Moderated Discussion Areas
  ContinuousWave: Small Boat Electrical
  VHF Marine Band Transmitter Problem

Post New Topic  Post Reply
search | FAQ | profile | register | author help

Author Topic:   VHF Marine Band Transmitter Problem
prj posted 06-15-2009 09:19 PM ET (US)   Profile for prj   Send Email to prj  
My Humminbird marine band radio is not able to transmit. This is my root problem, but a couple others have emerged that may drive the corrective action.

I have the following equipment mounted on my 18' Outrage:
Shakespeare 5206-C 8’ antenna.
Humminbird marine band radio, model DC 25 R

Transmission problem
My radio doesn’t transmit and I can’t tell you when it last did effectively. It receives well and from afar, I’m fairly certain of this. That seems to rule out the antenna as the problem.

I’ve had both reports of “garbled transmissions” from reasonably distant communicators and “nothing at all” from immediately adjacent radios. When the transmission key is depressed, the parenthetical flashing “T” appears on the LCD readout suggesting response and power between the handheld mic and the radio unit. I’ve looked at the rudimentary manual and checked connections with nothing apparently amiss or corroded.

What recommendations can be made regarding troubleshooting this problem?

In addition to the lack of transmission, I’ve broken and must replace my ratchet mount. Embarrassingly enough, this happened during the Michigander's visit to Milwaukee while I was assuring forum member Hoosier that there was sufficient overhead clearance on a railway bridge.

I have to remove the dangling antenna by Thursday for a camping trip and can’t find an obvious manner for doing so without cutting the antenna’s connection to the marine radio. This is accomplished by severing the threaded female single pin connector off in order to pass the cable through a small hole in my teak gunnel. Is this an acceptable and reparable cut? There isn’t a readily visible method for reattaching this connector to the cable. Advice on this would be appreciated.

jimh posted 06-15-2009 10:09 PM ET (US)     Profile for jimh  Send Email to jimh     
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.

jimh posted 06-15-2009 10:21 PM ET (US)     Profile for jimh  Send Email to jimh     
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.


A Shakespeare 5206C antenna is not a particularly good or bad antenna. It will work. 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.

jimh posted 06-16-2009 08:34 AM ET (US)     Profile for jimh  Send Email to jimh     
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.

prj posted 06-16-2009 09:58 AM ET (US)     Profile for prj  Send Email to prj     
Excellent responses, Jim.

I have a boat camping trip planned on an inland lake for the upcoming Solstice Weekend with no need for the radio, so I'll cut off the connector, fish the cable/wire/feedline back through the small diameter hole in the teak gunnel and remove the dangling antenna and broken ratchet mount.

Upon my return, I'll replace the ratchet mount, install a new connector (noting your estimates of success rates) and execute the testing procedure you've outlined below to establish whether my mic is working. When completed, I'll report back to this article with the results.

David Pendleton posted 06-16-2009 07:40 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
Drill a larger hole, and use a cable clam when you reinstall your antenna so you won't have to cut connectors off.

Post New Topic  Post Reply
Hop to:

Contact Us | RETURN to ContinuousWave Top Page

Powered by: Ultimate Bulletin Board, Freeware Version 2000
Purchase our Licensed Version- which adds many more features!
© Infopop Corporation (formerly Madrona Park, Inc.), 1998 - 2000.