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Author Topic:   Diagnosing VHF Marine Band Radio Communication Problems
Mr T posted 02-06-2011 01:36 PM ET (US)   Profile for Mr T   Send Email to Mr T  
I have been using a [Standard-Horizon] ECLIPSE VHF [Marine Band] radio on my boat with a Shakespeare model 5240-R antenna for the last three years. For that time the combination has worked out well in the [California Sacremento River] delta region and in saltwater. About a month ago, I went out to the delta for the day. As I usually do, I performed a radio check on two channels I use on a regular basis, and did not receive a reply. I checked the connections for power and antenna, and found no [problems or defects] with corrosion or voltage. On two subsequent trips on inland waters, I have tried to contact friends who I know are available and according to them they are not receiving my transmissions.

The radio is at least 10-years old and has been on at least three different boats since I got it used. It is not DSC ready, so I decided to replace it.

I replaced it with another Standard Horizon QUEST series VHF. Installation was done, and, upon powering-up the radio, I could pick up weather channels using the WX function on the radio. I was able to receive six to seven channels from Monterey [California} to the north coast near Oregon very clearly. I then checked channel 16 for Coast Guard and did not pick up anything at all. I live inland, so the nearest Coast Guard station is at least 50 miles away; based on this I did not feel concerned at the time about the lack of reception. I did not try to transmit from home since I doubted that anyone would happen to have their VHF Marine Band radio on in the middle of an inland suburb. I checked at that time to see if the old radio picked up the WX channels as well, and it did not, which led me to believe that the [cause of the former problems] was not the antenna, but the radio.

Last week I took the boat out to San Pablo Bay for some sturgeon fishing. I attempted a radio check and found that I still got no response on two different channels. I tried the scan function and it did not receive any channels. I checked the weather channels and they came in loud and clear for the same stations I had picked up inland. Later in the day, we got checked by California fish and game for licenses and I asked them to assist with a radio check; they did not hear me transmit, and I could not receive transmissions from them. Based on the lack of reception I suspect the antenna. What I am puzzled by is the ability to receive weather channels on the new radio. My limited knowledge of [VHF radio communication] would incline me to believe that the ability to receive these channels indicates a functional antenna, but I cannot receive or call out.

Are the weather channels on VHF frequencies not a good indication of antenna performance?

If I have to get another antenna I will, but I’m still puzzled here. Any thoughts? Thanks,

jimh posted 02-06-2011 01:54 PM ET (US)     Profile for jimh  Send Email to jimh     
I do not understand your test regarding "picking up the Coast Guard" on Channel-16 when you were inland 50-miles. There is no radio coverage for Coast Guard radio watch standers that extends 50-miles inland. Coast Guard coastal radio stations have antennas oriented to cover the sea, not the land.

A VHF Marine Band radio that can receive transmissions from NOAA weather radio stations that are a long distance away should be able to receive regular VHF Marine Band transmissions from nearby boats. There is no reasonable explanation why an antenna could permit reception of NOAA broadcasts but not permit reception of local ship transmissions. The antenna, whatever its condition, would affect both in the same way. The antenna is not likely to perform differently at the NOAA radio frequency than it does at the VHF Marine Band, as the two are close together in frequency.

On transmit, an antenna problem can have a severe reduction in range of transmission. If an antenna has become damaged, the transmitter often contains a sense circuit which reduces power to prevent the transmitter from being damaged. The transmitter then transmits at much lower power into a broken antenna. This combination results in a reduction of transmitter range to a very short distance, perhaps less than a mile.

One element of the installation which is still in common--besides the antenna--is the power circuit. If the power circuit is faulty it cannot maintain proper voltage on transmit, and this will drastically reduce the power.

There are three elements in this system:

--power circuit

You have only changed one element. Check the power. Try a different antenna.

In order to make useful radio checks with another station, you need the other station to have a reliable radio and a reliable radio operator. Your test transmission may have been fine and the other station had a problem with his receiver. To prevent this sort of confusion, use three stations in any radio check. A third station can help reduce ambiguity regarding which station has a problem.

