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ContinuousWave: Small Boat Electrical
Radio Frequency Interference
|Author||Topic: Radio Frequency Interference|
posted 06-22-2010 10:33 AM ET (US)
This is likely something simple but, unlike mechanical problems, electrical problems often find me stumped. The problem I am having is that when I receive a broadcast message on my VHF and the motor is running I get static in the rhythm of the motor RPM. When the motor is off the radio receives messages clear as day. Upon doing radio checks I am told my outgoing broadcasts are clear. I am guessing that I have a grounding problem? If so how do I track it down?
Here is an image of the boat that shows the antennae placement, as well as the radio mounting. As you can see the antennae is mounted using a rail mount unit attached to the console grab rail.
posted 06-22-2010 11:23 AM ET (US)
What engine are you using? Older two-cycle engines can have very hot capacitor discharge spark ignition, which will generate a lot of radio frequency interference. Also, you should install suppressor spark plugs.
posted 06-22-2010 11:37 AM ET (US)
I am using the same old 1974 85hp Evinrude I have always used.
posted 06-22-2010 02:08 PM ET (US)
You might also try a torroid ferrite (or ferrite beads) on the power cables of the radio.
I have a bunch if you can't find any locally. Send me an email.
posted 06-22-2010 03:15 PM ET (US)
Second with earlier posts. For what it's worth, I've always mounted my antenna on the stern, starboard side and have not had your problem.
Retards - Don
posted 06-22-2010 04:07 PM ET (US)
What exactly to those do?
The good thing is I have it mounted in such a way with that I can undo the cable ties, dismount the antenna from the railing and move it to the stern to see if that helps.
posted 06-22-2010 04:48 PM ET (US)
Jeff, a toroid ferrite or a split ferrite bead will stop the transmission of RF on the outside of a wire, thus reducing or eliminating RF interference.
Here's a pretty good explanation:
I have both beads and toroids if you can't find anything locally.
posted 06-22-2010 08:10 PM ET (US)
Radio interference can be conducted to the radio in two basic ways:
--via the antenna
To test to see if the interference in coming into the radio via the antenna, first establish a condition in which there is interference heard in the radio. Then, disconnect the antenna. If the interference goes away, the path for the interference to enter the receiver was via the antenna. If the interference persists even with the antenna disconnected, the path for the interference to enter the receiver was via some other means, and probably was via its power cord or other cables.
To suppress interference entering the radio via the antenna, move the antenna farther away from the source of the interference, or, work on the source of the interference to reduce its strength. The spark ignition in an outboard motor will typically generate lots of radio frequency interference (RFI). To suppress RFI in a motor's ignition circuit, use suppressor spark plugs, use shielded spark wiring, and add shielding to the spark coils. Generally only the first method is employed.
To suppress interference entering the radio via other conductors, add filters to those conductors to block or shunt radio frequency energy. Filter power line circuits with RFI suppressors.
posted 06-23-2010 08:06 PM ET (US)
I know you are the expert on RF but couldn't he also be picking up noise on the cable between the antenna and the radio? Especially if the cable is a coil of long wire with a poor ground.
When you disconnect the antenna, be sure to disconnect the cable from the back of the radio and not just the antenna itself.
|L H G||
posted 06-23-2010 08:26 PM ET (US)
Most radio manufacturers recommend that the radio power source come from it's own set of wires, usually about #10, directly to the battery terminals, and not be wired off the boat's main power distribution panel/buss bar. In your case, the engine's ignition harness is probably the source of the problem, affecting either the antenna cable or the boat's main power cable.
posted 06-23-2010 09:50 PM ET (US)
The antenna could certainly pick up signals on its coaxial transmission line, particularly with the so-called "marine grade" coaxial cable that is often used, which is not particularly well shielded. So, yes, you have to disconnect the antenna from the radio at the radio. If you leave the transmission line connected to the radio, it acts as an antenna. Ergo, yiou have not disconnected the antenna; you just changed antennas.
Using 10-AWG wire to power the radio is total overkill. The radio only draws about 1-ampere on receive. The gauge of the power conductor could never affect the tendency to have RFI in the radio during receive. Even on transmit, the radio only draws about 8-amperes, which can be easily handled by a conductor of 14-AWG.
The notion that the radio manufacturer can dictate how electrical power is distributed in the boat is held only by radio manufacturers. As long as the radio is connected to the power distribution in a manner which affords the radio a well-regulated source of power with little common-mode noise, the radio will be happy.
Common mode noise occurs typically when the negative return circuit has high current flow and thus a voltage drop. In my Boston Whaler boat the feed to the secondary distribution panel is with 8-AWG wire. The conductor is perhaps 15-feet long. We can calculate the resistance in the return circuit as follows:
0.6401-ohm/1000-feet x 15-feet = 0.0096015-ohm
If the secondary panel feeding the radio has a total load of 15-amperes (which is probably higher than realistic) which was cycling on and off at a rate which could modulate the audio of the radio receiver, the voltage drop in the return would be
0.0096015 x 15 = 0.14-volts.
This is the maximum variation in the return power. The common mode noise thus created in the power distribution would be 0.14-volts in 14-volts, or -40-dB. This means the common mode power noise would not be heard until the signal to noise radio was greater than 40-dB. When Jeff says he "hears interference" I am sure he is not talking about hearing something 40-dB down in the recovered audio of the radio receiver
posted 06-23-2010 09:53 PM ET (US)
A simple test to see if the interference is being conducted into the radio via its power lines is to temporarily disconnect the radio from the boat battery and power it from its own isolated battery which has nothing else connected. If the interference is reduced significantly, then you can explore the original power line wiring as a possible source of the interference.
The KILL switch circuit on older two-cycle outboards often has a rather high-voltage on it--as much as 300 volts I am told--and this voltage is a rather noisy pulsating DC which could generate interference. You should avoid running the power or antenna wiring for the radio anywhere near the engine harness with the KILL switch circuit.
This separation should also be maintained for any SONAR--keep it away from the engine harness and KILL circuit.
posted 06-24-2010 12:30 AM ET (US)
Jeff--Next time you are on the boat, try an experiment:
Test the radio for interference from the engine. If you have interference, disconnect the antenna mount from the hand rail on the console. See if the interference increases or decreases when the antenna mount is not in contact with the metal railing. I think the railing is possibly becoming part of your antenna system.
posted 05-31-2011 12:30 AM ET (US)
After pulling the boat out of storage this year and after my first outing I decided to finally take a couple minutes to do something about this problem.
I ended up using a piece of rubber tubing as an insulator between the rail mount and the hand rail. Upon running the motor and receiving a transmission it was clear that GREATLY reduced the interference I was receiving. There was just the faintest trace of it left. After searching and searching for a place where the grab rail and it's mounting could be making contact with the electrical system of the boat I could not find anything and was left wondering...By mounting the metal ratchet mount directly to the stainless railing was a just creating a larger antenna to pick up the electrical discharges from the motor, or is there a place in the railing that was (and likely still is) making a connection to the electrical system?
posted 05-31-2011 08:43 AM ET (US)
Try bonding the railing to the battery negative terminal, as a test, to see if this affects the radio interference. If bonding the railing to the battery negative helps reduce the interference, you may want to install a permanent connection.
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