages of time, the local mode of operation measured higher SINAD

values than distance mode and the radio had audibly better performance. As in the case of Savannah,

the SINAD measurements of the recreational grade radios in New Orleans was in the 0-5 dB range

and the 1 kHz test tone was inaudible due to the interference. 

A spectrum analyzer was used to sweep 150-174 MHz band and record the power of the

emitters when the SINAD of the radio under test was 14 dB or less. These spectrum recordings

showed that the EM environment of New Orleans was denser than Savannah with more emitters

present. The receiver de-sensitization effect and the audible interference caused the SINAD to be

below the trigger point so that data was collected for each type of interference event. The dense EM

environment made it difficult to identify specific emitters during the audible interference.

Sample spectrum plots are shown below in Figures 2-7 to 2-9 They were taken during the

testing of radio L at location one. These plots are representative of the EM environment at that

instant that this particular SINAD measurement was taken. They should not be considered to be

indicative for all cases of interference events.
-100 

-80 

-60 

-40 

-20 

0 

Received Power, dBm

151 152 153 154 155 156 157 158 159 160 161 162 163 

Frequency, MHz

New Orleans

Spectrum Plot

-100 

-80 

-60 

-40 

-20 

0 

Received Power, dBm

151 152 153 154 155 

Frequency, MHz

New Orleans

Spectrum Plot

2-8

Figure 2-7

151-163.5 MHz Band

Figure 2-8

151-156 MHz Band
-100 

-80 

-60 

-40 

-20 

0 

Received Power, dBm

157 158 159 160 161 162 163 

Frequency, MHz

New Orleans

Spectrum Plot

2-9

Figure 2-9

157-163.5 MHz Band

The figures show numerous maritime and land mobile emitters in this band in New Orleans. The

transmitter that produced the highest received power on the fireboat in New Orleans was weather channel

WX1 at 162.55 MHz. The received power at the radio RF input jack from this transmitter was

approximately -15 dBm. The IEC blocking tests showed that this power level would saturate the radio

receivers. The power level fluctuated as the ship traversed the river due to shielding from the downtown

buildings and bridges. This would explain why some spots on the river are more prone to interference than

others. 

Additional tests were performed by using the RF step attenuators to decrease the power of the

interferers till the audible periodic interference was no longer heard in the radio receiver. This test was

performed aurally and did not involve a SINAD measurement. The amount of attenuation each radio

needed to effectively eliminate the audible interference is shown below in Table 2-2. 

Table 2-2

Receiver Attenuation Values

Radio Receiver Attenuation (dB)

B 20

E 20

F 10

G 30

H 10

I 30

K 20

M 10
2-10

As shown in Table 2-2, the range of attenuation needed to lower the power of the land mobile

transmitters and weather channel emissions into the radio receiver to reduce the audible interference so that

the 1 kHz test tone was audible ranged from 10 to 30 dB. The attenuation also reduced the sensitivity of

the radio. The resolution for these tests was 10 dB. It was difficult to “hear” the change in the level of the

audible interference using smaller steps of attenuation.. 
3-1

SECTION 3

INTERFERENCE COUPLING MECHANISMS

3. Blocking and Receiver Front-end Overload/Saturation

The receiver front-end overload/saturation and subsequent SINAD reduction at both Savannah

and New Orleans would seem to be due to proximity of NOAA VHF weather broadcast transmitters to

mariners on the Savannah and Mississippi rivers. At Savannah the NOAA weather station broadcasts at

162.4 MHz and is identified as WX2. The antenna used by this weather broadcast station is located on top

of a 10 story building located under half a mile from the river’s edge near the tugboat company dispatch

office. During testing, the distance from the weather broadcast antenna to the tugboat ranged from one-half

to five miles. The highest the received power of the weather station as measured at the input of each radio

was approximately -10 dBm..

The tugboat company dispatch office had two antennas mounted on a tower that was

approximately 100 feet high for their base station radio operations. One antenna was mounted about 50

feet up the tower and the other was mounted close to the top. The radio in the dispatch office did not

experience audible interference and had a good SINAD measurement when connected to the lower

antenna. When connected to the top antenna the dispatch radio experienced interference similar to that of

the radio tested on the tugboat in the river. Visual inspection of the geometry between the lower antenna

on the dispatch tower and weather antenna on the bank building showed that the weather signal was

diffracting over the edge of an adjacent building that was taller than the lower antenna. This signal diffraction

lowered the received power of the weather broadcast by about 20-30 dB over the top antenna and almost

eliminated the interference in the dispatch office radio. This effect clearly demonstrated that the weather

broadcast was partly to blame for the interference in some radios. Other test radios were also tested in this

manner with the same result. 

