Can I shorten the cable?

Billjratt

Billjratt

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I have ST60 instruments that interfere with the VHF and domestic radios. The tranducer wiring is coiled up in the ceiling cavity and when moved, the interference alters. I can shorten the masthead and paddlewheel cables I believe, but can I shorten the depth transducer cable or is it part of a tuned circuit ( like the old whirlie ones). I'm pretty sure it's the echosounder that's causing the problem anyway.
I've tried googling and searching this forum to no avail.
 
read any teckie book and it will tell you to Z-Zag excess cable so that any inductive fields tend to be cancelled out. You are lucky
 
Don't shorten it!
try to coil it away from other cables, in alternate loops and figure 8's perhaps, this helps the fields cancel. You have to watch the minimum bend radius of the cable, if you bend it too tight it may upset the match in extreme cases.
If there is a handy metal plate such as a water tank, coil it next to that, it will give a bit of shielding. If moving the coils doesn't change things much, suspect that the interference is power supply borne and add suppressor caps.
hth.
 
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read any teckie book and it will tell you to Z-Zag excess cable so that any inductive fields tend to be cancelled out. You are lucky

[/ QUOTE ]...But don't be tempted to fold one half of the zigzag over the other - or the fields will add instead of oppose! The easiest way I know is to coil half the excess in a longish coil then do a 'half figure eight' turn (to reverse the direction) then coil the other half the other way. Alternatively hold the excess cable in a bight and coil that up (it does exactly the same thing, in effect). This produces a tighter coupling than a zigzag and therefore better self-cancellation of any induced fields. Hold it all together with a few cable ties.
 
Just to buck the trend - I am quite happy to extend or shorten depth transducer cables and haven't had any problems so far.

I'm really not sure what problem people would expect to experience - we are only talking about 150-450KHz here, it's not like Radar or even VHF. Assuming you are careful about making connections or joints you really shouldn't experience any problems.
 
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Just to buck the trend - I am quite happy to extend or shorten depth transducer cables and haven't had any problems so far.

I'm really not sure what problem people would expect to experience - we are only talking about 150-450KHz here, it's not like Radar or even VHF. Assuming you are careful about making connections or joints you really shouldn't experience any problems.

[/ QUOTE ]...And indeed you can buy 7m extension cables for some e/s manufacturers! I asked NASA techies about this a while ago and AFAIR they said that, although the kit uses frequencies around 150kHz, the problem is to do with the expected capacitive load provided by the cable and that they shouldn't be shortened.
 
Thanks guys. I agree with all of you if that's possible! I had considered the best solution was to double-and-coil to produce a "bucked" coil and try to put it somewhere far away from civilization. I was just wondering if the modern instruments were clever enough to work with a slightly altered tuned circuit. The Raymarine tech has come back with the "no" answer as well as some of you, but others amongst you have got away with it. I'll try the coil first then if I get frustrated - out with the snips ( I can always re-instate the spare bit if it goes to rats)
 
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Where did you get the (strange) idea that this was a tuned circuit? If you are concerned about the impedance, then coiling it will change that anyway.

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Exactly - all these people who are afraid of cutting the cable seem to see no problem with all sorts of coiling, wrapping in foil and so on that could introduce all sorts of stray capacitance and inductance into the circuit /forums/images/graemlins/smile.gif
 
It is a tuned circuit. the transmitter fires pulses by twitching a peitzo-xtal at a pre-determined frequency and expects to hear an echo using the same transducer. However, I have reconciled my quandry as to why some do get away with cutting and others don't. The result of de-sensitising (cutting cable) will be to reduce the range (depth) of the instrument. So, if you spend your time "magicing the swatchways" or "riddling the sands" you probably won't notice the difference. We who frequent the jaggier, steeper bits of the country may prefer to stick to manufacturers advice.Therefore, I will be coiling rather than cutting. As for the two previous entries - it's a coax cable - the capacitance is inside....and don't start me on time-domain-reflectometry, 'cos that's another way of proving the cut makes a difference.
 
But realistically cutting a few feet off a coax cable is not going to make a measurable difference to the circuit, particularly not in the Ultrasonic spectrum. The circuit is more akin to a loudspeaker than an aerial.

If the impedence of the cable is correctly matched to the circuit then you an add or remove almost indefinite lengths of the cable without altering the circuit. This is common even in the GHz range (e.g. Satelite TV).

I think that the reason some people have problems with cutting the cable is that they do not make good connections. That can drastically reduce the performance of the system.
 
[ QUOTE ]
[ QUOTE ]

Where did you get the (strange) idea that this was a tuned circuit? If you are concerned about the impedance, then coiling it will change that anyway.

[/ QUOTE ]
Exactly - all these people who are afraid of cutting the cable seem to see no problem with all sorts of coiling, wrapping in foil and so on that could introduce all sorts of stray capacitance and inductance into the circuit /forums/images/graemlins/smile.gif

[/ QUOTE ]I did some research /forums/images/graemlins/smile.gif. At the frequencies concerned the transducer/cable combination appears as a capacitive impedance at the output of the e/s. The capacitance is largely due to the cable. This is 'tuned out' (somewhat approximately) by an inductor in the e/s. I assume that the manufacturer chooses an inductance to suit the length of cable he supplies. If you increase or decrease the length you'll get a poorer load match which will reduce sensitivity and increase bandwidth. It'll probably still work but not as well.
 
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