Antenna Testing

You can make a very rudimentary test of the antenna by measuring its DC resistance. You need to know what the normal reading should be, and then you can compare what you get. Typically the normal reading is usually either just about a dead short or an infinitely high open circuit. It is hard to say which is appropriate for your antenna. Even if you know what the expected readings should be, this is not a fool proof test, as either an open or a short in the transmission line could mimic a working antenna.

A second method to test whether or not the antenna is working at all is to use the receiver as a detector. Generally the noise from the receiver will increase when an antenna is connected. Turn the squelch off so that the receiver is putting out noise. Disconnect the antenna. Adjust the volume to a comfortable audio level on the noise and listen carefully to the receiver output. When you connect the antenna to the receiver the noise level should increase very slightly. This is also not a fool proof test.

A third method to test the antenna is to use the transmitter and a good quality directional wattmeter or directional bridge indicator. Measure the VSWR on the transmission line. If the VSWR is not below 2:1, the antenna is very likely not working properly. Most boaters will not have a directional wattmeter or directional bridge, and in all honesty I do not recommend buying one of those cheap $35 meters. Borrow a decent meter from a radio technician who works with VHF radios, or ask him to check the VSWR for you.

You have already conducted one of my favorite tests: reception of distance weather stations. In most locations you should be able to receive at least one NOAA Weather Radio broadcast. These stations usually transmit with impressive antennas and high power, so should be able to receive them at ranges of up to 60 miles.

The best test is probably to conduct a two-way test with another station which is at least five miles away. Do not be fooled by tests at short range. Even a radio which has practically no power output at all or an antenna which is completely useless will be able to communicate with another station at short distances.

ASIDE: Anecdote from many years ago: We were cruising with another boat for a week in Lake Huron. We were in frequent radio contact, but generally only a few hundred feet apart. One afternoon we became separated by a few miles, although still in sight of each other. We were sailing along a stretch of rugged coastline heading up the Bruce Peninsula in early June. On the other boat, unknown to us, there was a serious health emergency. They tried to contact us via radio. They discovered we could not hear their calls because their VHF radio was broken and was transmitting with only a few milliwatts of power. This was a total surprise because we had been chatting with them all week on a routine basis, but never more than a few hundred feet apart. We kept sailing on, wondering why the other boat was falling farther and farther behind. We kept calling them on the radio, and getting no reply. Fortunately, the health emergency was resolved, no one was seriously hurt, and they eventually caught up to us to tell us about their radio problem. It was quite a surprise because we had been working each other via radio without any problem. The Moral of the Story: if you get a radio check, be sure it is with another station a few miles away.

SECOND Anecdote: Just a year or so ago we had a similar instance. We were out in Lake Michigan with another boat, the wind picked up, and the waves grew. They developed a problem with their fuel system and tried to hail us on the radio. It turned out their radio was also defective, and, again, we had been working them just fine from a short range. But once they got about a mile away--we could see them clearly--they were out of radio range and we just barely picked up their call about the fuel system problem.

On open water with a good radio and antenna on each boat, it should be very simple for two boats to communicate at five mile separation. If that does not work, one of the stations probably has a problem. So that is a good test, as long as you have another boat to work with and that boat has a good installation. It is really quite normal for two boats to be able to communicate when at sea for a range of at least ten miles, assuming each boat has a good radio and a decent antenna installation.

Transmitter Checks

To check your radio transmitter, I suggest the following:
--find a cooperative remote station to check with, preferably someone familiar with VHF FM radio communication;

--the remote station should not be too close. Most VHF Marine Band radio receivers will overload if they are close to a VHF Marine Band transmitter. The remote station should be a mile away;

--have a secondary means of communication with the remote station, like a cellular telephone, so that you can communicate during the test, even if there is a problem with the radio under test.

--establish the test procedure so that both stations will be aware of what is going to be done.

You can perform several tests of your radio with just a bit of cooperation. We will call the station testing his transmitter as "A" and the remote station cooperating in the test "B". The procedure goes like this

--A and B agree on a channel and time for testing. B is located a mile away from A. We assume that B's radio works perfectly on transmit and receive.

-- A calls B and stands by for a reply.