 In New Orleans the NOAA weather station broadcasts at 162.55 MHz and is identified as WX1.

The antenna used by this weather broadcast station is located on top of a 20 story building located about

half a mile from the river’s edge. During testing, the distance from the weather broadcast antenna to the fire

boat ranged from about half a mile at location one to under two miles at location two. The highest the

received power of the weather station as measured at the input of each radio was approximately -15

dBm.. The weather broadcasts were continuously on during the testing. The received power level changed

as the boat moved in the river due to shielding and diffraction.

The IEC specification for blocking is -23 dBm at the radio RF input, which would result in a

minimum SINAD of 14 dB. In Savannah the received power of the NOAA signal, which resembles the

unwanted signal in the IEC blocking test, was 13 dB above the specification. In New Orleans the received

power of the NOAA signal was 8 dB above the specification. 

Some RF filtering in the front end of marine radios might be expected because the desired signal

channels and the weather broadcasts were separated by at least 700 kHz or more. However, the marine

radios that were tested were designed to receive all VHF marine weather broadcast channels and therefore

do not have any RF rejection to NOAA weather broadcasts. In addition, the marine VHF radio receivers

also do not have any RF rejection to land mobile transmitters operating within the band.

Receiver front-end overload/saturation may not be audible or apparent to the radio operators. The
3-2

weather broadcasts were audible in only one of the radio receivers while it was tuned to a desired test

channel. Non-audible interference would cause a reduction in the reception range for the radio receiver

and would not be obvious to the radio operator. For example, if the radio operator was unable to discern

a weak signal in the presence of the weather broadcasts or any other strong in-band signal, they might say

that the radio had gone “deaf”. The radio operator would not be able to hear the person that they were

trying to communicate with, but also would not hear any interference coming through the radio receiver.

Hence, the radio was quiet and had gone “deaf”.

3.2 Intermodulation and/or Cross-modulation

In addition to maritime communications, the VHF maritime mobile band is used by land mobile

systems for a variety of functions. The functions and the frequencies that these land mobile systems operate

on are listed in Parts 22 (Public Mobile Service) and 90 (Private Land Mobile Radio Services ) of the

Code of Federal Regulations (CFR). The spectrum recordings that were taken in Savannah and New

Orleans showed that numerous land mobile systems that operate according to these parts of the CFR were

active in both areas.

 In Savannah and New Orleans, the audible intermodulation interference in the radio receivers

seems to be attributable to land mobile transmitters and the NOAA VHF weather broadcasts. The land

mobile transmitters and the weather broadcasts were identified as possible sources of interference by

listening to the interference and observing the display on the spectrum analyzer. However, the interference

mechanism may be a combination of intermodulation and/or cross modulation products. An intermodulation

product is due to two or more signals driving the radio receiver circuitry into non-linear operation. Cross

modulation is the result of an interfering signal that is strong enough to act as a local oscillator in the radio

receiver. 

The land mobile transmitters seemed to initiate the audible interference events. This was observed

by listening to the radio and watching the spectrum analyzer’s display. The audible interference only

occurred when the land mobile transmitters were active. During the audible interference events, receiver

addressing tones could be heard followed by land mobile data transmissions. During the land mobile

transmissions the weather broadcast could also be heard as somewhat garbled but identifiable voice

interference in the radio receiver. 

Previous studies done by the FCC have shown that when WX2 is not broadcasting in Savannah,

the audible interference does not occur (this test was not performed by NTIA). According to the FCC the

WX2 weather broadcast has been transmitting from the same antenna location for over 10 years and

complaints from mariners about the interference only started to be reported in the past few years as land

mobile systems were deployed in the area. During our tests, WX2 was continuously transmitting and the

audible interference only occurred when the land mobile transmitters became active. The results of these

tests show that the combination of land mobile transmitters and weather broadcasts in the same EM

environment can generate interference in marine VHF radio receivers. 

