Shorten some coax

Discussion in 'General Electronics Chat' started by tom66, Jan 19, 2012.

  1. tom66

    Thread Starter Senior Member

    May 9, 2009
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    I've got a GPS antenna with about 5 metres of coax (I think RG-59) plus an SMA connector. The antenna is quite light, but the cable is heavy, so it makes it tricky to put on my model aircraft.

    Is it possible to shorten the cable to around 10 cm whilst maintaining its RF integrity and keeping the SMA connector?
     
  2. colinb

    Active Member

    Jun 15, 2011
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    Is the antenna end of the coax fixed to the antenna? If so, maybe it is soldered directly and you can unsolder it, cut the coax to the desired length, strip the new end, and re-solder to the antenna.

    If you need connectors on each end, I wonder how hard it is to install new SMA connectors? Are they crimp or solder connection? Crimp might mean a ridiculously overpriced tool is required.
     
  3. tom66

    Thread Starter Senior Member

    May 9, 2009
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    The antenna looks like this picture. I have two which are similar, so I'm okay to try something with one which might damage it, just for a test.

    The cable is not removable on the antenna end (well, not without damaging the antenna.)
     
  4. colinb

    Active Member

    Jun 15, 2011
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    The image isn't high enough resolution for me to zoom in for detail, but is that heat-shrink tubing on the coax by the SMA connector? If so, it would be interesting to carefully cut the heat shrink tubing off to reveal how the coax is terminated to the SMA connector -- maybe by soldering? If you can't remove/re-terminate the coax, you may or may not be able to get proper heat shrink over the SMA, so you might have to suffer with electrical tape to restore it....

    On the antenna end, it looks like you might have to dissect a plastic box. A job for some careful work with a Dremel and cutting wheel? Or, some careful Exacto-knife work? Or, can you crack it open at a plastic-weld joint? If you cut the antenna plastic enclosure cleanly, you can easily use some super-glue to put it together nicely at the end.
     
  5. tom66

    Thread Starter Senior Member

    May 9, 2009
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    Inside all GPS antennas is a ceramic dielectric of some kind. It's quite delicate, so I can't go chopping anything near or inside the antenna. I'm prepared to buy some connectors and re-fit a connector on 10 cm of cable, if I need to.
     
    Last edited: Jan 20, 2012
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    RG 59 is 75 Ohms as I recall.
    RG 58 is 50 Ohms (thick).
    RG 174 is also 50 Ohms (thin).
    Maplin has crimp connectors for the cable:
    http://www.maplin.co.uk/sma-crimp-plugs-26627
    You will need the crimping tool too if you want to use these.

    Bertus
     
  7. tom66

    Thread Starter Senior Member

    May 9, 2009
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    Oh it's probably RG 174 then. Any guide on wire thickness?

    What type of tool is a crimping tool, is it expensive?
     
  8. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Could you just get rid of the connector and solder cable directly to the board?
     
  9. tom66

    Thread Starter Senior Member

    May 9, 2009
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    I want to keep it expansible in case I need to change the GPS receiver or antenna.
     
  10. bertus

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  11. tom66

    Thread Starter Senior Member

    May 9, 2009
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    Ouch. That's very pricey :(. No easy way to do it without that, I guess?
     
  12. bertus

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  13. tom66

    Thread Starter Senior Member

    May 9, 2009
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    Yeah, it's just a matter of economy... I can buy a GPS antenna with a fitted 10cm cable and SMA end for about £10, so I'll probably do that. I'd like to learn how to do it myself later, though.
     
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