Cutting co-ax

[JeremyF]

Ive seen a few postings stating that co-ax should not be cut, whether from VHF antenna or from an echosounder. The trouble is that it is nigh on impossible to track the cables through with the existing plug in situ.

If you join the co-ax with proper fittings ( rather than tape) I doubt more then 1 or 2 dB of signal strength is lost. So what's the problem, or what's the alternative?

Jeremy Flynn

www.yacht-goldeneye.co.uk
New pics 11.02.02

 

[Bergman]

No problem with VHF antenna provided that connectors are good quality and correct impedance.

Not sure about sounder I have cut and jointed mine - screened twin not co-ax - without problem but nature of signal may cause reflections from connector which could upset sounder. Probably best avoided if possble.

If not possible best option would be to cut as near connector to sounder as possible and to replace with new connector - if you can get one. Not always easy because modern trend is to use moulded on connectors.

 

[ccscott49]

VHF no problem, use correct VHF fittings, PL259 etc. Echo sounder, do not cut, the legth is matched to the sounder, although I've now been told you can extend the cable by a certain amount??? I woulod check with the manufacturer.

 

[billskip]

I think it depends on the ability of the person fitting it ..also manufactures are protecting them selfs against people sending stuff back due to stupid conections..that said I have asked suppliers and they say its ok to cut but I proberbly spoke to someone who was answering phone while sweeping the floor...some of the cables today have screening material that is difficult to re join..so I think its importent to make the join where it will be least affected by other electrical/magnetic interferance .IMHO

Bill
www.aegeansailing.co.uk

 

[oldharry]

Some co ax cables have a foil sleeve rather than the traditional braided sleeve outer, which can be a pig to rejoin properly without the proper connectors (which should always be used on any signal carrying cable anyway)

Some echo sounders are matched to their cable/ transducer set ups very precisely - the length of the cable being an integral part of the electrical impedance and capacitance matching, and the length of cable supplied may be matched to the operating frequencies of the transducer. Cutting or shortening such cables may well substantially reduce the performance of the gadget to which they are attached.

Only the maker, or a good electronics engineer can tell you whether your gear would be affected by changing the cable length.

 

[HaraldS]

In principle you can cut and join any coax cable. Equipment manufacturers often tell you not to, because the joints are often the most common source of trouble.
If you use a good connector you will hardly have any noticable losses, but you have to find the ones that match the cable size and construction.

If you want to make a cable longer, you need to pay attention to the right kind of cable. Most important is to match the impedance. The second think to look for is signal attenuation for the frequecy range you are looking for, as low as you can find or afford. You will find that most things on the boat are 50 Ohm impedance.

HAM radio shops have the biggest selection of connectors and cables for usually a much better price than marine.

 

[LORDNELSON]

I had to cut my clipper log coax when fitting it. Manufacturers said it was ok to cut. I cut it about 1 metre from the paddle wheel and joined it with a proper connector surrounding it with self amalgamating tape. Has worked perfectly for the last year.

 

[ParaHandy]

I am afraid I always cut as otherwise impossible, like Jeremy, to feed cables thro' etc. I have never used coax connections (not sure whether 50-75-100 ohm cable and neither, oftentimes, is the manufacturer) and simply solder joints together except for radar which was to terminal block thro' soldered tags. All still working.

Putting joint away from interference wise advice, as Bill says, but keeping whole cable away from interference better.

 

[JeremyF]

Thanks for the advice and experience.

As you state, Parahandy, there's often no choice, and certainly in this case.

I wouldnt have thought that the resitance is going to be massively effected by a connector, compared to the cable and the device at the other end. In this case Ive got to add a connector 1ft away from the echo transducer. I hope that interference wont occur. Surely the inner core remains shielded by the outer screen even in a socket.

Anyway, I'm now commited to try, and will report back on the result!

Jeremy Flynn

www.yacht-goldeneye.co.uk
New pics 11.02.02

 

[bedouin]

There's a huge difference between VHF and echosounder.

VHF carries much more power at much higher frequencies. It is fine to cut VHF cable, but important (as mentioned above) that it is well joined with connectors of the correct characteristic impedance. (Note that characteristic impedance is not the same as resistance, even if both are measured in Ohms).

For depth sounders it is much simpler; they operate in the 150-200kHz range at much lower power, so the nature of the join is much less critical. I cut and re-soldered the lead to my depth sounder on Saturday and it still works fine.

 

[WayneS]

There is no problem cutting the echo cable and fitting a new connector.

If you were going to remove/add multiple feet to the co-ax you may get some problems with false readings. Cut the cable as close to the old plug as possible and fit an ordinary TV aerial connector (It's unlikely that you will be able to use the old plug as they are moulded on and joining the cable can only cause you problems).

I have done it this way on several boats after getting advice from a marine electronic chap and never had a problem.