--If B hears A, he calls back with a report and stands by for a reply

--If A hears B and acknowledges. Now we have communication. The test can begin.

--B switches his receiver momentarily to a NOAA Weather Radio Broadcast. B sets the volume on his receiver to a comfortable level. B switches back to the original test channel and calls A to instruct him to begin a modulation test transmission.

--A begins the modulation test. A speaks clearly and directly into the microphone at a slightly louder than normal voice and makes a 30-second transmission. A holds the microphone about one inch from his mouth and keeps wind from blasting into the microphone. The proper microphone technique is as follows:

--Point the microphone at your mouth, but
--Do not point your mouth at the microphone

--B listens to A's transmission and compares the volume with the level from the weather radio broadcast. If A's transmitter is working properly, the level and clarity will be nearly the same as on the NOAA transmitter.

--B assesses the modulation from A for volume and clarity. B informs A of his transmitter modulation test results.

--Next, a second test is begun. This time A transmits with no modulation. To do this the microphone is wrapped with a cloth covering to prevent any pickup. B listens carefully for any hum or stray noise on A's signal. B also listens to evaluate how much quieting there is in this receiver output. At one mile the signal should be strong enough to cause the recovered audio to be very quiet. No static or hiss should be heard by B. If B hears a lot of hiss, the transmitter carrier signal is weak.

Typical Problems and their Causes

Low Modulation: low recovered audio at B's receiver means A's transmitter was not modulated very much. This could be due to a defective microphone, poor microphone technique, or low microphone gain in the transmitter.

Garbled Modulation: if the transmitter antenna is too close to the transmitter, the radio signal can interfere with the transmitter. This usually results in garbled modulation. Repeat the check on 1-WATT power and see if the modulation sounds better. If the modulation is clear at 1-Watt and garbled at 25-watt, your transmitter is too close to the antenna.

No Modulation: lack of any audio modulation is usually a broken microphone, a bad connector, or a broken wire in the cord.

Weak Signal (not weak modulation): the transmitter output is below 20 to 25-watts due to malfunction of the transmitter, poor antenna, poor transmission line.

Antenna feedline

Typically the feedline is permanently attached at the antenna. The feedline is typically 15-feet long. The usual connector is an PL-259 plug or PL-259-type plug. In the original installation it is typical that the feedline is installed without the PL-259 at the end, which allows for smaller holes where the feedline has to pass through a bulkhead. Once the PL-259 is installed the feedline is captive.

If you have to replace an antenna mount, you can unbolt the mount from the boat, then you can generally unthread the mount from the antenna. If the feedline is also captive by passing through a hole in the mount, you will have to remove the PL-259 from the feedline, fish out the feedline from all the captive holes, and remove the mount.

Installing a PL-259 successfully seems to be about a 50-50 success rate among boaters who are not also skilled in radio electronics. Among those skilled in radio electronics the success rate moves up to 70-30. That is to say, installing a PL-259 can be a problem for many boaters. Consider getting a new connector. Consider getting a solderless connector if you are not great at soldering or lack the proper soldering equipment.


My preferred antenna set up for small boats where antenna height is important is the GAM SS-2. See

The article explains in detail my reasonings and preferences.

On-air testing of a VHF Marine Band radio with another station requires that the other station's radio be known to work perfectly on both transmit and receive. If this is not known for certain, employ a second remote station. With three stations, A, B, and C, you can generally determine who has the problem by comparison of the three signals.

For example, A and B transmit and C listens. C can compare the modulation of A and B. Next B transmits and A listens. A can deduce his modulation level because he knows how it compares with B whom he has heard. Using three stations in the test reduces ambiguity regarding what station has a problem.

Mr T posted 02-06-2011 02:27 PM ET (US)     Profile for Mr T  Send Email to Mr T     
I mentioned the test of channel 16 only to indicate that the initial test included a check of that frequency. I did state that I was not suprised to find lack of reception. In the past when weather condition have been favorable I have in fact picked up the bay area coast guard transmissions from this location inland. I understand that they are not directed inland. It was simply another test that I made at the time to see what would happen. If I did receive on that channel, it would be confirmation of reception on a channel other NOAA broadcast frequencies. If it did not, it would be inconclusive.

