Extending Depth tranducer cable

[Stemar]

I've acquired (legally) a set of integrated instruments including depth sounder. I know one isn't supposed to cut the cable for this, but it has been cut to remove it from the boat, which was getting a set of new toys.

The cable I have, which may or may not be all there was, is too short to reach from where I want the transponder (existing hole) to where I want the display. If I use proper coax (vhf?) cable and connectors, is there really likely to be a problem?

I know it will take longer for the signal to go down the wire and back, but from what I can remember from my physics studies, a signal goes down a wire at around 1/10 the speed of light - 1000s of miles a second, and sound through water at less than 1000mph, but surely this isn't going to be significant is it?

 

[john_morris_uk]

The problem as I understand it is not that the transmission time down the cable is significant, but the fact that many if not most echo-sounders seem to use the capacitance of the cable as part of their circuit. Don't ask me to justify the odd design - but altering the length of the cable will make the sounder produce incorrect readings.

Actually, I will attempt to justify it. When I start thinking about it, the transducer does not present an impedance of 50 ohms at the frequency of the pulse (typically 100 or 150khz). As the load and receiver (the transducer) is a mismatch, the cable will act as a transformer for the mismatch and thus added inductance or capacitance will be needed at the e/s end to effect a result.

Altering the cable length jiggers the whole thing up.

Will probably be shot down over this as I haven't run through any calculations for such low frequencies and I have no idea what the impedence of the transducer looks like, but I will put a pound to a penny its not 50 ohms!

 

[VicS]

The answer is of course: suck it and see. If it works ok then you have no more worries. If it doesn't then you sort it out, relocating the transducer if necessary. Have you tried using your existing transducer, or won't that reach either? If the original was a Seafarer then the cable length was 25ft.

BTW I knew the owner of Jissel back in the mid '70s.

 

[Guest]

2 thoughts ..............1) Electronics can calculate at incredible speeds and the slight change to length COULD be significant. Why not experiment, in a section of water of known depth, example alongside a tide gauge ? - if the cable is already cut - join as is ..... switch on and register depth ..... now un-join cable and insert a length = to the extra you were considering .... now check depth again and see what happens ? If you do test out the idea - can you post the results ??? Would be interesting to see them

As to original cable length - most sets have very similar transducers etc. and the length of cable is normally about same. So measurement of another cable on a set could give you the answer as to what it should be.

Another point is that joint must be absolutely secure and well made ..... suggest a bnc or similar connector to ensure good contacts.

Finally manufacturers do say quite clearly that cables should not be cut, joined or otherwise altered.

 

[simonfraser]

i have soldered & taped cut coax with no probs, same lenght is important

 

[John_Walker]

The cable capacitance is part of a tuned circuit in the instrument. Changing the cable length results in the instrument no longer being tuned to the frequency of the signal returning from the sea bed. The result is a loss of sensitivity. The device will still indicate the correct depth when it is working, but will stop working at less depth than normal. As others have said, the only thing to do is suck it and see, including whether it works at adequate depths for your needs.

 

[tugboat]

According to the Navman catalogue, their fishfinder kits come with a 10metre cable on a transom mount transducer as standard. The optional thru-hull transducers have either 8metre or 9metre cables and they offer a 4metre extension with the stipulation that max length is 15metres. Had wondered if the length had to be multiples of something (e.g 4metres) but apparently not. Thanks for your reply to my post. /forums/images/graemlins/smile.gif

 

[VicMallows]

It's possible to alter the coax length (either shorten or lengthen), but as John Walker explains you need to maintain the resonance of the circuit. This could be done by internal adjustment on many older sets but is probably not povided for on current sets. It can also be achieved externally with a simple circuit, but either way you'd really need test equipment to set it up accurately.

The principle is the same as on old MW/LW car radios which had a trimmer for just this purpose (but could easily be adjusted by ear).

Vic