Cable Length Question for Eric

[Anonymous]

Hi, I want to put 150feet to 300feet of some sort of coax on a 1meter coil to pull behind a boat on a fish attached to a Towtec on the boat. Is there any problem with doing this? Being that we are reading such a small signal (around a millivolt I guess) with the coil and sending it up such a long cable run is there any concern with noise, resistance, or capacitance that prevents this from being possible or does it work ok? I guess the alternative is the front end at the fish and running the output up the cable. Have you done this? I really don't want to butcher a Towtec board if I don't have to. If this works ok, do you recommend a particular coax cable for this? Your help is appreciated. Thanks, Fred

 

[Anonymous]

Hi Fred,
Yes, you can put long lengths of coax between the electronics and the coil but you will have to adjust the damping and also the sampling pulse delay. You will need a heavier coax otherwise the dc resistance will be too great and the pulse current will suffer. I have used RG213 which is just over 10mm OD, but you really want to run the coax inside a flexible plastic tube (like a hosepipe) otherwise the pvc jacket will soon get cut and once water gets in everything goes haywire. You would need to adjust the damping using a scope on the output of the first amplifier (709 of course). Adjust resistor value for just no ringing. You will most likely have to move the sample delay back a bit but it is easy to see where it is by the positive bump up caused by the auto-zero. Shift this back until you are just clear of the recovery waveform. Having the front end at the fish is likely to trade one set of problems for another and is best avoided.
Eric.