Being able to receive NOAA broadcasts is why I feel the antenna is functional; to my eye, if it can receive, it should be able to transmit, at least for short distance testing purposes.

I checked power at the time of installation of the new radio, and found the voltage at the power connector going to the VHF to be 12.1 Vdc when not transmitting, and a drop to 11.9 Vdc when the transmit key was pressed on the mic. I do not know what is an expected voltage drop on transmit, but it did not appear to be a significant drop in my opinion.

My radio checks were done with another boat using a fixed mount VHF, a handheld VHF radio, and a State fish and game boat fixed mount VHF radio, on different days. In all three cases, the checks were done within 100 yards of my boat. I am of the opinion that the probability of failure on all three radios on two different channels each on different days with different operators is remote at best.

I do not have another antenna at this time to test with, so my intent was to confirm whether an antenna that could receive shoud be able to transmit, even if at a low power, which your reply seems to confirm.

jimh posted 02-06-2011 03:02 PM ET (US)     Profile for jimh  Send Email to jimh     
"Being able to receive NOAA broadcasts is why I feel the antenna is functional; to my eye, if it can receive, it should be able to transmit, at least for short distance testing purposes."

This is not a correct conclusion. In receiving there is an enormous amount of reserve gain available in the receiver itself which will make up for any deficiency in the antenna. In transmitting there is only the fixed gain or power of the transmitter, and, as I explained earlier, if the antenna has a defect, the transmitter power is reduced by protection circuits. Ability of an antenna to receive is not at all a test of ability of the antenna to transmit.

" In all three cases, the checks were done within 100 yards of my boat."

This is not correct procedure. Please read my recommendation above for how to conduct testing. You cannot conduct testing at close range. Many boats have VHF Marine Band radios which will be easily overloaded if the other station is too close. If you transmit at 25-watts to most boats that are at close range their receivers will malfunction.

jimh posted 02-06-2011 04:39 PM ET (US)     Profile for jimh  Send Email to jimh     
"...the voltage at the power connector going to the VHF to be 12.1 Vdc when not transmitting, and a drop to 11.9 Vdc when the [PTT] was pressed...."

Your power distribution has some voltage drop in the system, but a shift of only 0.2-volts on transmit is not a problem.

Mr T posted 02-07-2011 12:09 AM ET (US)     Profile for Mr T  Send Email to Mr T     
Actually the 12.1 is a typical number for this system. The battery is now two years old and there is no charging system on board, so it gets drained when I do not use the boat for a week or more. A side effect of the GPS module is that is always draining the battery slowly as long as power is connected. It is noted on the owners manual as such and always struck me as a strange thing to have wired this way.
jimh posted 02-07-2011 09:23 AM ET (US)     Profile for jimh  Send Email to jimh     
Actually 12.1-volts represent a battery which is more than 50-percent discharged. The power output of most VHF Marine Band radios is rated for an input voltage of 13.2-volts, which is the nominal voltage of a 12-volt system when the battery is being charged by the engine charging current. If the radio only receives 11.9-volts during transmit, the power output of the radio will decrease from the rated 25-watts. It is hard to say with certainty how much the power output will decrease, but it will definitely not be at the rated 25-watt output when the supply voltage is down to 11.9-volts. I'd guess perhaps as low as 15-watts.

Reduced power output to 15-watts from 25-watts can not explain why the radio's transmissions could not be heard by boats less than five miles away. That power output should be easily able to transmit that far.

Mr T posted 02-07-2011 11:32 AM ET (US)     Profile for Mr T  Send Email to Mr T     
Update, got word from Shakespeare that they will replace the antenna under warranty, so it is going out today.

I agree about the battery being discharged, it is a stand alone group 24 deep cycle used for electronics only. The engine is connected to the starting battery only. I had problems with the electronics giving error messages when using one battery for starting and electronics when they were wired together. When I started the engine, the voltage drop would cause a low voltage error on the raymarine head unit. I felt it best to have the electronics on a different battery to avoid potential issues resulting from voltage drops.

