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Author Topic:   Solving Interference Between VHF Radio and Tachometer
Ridge Runner posted 04-27-2009 02:48 PM ET (US)   Profile for Ridge Runner   Send Email to Ridge Runner  
I have a Yamaha analog tachometer mounted in the port side of the console in a 16-foot center console boat. I flushed mounted an ICOM VHF in the starboard side of the console about two feet from the tachometer. On the far starboard front of the console I mounted a Shakespeare Style 5250 “Skinny Mini” three-foot VHF Marine Band antenna. The antenna is about three feet from the tachometer. With the engine running, and when I key the transmitter, the tachometer needle sweeps all the way to max redline. I assume it is reacting from the broadcast energy radiating from the antenna. I really don’t have another place for mounting the antenna, so do I just live with this? Will a digital tachometer solve this [interference]?
Fishmore posted 04-27-2009 04:37 PM ET (US)     Profile for Fishmore    
The first step I would take is to check and/or improve all of your ground wires. Check the grounding for all affected equipment tach, radio and antenea. Is your extra antenea wire coiled anywhere near the wires going to the tach? If so then move them.
Ridge Runner posted 04-27-2009 04:53 PM ET (US)     Profile for Ridge Runner  Send Email to Ridge Runner     
All the grounds seem good, but I will recheck. I only have 1-meter of RG-8 cable with PL-259's going directly from the back of the VHF to the antenna, no coiling at all.
jimh posted 04-27-2009 09:12 PM ET (US)     Profile for jimh  Send Email to jimh     
What effect will coiling of the coaxial transmission line have on the tendency for the tachometer to be affected by the presence of radio frequency energy in the vicinity?
jimh posted 04-27-2009 09:15 PM ET (US)     Profile for jimh  Send Email to jimh     
Install a three-foot extension mast on the antenna to raise it higher.

Check the coaxial transmission line to be certain you have a good connection to the transmitter.

Fishmore posted 04-28-2009 01:39 AM ET (US)     Profile for Fishmore    

RE: the coiled wire. I have seen where a coiled VHF cable laying on top of or bundled with other cables caused induced static I assume that this could also cause a tachometer to behave erratically. Seperating the coiled antenea wire from the other devices cables solved the problems I have seen before so it might help here. It certainly couldn't hurt.

jimh posted 04-28-2009 10:08 AM ET (US)     Profile for jimh  Send Email to jimh     
In radio communication the term "static" is usually associated with the radio noise generated by thunderstorms. Static is not heard on FM systems because static is a form of amplitude modulation. VHF Marine Band radios use FM modulation, so static is generally not a problem.

In a coaxial transmission line there should not be any current flowing on the outer surface of the coaxial transmission line, and the cable can be bent or formed into any shape without affecting the property of the cable as a transmission line, so long as the required minimum bend radius is observed.

If you observe that other devices are affected when their wiring is brought close to a coaxial transmission line, this is generally an indication of poor quality in the transmission line, likely due to poor shield coverage in the braided shield. Many of the products sold as "marine grade cable" for coaxial transmission line are not particularly good products from a radio point of view. I generally prefer to use coaxial transmission lines which are qualified to military specifications, such as RG-58C/U cable. This cable has excellent coverage in its braid and should not have much leakage.

The notion that a coil form somehow aggravates or enhances interference may have some merit if you are using a poor quality coaxial transmission line that has a lot of leakage to begin with, but the fundamental problem is poor transmission line quality, not that you have made it into a coil.

In most antenna installations I have made I intentionally form the excess transmission line into a coil in order to make a choke-balun to suppress the flow of any antenna current on the outer surface of the coaxial transmission line.

I don't doubt that there is anecdotal experience in which moving a transmission line away from other signal lines caused a change in interference that was experienced, but in general you can think of a proper coaxial transmission line as being a closed system. It is not radiating power on its surface.

jimh posted 04-28-2009 11:37 PM ET (US)     Profile for jimh  Send Email to jimh     
With the engine running, and when I key the transmitter, the tachometer needle sweeps all the way to max redline. I assume it is reacting from the broadcast energy radiating from the antenna.