Personnel from the FCC Compliance Bureau field office based in New Orleans have investigated

the reports of interference in Savannah and New Orleans and stated that the land mobile systems and

NOAA weather broadcasts in Savannah and New Orleans are operating within Federal guidelines with

respect to output power, signal deviation, and spurious emissions
3-3

Although land mobile transmitters have been identified as a component of the audible interference,

determining which land mobile transmitters are causing the interference on specific marine channels is

difficult. Inspections of the spectrum recordings show that numerous land mobile transmitters were active

when these recordings were made and the interference was present. With so many land mobile transmitters

active when the interference was heard, it was difficult to determine just exactly which ones were

contributing to the interference on the desired test channels. 

The received power levels of the land mobile transmitters at the radios RF input were

greater than the unwanted signal power levels that were used in the IEC intermodulation rejection ratio

test. The received power levels of the land mobile transmitters was up to -27 dBm at the radios RF

input and generated intermodulation interference in all receivers except radio M and Radio L operating

 in local mode. Testing the radios in the laboratory to the IEC intermodulation rejection ratio procedures

revealed that unwanted signal powers ranging from -55 to -34 dBm were sufficient to generate the

intermodulation interference and lower the SINAD to 14 dB. The only radio which had laboratory test

values equal to the land mobile received power levels was radio L operating in local mode. Radio was

not available for testing when the laboratory test were performed. 

3.3 RTCM RECEIVER STANDARD

RTCM has developed a draft receiver standard for marine VHF radios that is based on the

received powers of the land mobile and NOAA transmitters that were measured at the RF input of the test

radios. The standard is a combination of blocking and intermodulation rejection ratio tests which are similar

to tests and procedures outlined in IEC 1097. The major difference between the RTCM test and the IEC

tests is that the unwanted signal powers in the RTCM standard are set at specific levels while the IEC tests

are referenced to the receivers maximum usable sensitivity. In the RTCM standard, the blocking signal is

set at -15 dBm at the receiver input and the two unwanted signals that simulate the land mobile transmitters

are set at -27 dBm. The RTCM performance requirement is that the receiver have a minimum SINAD of

14 dB with a wanted signal power of -107 dBm at the receiver RF input and the afore mentioned

unwanted signals also being present. 
4-1

SECTION 4

EVALUATION of RADIO PERFORMANCE

The performance of the radios was evaluated using the SINAD histograms and by listening to the

radio during the tests. Radios that had histograms which showed a distribution of SINAD measurements

to higher values for a greater percentage of time performed better than those radios that had low SINAD

values for higher percentages of time. Some radios performed better in Savannah than New Orleans.

A brief discussion of each radio’s histograms from the New Orleans and Savannah tests is given

in the following paragraphs.

Radio A

Radio A is a recreational grade fixed mount radio.The histogram for this radio (Figure B-7) shows

that this radio receiver never had a SINAD above 4 dB and that most of the SINAD distribution was in

the 1-3 dB range. The radio was in a constant state of saturation and the 1 kHz test tone was almost

inaudible due to the intermodulation interference. This wasthe only radio in which the weather broadcast

could be heard when the radio was tuned to the test channel. The radio broke and was not tested in New

Orleans. This radio performed poorly. 

Radio B

Radio B is a commercial grade fixed mountradio. The histogram for this radio (Figure B-6 and

Figure C-4) shows that in Savannah the radio had a SINAD distribution of 0-13 dB while in New Orleans

the values were never above 4 dB.

Radio E

Radio E is a recreational grade handheld radio. The histograms for this radio (Figures B-9 and C10) show that this radio never had SINAD measurements above 4 dB and that most of the distribution was

in the 0-3 dB range. This radio had a plastic housing and case penetration may have added to its poor

performance.

Radio F

Radio F is a commercial grade fixed mount radio. The histograms for this radio (Figures B-3 and

C-7) show SINAD measurements from 3-14 dB in Savannah and 1-13 dB in New Orleans.This radio

seemed to function well in Savannah but was more degraded in New Orleans. It had better performance

than the recreational grade radios but not as good performance as the other commercial grade radios.