The electronics battery never charges over 12.3 Vdc even when the charger is applying a maximum charge of 13.2 Vdc to it. The charger will indicate a 100% charge at 13.2 Vdc, and once it is turned off and removed, a check made at the battery terminals indicates 12.2-12.3 Vdc.

For this intended usage it does suffice. but it is not optimal.

jimh posted 02-12-2011 09:28 AM ET (US)     Profile for jimh  Send Email to jimh     
Again, I cannot envision any antenna problem which would produce the observations you mentioned, viz.:

--radio can receive NOAA transmissions
--radio cannot receive VHF Marine Band transmissions

If the antenna functioned sufficiently to permit reception of a NOAA Weather Radio broadcast that was 25-miles away, the antenna would also function to deliver signals from transmitters in the VHF Marine Band. The separation in frequency between these signals is not particularly great.

Comparison of Frequency of Signals

NOAA Weather Radio Broadcast = 162.550-MHz (typical)
Marine Band Safety Channel 16 = 156.800-MHz

If the antenna provided sufficient gain to permit the radio to hear a signal from a NOAA Weather Radio Broadcast station, the antenna should also deliver a signal of similar strength from a transmitter in the VHF Marine Band.

If we assume the observations reported are accurate, it seems to me that there is likely some cause for the radio under test to fail to receive signals in the VHF Marine Band other than the antenna. However, we cannot say that the antenna is working perfectly from these observations. As noted, it is entirely possible for a typical radio to be able to effectively receive signals on an antenna which has malfunctioned, but not be able to transmit effectively on that antenna. Again, this is because in the receiver circuit there is a tremendous reserve of receiver gain or amplification which is automatically applied to compensate for low received signals, while on transmit the gain is fixed, and, even worse, likely to be reduced to prevent damage to the transmitter output stage from the effect of the malfunctioning antenna.

In my experience, difficulty in receiving signals from VHF Marine Band stations which are very close by is often seen as a result of receiver overload from the extremely strong signal that is produced by the near by transmitter in the receiver.

jimh posted 02-12-2011 10:58 AM ET (US)     Profile for jimh  Send Email to jimh     
In regard to testing a radio communication link, as I mentioned previously, it is important to have another means of communication between the stations involved in the test, other than the link being tested.

A good example of this comes from history. When Marconi built his wireless transmitting station at Table Head in Glace Bay, Nova Scotia, he had no receiving equipment. The test of trans-Atlantic radio transmission was conducted on a pre-arranged schedule. Marconi transmitted from Table Head according to a certain schedule. On the other end of the circuit, at Poldhu in Cornwall, England, a receiving station listened for Marconi's signals. They probably notified Marconi he had been heard via telegram. This occurred in 1901, but the principal remains today:

If you want to test your radio with another station, you will need a secondary communication link to that other station so you can communicate with them in the event that the radio under test does not work.

Also, when testing with another station, if there is a failure in the communication link it is impossible to resolve the cause as being from a particular station. If A transmits and B does not hear, one cannot say that for certain that the problem is either A's transmitter or B's receiver. To resolve the ambiguity a third station can participate. Now if A transmits and neither B or C can hear the transmission, there is more chance the problem is with A's transmitter than with both B's receiver and C's receiver.

Mr T posted 02-14-2011 08:36 PM ET (US)     Profile for Mr T  Send Email to Mr T     
Until I receive word back from the factory on what they find upon receipt of the antenna, I am going to hold off on speculating as to a cause here.

All the points raised here are valid, but without an antenna to attach to a radio, I cannot test anything.

Mr T posted 02-23-2011 08:43 AM ET (US)     Profile for Mr T  Send Email to Mr T     
New antenna was on the door step today. I will attempt to get it installed and tested thsi week. The weather forecast is for rain starting tomorrow through the weekend, so not a done deal.

There was no information on what (if any) investigation was done on the antenna I returned. I suspect it is in a dumpster in the back right now. Testing the old one is going to cost more than a new one to the factory for sure.

Mr T posted 03-01-2011 11:00 AM ET (US)     Profile for Mr T  Send Email to Mr T     
Update on testing: the new antenna is in place and I did testing on channels 68 and 69, both of which transmitted and received communication from other boats in the area from 1-5 miles distance.

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