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Transponder cable length

Discussion in 'Boat Electronics' started by Rob Mills, Aug 31, 2004.

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  1. Rob Mills

    Rob Mills Guest

    I'm rigging an old Lowrance LFG300 flasher to use in a pedal boat (I know,
    I'm nuts) that's to be used on a small residential lake. I would like to
    shorten the cable to about 6 or 8 ft. Will this have any or much effect on
    the unit? I'm only interested in checking depth of lake as it's been years
    since the lake was dredged.

    Also, does turning this unit on with out the transponder connected do any
    harm to this unit.

    Rob Mills ~ Tulsa
  2. Vito

    Vito Guest

    Theoretically yes but not enough to notice.
    Dunno but might ruin it so I wouldn't.
  3. Rob Mills

    Rob Mills Guest

    That's what I was hoping. Thanks, RM~
  4. Vito

    Vito Guest

    The transmitter sends a pulse of electromagnetic energy down the coax to the
    transducer at the speed of light. The transducer converts that to sound
    which travels at (yup) the speed of sound in water; then it listens for
    echos and converts them back to electromagnetic signals sent via the same
    cable to the receiver. The receiver measures the total time required by all
    this to determine depth. If you cut 10' off the cable that determination
    will be off by 2x the time required for a speed of light signal to do that
    10'. Since the speed of light (186,000 miles PS IIRC) vs sound (about 1100
    feet PS) that error is small.
  5. I hate to rain on your parade here Meindert, but most, if not all,
    consumer Marine Depth Sounders, have Transformer Coupled outputs that
    are resonate in and of themselves, and really don't care what is
    connected to their output link as long as it isn't shorted.
    The transducer is a Barium Titante Crystal that looks like a
    capacitive load to the sounder transmitter, and if it isn't actually
    connected, the few watts of transmitter power is just burned up in the
    transformer as heat, as the average power is very low, similar to a
    radars average power compared to it's Peak Power. Now if we are
    talking commercial Depth Sounders, we are talking a horse of a bit
    different color, but again operating them into an open transducer
    line isn't very likely to cause any problems with the transmitter,
    you just don't get any return. The result of all the above is that,
    one "could" actually go and tune the transmitter for the best coupling
    to the transducer connected, but the gain in signal/noise by doing so
    isn't really significant. (Like less than 2db or so) The length of the
    transducer cable isn't critical to the operation of the souder, and
    many of these installations in the North Pacific Fleet have splices
    in them due the cost of hauling the vessel to change transducers,
    compared with just connecting a new sounder to what is already in the
    hull. (Frequencys all being the same, between the old and new units)
    There is Theory, and then there is 30 years of Practical........

    Bruce in alaska
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