This is the most reasonable analysis of the interference problem. Your transmitter's antenna is just a few feet away and directly in line with your engine tachometer. The engine tachometer is apparently sensitive to radio frequency energy, and when you key the transmitter, the tachometer reacts.

The remedy for this sort of interference is to move the transmitting antenna away from the tachometer. The best direction to move the transmitting antenna is upward. This is very effective in reducing the interference to the tachometer, and it will also improve the performance of the radio on both receive and transmit.

I do not recommend seeking a remedy to the interference by buying a new tachometer which uses a different display type.

If you are not able to raise the antenna vertically to get more separation between the antenna and the tachometer, there are some other techniques which could be used to reduce the sensitivity of the tachometer to interference from radio transmitters which are close by. Let me know if moving the antenna is out of the question.

jimh posted 04-29-2009 07:51 AM ET (US)     Profile for jimh  Send Email to jimh     
I had precisely this same interference problem between my VHF Marine Band radio transmitter and my outboard engine tachometer when the antenna was located near the tachometer. I resolved this problem by changing to a different type of antenna and raising it several feet above the tachometer.

See the article I wrote about revamping the radio antenna installation at

VHF Marine Band Antennas For Small Boats

A dial indicator tachometer measures the speed of the engine by measuring the frequency of the pulses which are applied to its input. I suspect that most of these tachometer circuits contain diodes which rectify the incoming pulses and then integrate their voltage onto a capacitor. The more pulses per second, the higher the integrated voltage that results. The tachometer dial indicator is a voltmeter that reads the voltage on the capacitor. With proper calibration the engine speed is read from this voltage reading.

When there is a strong radio frequency signal present, it can induce current into the tachometer circuit, and the diodes will rectify and integrate that voltage. This is what causes the tachometer meter to drift upscale when the radio is transmitting.

I do not think the interference is carried to the tachometer on the sensor input lead. The sensor input is typically a tap into the rectifier on the engine battery charging circuit. This is a very low-impedance circuit. On one side there is the winding of the alternator which is typically only a few ohms. On the other side is the battery whose resistance is typically less than 5-milliOhm, that is, 0.005-ohm. On this very low impedance circuit there is already a voltage swing of about 12-volts. It seems very unlikely that the radio frequency energy could induce much voltage across this extremely low impedance circuit, at least not anywhere near as much voltage as the engine is supplying by being wired directly to it.

It is much more likely that the radio frequency energy is being picked up inside the tachometer instrument itself in some part of its circuitry. In order to integrate the voltage onto the capacitor, the impedance has to be fairly high. This a likely point of entry for the radio frequency interference.

Ridge Runner posted 04-30-2009 01:31 PM ET (US)     Profile for Ridge Runner  Send Email to Ridge Runner     
Jim - Thank you for the thoughtful and detailed reply. The current console configuration does not allow me to move the antenna to a distance that would correct this issue. Also I’m not able to increase its height; with the way the console is configured, I cannot install a “standard” antenna mount that would allow me to lower the antenna. Is there any way to “shield” the tachometer?
jimh posted 05-01-2009 01:18 AM ET (US)     Profile for jimh  Send Email to jimh     
To reduce the interference, I suggest your proceed as follows:

First, determine how the radio frequency interference enters the tachometer. You can do this by temporarily disconnecting the SENSOR lead from the tachometer. If the interference goes away or is significantly reduced, this implies the interference is coming in on the SENSOR lead. To suppress this, try installing a 0.01-uF ceramic disc capacitor from the sensor lead to ground. You can get a 0.01-uF ceramic disc capacitor from Radio Shack, I think. This will bypass the RF energy to ground. Another approach is to shield the SENSOR lead with some copper braid. The outer jacket of an old piece of RG-58C/U coaxial cable would be perfect.

