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Thread: A guide to a 5 element coco

  1. #1
    Purser
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    A guide to a 5 element coco

    A 5 element coco made from coax (soldered) with a shorted stub.

    I picked the solder method because after building a few coco's aerials and comparing the results. With a coco made by pushing the elements together they are very unstable and vary massively on the network analyser by many dB's with the slightest movement unlike the soldered ones which remain fairly constant.

    Sorry there is just a little bit of maths to know

    to get the wavelength of 1090MHz you need to use the following formula

    λ=C / f Where C is the speed of light and f is the frequency.

    this works out to be 299792458 / 1090000000 which equals 0.275m or 275mm (i worked to more decimal places than displayed here)

    however because we are using coax cable, we have to account for the velocity factor of the cable.

    For the cable i am using.

    cable.jpg

    If you look up the data sheet (website wouldn't let me link the PDF) it has a velocity factor of 85% so taking our length of 275mm and multiplying it by 0.85 we end up with a wavelength of 233mm now for the coco we will be working with half and quarter (for end piece) so they are wavelengths of 117mm and 58mm.


    I would recommend buying a pre-made BNC to BNC coax cable I will explain at the end of the trio of posts. However i will start with how I make each element.

    Taking a piece of coax remove about 15mm of the PVC outer, I personally you a scalpel nice and sharp so you dont have to use any pressure. Taking your knife roll the knife around the outer of the cable remember you dont have to press hard because when you bend the cable it will tear the outer.

    roll.jpg
    tear.jpg

    Remove the outer PVC and twist the braid together off to one side.

    braid.jpg

    tin and trim the braid and centre core making sure that the braid is trimmed to the length of white centre of the cable. I aim to have 5mm of dielectric showing and then a further 5mm of the centre copper showing.

    prepped.jpg
    Last edited by SpaxmoidJAm; 2014-06-11 at 18:50.
    T-EGLF8

  2. #2
    Purser
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    Then cut off the prepared end of the coax approximately 15mm longer than the 117mm we are aiming for measuring from the white dielectric.

    CutLength.jpg

    Now using a rule to measure, trim of the PVC outer 5mm back from 117mm so from 112mm from the white dielectric and prepared end the same as before. It helps if the braid is twisted on the same side.

    CocoSection.jpg

    I know it looks longer than 117mm but it is a perspective error (i checked twice)

    Create the next four sections in the same fashion.

    For the stub at the end start with one the same as before, but for the shorted end cut the PVC back an extra 5mm off and trim the dielectric the same amount 5mm from the twisted braid as before this gives you a little bit of extra copper to bend around some pliers to make a neat short. So the length of this piece is 58mm from the end of the dielectric on the left to end of the copper centre bent round at the end.

    stub.jpg

    Now to join your sections together, lay two sections on top of one another like this and solder it is quite easy with this method to see any shorts.

    join.jpg

    keep going until all five elements and the stub are connected.

    sections.jpg
    Last edited by SpaxmoidJAm; 2014-06-11 at 18:03.
    T-EGLF8

  3. #3
    Purser
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    Back to why i said to use a coax cable with BNC's on if you were to get a 3m or more BNC to BNC it gives you enough coax to have a couple of goes a making an aerial and you already has a well fitted connector, plus if you get you self a MCX to BNC converter you can attach it strait to you dongle of choice.

    By Using the BNC tail like so
    bnc.jpg

    MCX_BNC.jpg

    Meaning that you only have to have a 30cm or so tail on your aerial, no losses through a long coax run.

    I am trialling a USB extender that works over a Ethernet cable. I have it running on a 5m cable and i will try a longer one when I have a longer Ethernet cable.
    edit: i found a 15m cable and its working still (that's 50 feet)


    http://www.maplin.co.uk/p/cat5-booster-usb-11-a44hg
    usbext.jpg

    I used this exact process earlier today at work to make an aerial, and when i tested it on a network analyser it produced a gain at 1090MHz of around 5.5 dB's I did not play around to and improve anything it was what it was, to prove the process if you like.

    My apologise for making this three posts had i know before i typed it out that i could only link 5 images in a post i would done it as a separate document, if this is a problem delete the thread and I will sort it out.

    Also if there is anything that is confusing, needs explaining better or plain wrong let me know and i will fix them. I'm sure when i re-read this i will keep finding things.
    Last edited by SpaxmoidJAm; 2014-06-11 at 19:07.
    T-EGLF8

  4. #4
    Purser
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    Great write up - thanks

  5. #5
    Captain abcd567's Avatar
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    Good work! Thanks for sharing.

  6. #6
    Passenger
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    I evaluated many versions oc CoCo proposed in this forum and I think Im gonna try this one as my first CoCo.
    Thanks to all for the invaluable support.
    Ill keep you informed
    T-GCLA3 - Orange Pi PC Plus + NooElec NESDR SMArt + Spider antenna

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