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Author Topic:   Repairing SONAR Electrical Cable
Perky posted 05-18-2008 01:21 PM ET (US)   Profile for Perky   Send Email to Perky  
I have recovered a fish finder from a old boat. The cable between the unit and the [transducer] has been cut. Can [I] repair [the cable connecting the transducer to the control head of a SONAR]? Can [I] shorten [the cable connecting the transducer to the control head of a SONAR]?
jimh posted 05-18-2008 01:33 PM ET (US)     Profile for jimh  Send Email to jimh     
Yes. Yes.

jimh posted 05-18-2008 01:51 PM ET (US)     Profile for jimh  Send Email to jimh     
For the curious:

Electrical signals propagate at almost the speed of light in a conductor. In transmission line cables there may be a reduction due to a velocity factor. For the wiring used in a SONAR which operates at 200-kHz, I doubt that the velocity factor exceeds 0.66. Therefore signals propagate along the cable at 0.66C, where C is the speed of light. This is approximately 200,000,000-meters per second.

(Cf.: )

Acoustic signals propagate in water at a much lower speed. The velocity depends on several factors:

--depth of the water
--temperature of the water
--salinity of the water

A typical speed is 1,550-meters per second. (Cf.: )

If we compare the speed of the electrical signals in the cable to the speed of the acoustic signals in water, we see that the electrical signals travel much faster, by a ratio of

200,000,000 / 1,550 = 129,000 to 1

As far as deducing the depth of the water from the time interval in the acoustic echoes, changing the length of the wire connecting the transducer to the control head has little effect. If you cut out ten feet of wire it might cause a change in calibration of the unit by one-ten-thousandths of an inch. The unit does not have anywhere near the resolution necessary to be affected by such a change.

In terms of the operation of the electrical circuit, the wire has very little lumped capacitance or inductance which might affect the signals traveling on it. The excitation to the transducer is primarily a DC pulse, and the return echoes are mainly concentrated around the transducer resonant frequency, typically 50 or 200 kHz.

If a splice is made in a workmanlike manner, it should bring minimal effect. As for shortening the cable, this is the opposite of lengthening it, an option which is offered by all manufacturers via extension cables. I doubt that any re-adjustment is needed based on a modest shortening of the cable.

I do not recommend modifying the cable unless unavoidable, as an intact factory original cable is preferred to one cut up by an installer. But there is no reason to expect that a well-made splice won't work just fine.

Perky posted 05-19-2008 03:25 PM ET (US)     Profile for Perky  Send Email to Perky     
How would i go about joining the cable, what type of connector
jimh posted 05-19-2008 06:19 PM ET (US)     Profile for jimh  Send Email to jimh     
I would not install a connector unless you were planing on disconnecting it often. To splice a broken electrical wire, make a nice in-line twist of the conductors. Cover them individually with heat shrink insulated tubing. Stagger the splices so they don't overlap. Cover the splice area with heat shrink tubing.

If you don't have the tools and materials on hand to do this, and you don't have experience in making workman-like splices in multi-conductor electrical cable, it might be less expensive to just buy a new transducer.

jimh posted 05-20-2008 01:00 PM ET (US)     Profile for jimh  Send Email to jimh     
For a quick test, just strip the wires and twist them together. This ought to reveal if the device will work and warrants a more comprehensive repair.
roloaddict posted 05-21-2008 01:19 AM ET (US)     Profile for roloaddict  Send Email to roloaddict     
We don't see much light out hera in Washington, when we do it is usually running at about 300,000,000 M / S.

When you test the connection, be sure to seperate the conductors so they have a good gap. A couple of inches from each other and anything conductive. Clean fiberglass is ok to set them on. You may have several thousand volts present at a sounder output depending on the output power. I would not recommend holding the wires either, unless you are into entertaining witnesses. Make sure you are in an area with good ventilation (no gas vapor) in case of spark.

Jim is correct. A nice, staggard, soldered connection will work fine. Extra points for hot melt glue filled shrink wrap over the connections then over the whole thing.

jimh posted 05-21-2008 01:50 AM ET (US)     Profile for jimh  Send Email to jimh     
The speed of light is a constant. Electrical signals traveling in a transmission line conductor do not travel at the speed of light. See

Jefecinco posted 05-21-2008 07:10 PM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     
Once upon a time the Lowrance web site had instructions for splicing a transducer cable. One of the steps was to cut the cable and the shielding offset an inch or so to facilitate soldering. After soldering the cable it should be wrapped in tape or shrink wrap as should the join at the shielding. I would attempt to make the splice at a point in the cable where it will be out of the bilge and the weather.

I had no [malfunction of the SONAR] when I [made a repair to the cable connecting the transducer and the control head] several years ago.


roloaddict posted 05-21-2008 11:31 PM ET (US)     Profile for roloaddict  Send Email to roloaddict     
Sorry Jim. I see were you were going now.

Therefore signals propagate along the cable at 0.66C, where C is the speed of light(300,000,000 meters per second). The speed of the signal through the cable is equal to approximately 200,000,000-meters per second.

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