Radio G

Radio G is a recreational grade handheld radio. The histograms for this radio (Figures B-10 and

C-9) shows that this radio receiver had SINAD measurements from 1-5 dB and that most of the SINAD

distribution was in the 1-3 dB range. The radio was in a constant state of saturation and the 1 kHz test tone

was almost inaudible due to the intermodulation interference. This radio performed poorly.
4-2

Radio H

Radio H is a recreational grade fixed mount radio. The histograms for this radio (Figures B-3 and

C-3) show that this radio receiver had SINAD measurements from 1-14 dB and that most of the SINAD

distribution was in the 6-12 dB range. This radio seemed to work better in Savannah than New Orleans.

For a recreational grade radio it had moderate performance.

Radio I

Radio I is a recreational grade fixed mount radio. The histograms for this radio (Figure B-5 and

C-6) show that this radio receiver never had a SINAD above 5 dB and that most of the SINAD

distribution was in the 1-3 dB range. The radio was in a constant state of saturation and the 1 kHz test tone

was inaudible due to the intermodulation interference. This radio had an unshielded plastic housing and case

penetration interference was possibly occurring. This radio performed terribly.

Radio K

Radio K is a recreational grade fixed mount radio. The histograms for this radio (FiguresB-8 and

C-8) show that this radio receiver never had a SINAD above 7 dB and that most of the SINAD

distribution was in the 1-5 dB range. The radio was in a constant state of saturation and the 1 kHz test tone

was almost inaudible due to the intermodulation interference. This radio performed poorly.

Radio L

Radio L is a commercial grade fixed mount radio that incorporates a local/distance switch in its

receiver that is accessible from the front panel of the radio. The histograms for this radio (Figures B-1, B-2

and C-1 and C-2) show that in distance mode (Figures C-1 and B-1) this radio receiver had SINAD

measurementsfrom 1-14 and in local mode had measurements from 8-18 dB. Using the local mode setting

on the radio receiver removed almost all of the audible interference. This radio performed well in local

mode and had marginal performance in distance mode.

Radio M

Radio M is a commercial grade fixed mount radio. The histograms for this radio (Figure C-5)

shows that this radio receiver SINAD values from 1-14 dB. This radio had an externally adjustable

attenuator for operations in harsh EM environments that was also connected to the radio’s squelch control.

Although the radio was able to operate in New Orleans when the attenuator was set to its full value, the

squelch closed and about 3 dB more of desired signal power was needed to restore the SINAD. The radio

was not in the test inventory for the Savannah tests. This radio worked well.
5-1

SECTION 5

CONCLUSIONS and RECOMMENDATIONS

5. Conclusions

 The following conclusions can be drawn from these tests:

1.VHF FM marine radios are experiencing receiver saturation and intermodulation/cross

modulation interference on numerous channels at Savannah, Georgia and New Orleans, Louisiana, including

those used for key ship-to-ship and safety communications.

2.The received power levels of the land mobile emitters in New Orleans and Savannah at the

radios RF input was up to -27 dBm, which exceed the levels of simulated interference set by the IEC 1097

intermodulation test by up to 28 dB. 

3. The received power levels of the NOAA VHF weather broadcats in New Orleans and

Savannah at the radios RF input was up to -10 dBm, which exceed the levels of simulated interference set

by the IEC 1097 blocking test by up to 19 dB.

4.The receiver saturation and intermodulation/cross modulation interference appears to be

attributable to CFR Title 47 Parts 22 and 90 land mobile operations in the marine VHF band and NOAA

VHF weather broadcasts.

5. The marine VHF radios do not have any protection against land mobile operations and weather

broadcasts that operate within its receiver passband.

6. NOAA weather radio and land mobile transmitter antennas are very close to the river’s edge

inSavannah and New Orleans resulting in higher received unwanted signal powers at the marine radios RF

input, which further exacerbates the interference problems. 

7. The FCC Compliance and Information Bureau field office in New Orleans has stated that the

land mobile systems in Savannah and New Orleans are operating within CFR Parts 22 AND 90 Title 47

guidelines with respect to output power, signal deviation, and spurious emissions.

8.Some high quality commercial grade radios have an internal attenuator selectable with a

local/distance switch or squelch control that reduces the sensitivity of the radio receiver. Such radios were

capable of operating satisfactorily in the New Orleans and Savannah electromagnetic environment.

