Coaxial vs Twisted Pair cable for a Side Scan Towfish. Are they interchangeable?

Third Party Products Hardware
#1

Does anyone know if twisted pair and coaxial conductor types are likely to be interchangeable in this application?

I’m shopping for a new tow cable for our side scan tow fish. The tow fish was supplied with a Kevlar-reinforced “soft” coaxial tow cable, and i’d like to replace it with stainless steel. But most of the new cables I’ve found use twisted pair conductors rather than coaxial.

On this sonar, the coax cable caries the data signal as well as the 150v@1a power supply from the topside unit to the towfish. The manual says the data signal is FSK format and is transmitted at 9600 baud on the high voltage power line.

I suspect there is a problem with twisted pair, because every towed sonar system I’ve ever used has been supplied with coaxial cable. But twisted pair would be quite a bit more economical if it would work. I’m at the edge of my knowledge on this one…

#2

Hi @StrikeLines,

Coaxial and twisted pair cables have different noise rejection, signalling performance, power transmission, and mechanical strength characteristics, so each will have different tradeoffs. They’re not equivalent designs, so are not completely interchangeable.

I expect the communication signal would likely get through ok (especially if it’s a shielded twisted pair, rather than bare), so my main concerns would be on

  • power loss (depending on wire gauge and cable length)
  • electrical safety (if the cable gets nicked, coaxial is much less likely to expose the high voltage power wire), and
  • breaking strength (depending on the cable’s physical composition)
#3

Hello @StrikeLines
Most coax based sonar equipment will not work with twisted pair, usually because the modem expects good controlled impedance and shielding. That data rate is so low though, you might be able to get away with it. Do you have any more information on the sonar?

#4

Thanks for the info. I ended up finding a source for armored coax that was way less expensive than I thought. (Less than $2/foot… Turns out sonar companies have been putting the screws to me for years, charging $6k for a 300 foot cable.)

So I went with coax. I’ll leave the experimentation for someone else. Many thanks though!!