If the interference does not go away when you remove the SENSOR lead, there is a chance it is coming in on the power leads. Try installing a bypass capacitor across the power lead positive to ground. Try shielding the power lead.

If the interference still persists, then it is being picked up inside the tachometer gauge. You will have to open up the instrument and modify it. Before you go to that point, try the remedies above.

Ridge Runner posted 05-01-2009 11:50 AM ET (US)     Profile for Ridge Runner  Send Email to Ridge Runner     
Thank you for the trouble shooting procedures. I now have a nice weekend project. Results to follow -
jimh posted 05-02-2009 07:54 PM ET (US)     Profile for jimh  Send Email to jimh     
Let us know what you find. I am guessing that the radio frequency interference gets into the tachometer some way other than on the SENSOR lead. The power lead may be more likely.

You can also try decoupling the power lead by inserting an inductance in series at the tachometer terminal marked "IGN" . Radio Shack might have an RF Choke, or you could make one by winding about 50 turns of small magnet wire on a 2-Watt composition resistor of 10,000-ohms.

Ridge Runner posted 05-04-2009 03:37 PM ET (US)     Profile for Ridge Runner  Send Email to Ridge Runner     
It seems like the radio frequency interference is being picked up directly inside the tachometer. I removed the SENSOR lead with no change in effect; I then disconnected the positive (purple wire) and negative (black wire) leads from the tachometer and hooked it up to a completely independent 12v bench power source. This also resulted in no improvement. I then removed the tachometer from the console and extended the SENSOR lead; then using the bench power supply moved in one foot increments away from the VHF antenna, with the VHF transceiver in the 1 watt mode at ~5 feet from the antenna the interference stopped, in the 25 watt mode at ~14 feet from the antenna the interference stopped. Is it possible to shield the tachometer from the radiant energy? Also this is an end-fed ½-wave stainless 3’ steel whip antenna, does the base load radiant more energy at the base of the antenna? Would changing to a 4’ center-fed ½-wave fiberglass antenna make a difference?
jimh posted 05-04-2009 08:36 PM ET (US)     Profile for jimh  Send Email to jimh     
Thanks for the report on your thorough testing. Your results tend to confirm my suspicions that the mechanism by which the radio frequency energy interferes with the tachometer is from propagation of the radio wave to the tachometer internal circuitry. In this regard, I can't offer much speculation about how a different feed system on the antenna would affect the problem. Because most of the boat is non-conductive, and the antenna is isolated from ground, I don't think there would be a difference.

To remedy the interference the easiest choice will be to move the antenna. I suggest you move it up three or four feet with an extension mast. If that is truly not an option, the alternative is to modify the tachometer to make it less sensitive to radio frequency energy. One approach is to place the tachometer into a shielded can. Try using a metal soup can that will fit over the tachometer rear body. Ground the metal soup can to the chassis of the radio. A copper shield might be even more effective. You could make one from a short section of copper pipe of the appropriate diameter.

jimh posted 05-04-2009 08:37 PM ET (US)     Profile for jimh  Send Email to jimh     
Another idea: get some conductive tape. Wrap the body of the tachometer meter in the conductive tape, and ground the tape to the radio chassis.
Ridge Runner posted 05-07-2009 05:51 PM ET (US)     Profile for Ridge Runner  Send Email to Ridge Runner     
Jim – I took a 10mil copper sheet, and formed a round covering around the outside of tachometer, then cut a circle of copper and soldered it to the “wrapped” sheet, drilling holes for the SENSOR, negative and positive leads. Then I soldered on a grounding lug. I ran 12 gauge wire from the grounding lug to the terminal grounding bus, this seems to have solved the interference issue, the tachometer needle no longer swings up to redline.
jimh posted 05-07-2009 05:56 PM ET (US)     Profile for jimh  Send Email to jimh     
Excellent solution! Thank you for the follow-up posting. It is always best when a discussion leads to a remedy.

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