8. Recreational grade radios were able to overcome the interference with an external attenuator

at the receiver RF input which ranged in values from 10 to 30 dB. The sensitivity of the radio was also

degraded by the attenuation. 
5-2

9. The RTCM SC-117 draft receiver standard for marine VHF radios, which is based on

interference power levels that were recorded in New Orleans and Savannah, should be sufficient to protect

marine radio receivers from saturation and intermodulation interference. 

5.1 Recommendations

It is recommended that the Coast Guard consider the following items in resolving reports of interference

in marine VHF radios at US ports and waterways:

1. In the interests of increased public safety and environmental concerns for maritime operations

on rivers, lakes, inland waterways, and coastal areas of the United States, the Coast Guard district offices

shouldinform mariners which radios are commercially available that can function at Savannah and New

Orleans when complaints of interference are sent to their offices.

2. Promoting the RTCM SC-117 receiver standard among different manufacturers so that more

radios are available at an affordable price to the general boating public which can meet the standard.

3. To reduce interference from occurring in marine VHF radio receivers from future deployments

of NOAA VHF weather broadcasts near ports and waterways, working with NOAA to develop antenna

siting guidelines for NOAA VHF weather transmitters. These guidelines could possibly be incorporated

inthe NTIA manual. The guidelines should be based on marine radio performance that is in accordance

with the RTCM receiver standard.

4. To reduce interference from occurring in marine VHF radio receivers from future operations of

land mobile systems in the VHF band, working with the FCC and NTIA in developing antenna siting

guidelines for land mobile transmitters that can be incorporated in the Code of Federal Regulations. The

guidelines should be based on marine radio performance that is in accordance with the RTCM SC-117

receiver standard.
A-1

APPENDIX A

The following procedures were used for the radiated tests with the test set-up shown in

 Figure A-1.

Figure A-1

Test Set-Up

1. The test equipment was placed on a workbench located on the maindeck or in the wheelhouse of the ships

and test personnel traveled to locations on the Mississippi and Savannah Rivers where the performance of marine

radios was reported to be degraded.

2. An RF termination was connected to the input of the first coupler. 

3. The signal generator and the radio under test were tuned to the frequency of the channel being tested. The

signal generator functioned as the desired signal transmitter in all tests.

4. The RF power of the signal generator was adjusted till the SINAD of the radio under test was approximately

20 dB.

5. The input of the first coupler was connected to a 10 foot long VHF whip antenna mounted on the bridge of

the ship, which was about twenty feet above the water. The whip antenna was used to introduce emitters from

the EM environment into the circuit. The emitters caused a degradation in the receiver’s SINAD measurement.

 

6. The computer monitored the SINAD of the radio under test and instructed the spectrum analyzer (which is

connected to the antenna through the coupled port of the first coupler) to take a “snapshot” of the 150-174 MHZ

band when the SINAD fell below 14 dB. The spectrum snapshot was then stored in the computer.
B-1

Appendix B

This appendix contains the radio histographs which were derived from SINAD measurements taken

in Savannah. Radios which have histographs with their major distribution of SINAD measurements towards

greater values for a greater amount of time had better performance and the 1 KHz test tone was more

audible during the interference events.
B-2

 

 

 

Figure B-1

Radio L distance Mode

Figure B-2

Radio L Local Mode
B-3

Figure B-3

Radio H 

Figure B-4

Radio F 
B-4

Figure B-5

Radio I 

Figure B-6

Radio B 
B-5

Figure B-7

Radio A 

Figure B-8

Radio K 
B-6

Figure B-9

Radio E 

Figure B-10

Radio G 
C-1

Appendix C

This appendix contains the radio histographs which were derived from SINAD measurements taken

in New Orleans. Radios which have histographs with their major distribution of SINAD measurements

towards greater values for a greater amount of time had better performance and the 1 KHz test tone was

more audible during the interference events.
C-2

Figure C-1

Radio L, Distance Mode

Figure C-2

Radio L, Local Mode
C-3

Figure C-3

Radio H

Figure C-4

Radio B
C-4

Figure C-5

Radio M

Figure C-6

Radio I
C-5

Figure C-7

Radio F

Figure C-8

Radio K
C-6

Figure C-9

Radio G

Figure C-10

Radio E