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Thread: best antenna

  1. #1271
    First officer 1090 MHz's Avatar
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    Quote Originally Posted by HermanZA View Post
    Today I made the bracket that must clamp onto the mast and give the antenna a stand-off of about 1m.
    Be sure to take lots of photos and show us the bracket you built..

  2. #1272
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    Nobody done any antenna work in over a fortnight ???????

  3. #1273
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    I should be posting a report, results and photos in the next few days.

  4. #1274
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    I'm conducting the comparative test that abcd456 proposed some time ago. Different Coco tip terminations, all of them made with the same cable type, same location, same receiver, etc. I'll prepare a specific webpage to show them all with gathered data.

    Just an image to make audience

    Prev-coco.jpg
    Northwest Spain: F-LECO1, F-LEST1

  5. #1275
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    Looking forward to your results! Might be making myself another CoCo this weekend, and can just maybe benefit from your tests.

  6. #1276
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    Quote Originally Posted by HermanZA View Post
    Looking forward to your results! Might be making myself another CoCo this weekend, and can just maybe benefit from your tests.
    It will take a bit longer, I'm trying each model over a long time period (a whole week, 168 hours test run) to avoid inconvenient situations: low density traffic, poor propagation, etc.
    Northwest Spain: F-LECO1, F-LEST1

  7. #1277
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    Quote Originally Posted by Breitling View Post
    I'm conducting the comparative test that abcd456 proposed some time ago. Different Coco tip terminations, all of them made with the same cable type, same location, same receiver, etc. I'll prepare a specific webpage to show them all with gathered data.

    Just an image to make audience

    Prev-coco.jpg
    Quote Originally Posted by Breitling View Post
    It will take a bit longer, I'm trying each model over a long time period (a whole week, 168 hours test run) to avoid inconvenient situations: low density traffic, poor propagation, etc.
    Thanks for your efforts which will benefit all of us. You are thorough & dilligent. Waiting for your results.

  8. #1278
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    It just occurred to me that coaxial collinear antenna made of coaxial cable is a compromise.

    For any element of the coco, the central conductor is used for phasing & should be 1/2 wavelength.
    As the phasing function is carried out by inner space of the coaxial where the electromagnetic field is totally confined in the dielectric between core & shield, the required length is 1/2 wavelength in air (138 mm) x Velocity factor of the dielectric. If VF is 0.85, the length should be 117mm

    The outer surface of the shield of that very element is used for receiving the incoming electromagnetic field. Its length should also be 1/2 wavelength. Since the incoming field is totally in the space between antenna & aircraft, and is in air, Velocity factor is 1. Hence the length should be 1/2 wavelength in air = 138mm.

    We are making CoCos with element length using Velocity factor of Coaxial Cable. This makes the phasing element (inner space) accurately dimensioned (117mm), but makes the receiving element (outer surface of shield) shorter than optimum (117mm instead of required 138mm).

    The CoCo will be precise & optimum only if inside dielectric is air.
    If the elements are made of hollow metallic tube with central conductor supported by few rings/washers of plastic or rubber, this can be achieved.

    Air Insulated CoCo.png

    .

  9. #1279
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    ABCD, you got me totally confused now. (admitting, you have done so on a few occasions in the past as well! )

    So you say that the VF we used was based on the cable specs, and not the fact that the majority of the EM signal is travelling through open space (with a VF of 1.0), so we are using elements that are shorter than what it should be. On the Antenna itself.
    Based on that, should we then not consider that the signal received by the antenna, travels through a cable to the receiver, and the VF factor rule comes into play, cause we are now constraining it in a cable? Me thinks: optimal reception on the antenna, then we 'throttle' it through whatver cable, messing the perfect signal up.

    So you are proposing we should look at new antenna specs, with 138mm element lenghts, built of say an aluminium tube (of what diameter?) with rubber (non conductive) spacers to hold a centre element (copper perhaps?) in place. Based on this, what number of elements, and also a 5 1/2 element design?

  10. #1280
    Captain abcd567's Avatar
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    Quote Originally Posted by HermanZA View Post
    ABCD, you got me totally confused now. (admitting, you have done so on a few occasions in the past as well! )

    So you say that the VF we used was based on the cable specs, and not the fact that the majority of the EM signal is travelling through open space (with a VF of 1.0), so we are using elements that are shorter than what it should be. On the Antenna itself.
    Based on that, should we then not consider that the signal received by the antenna, travels through a cable to the receiver, and the VF factor rule comes into play, cause we are now constraining it in a cable? Me thinks: optimal reception on the antenna, then we 'throttle' it through whatver cable, messing the perfect signal up.

    So you are proposing we should look at new antenna specs, with 138mm element lenghts, built of say an aluminium tube (of what diameter?) with rubber (non conductive) spacers to hold a centre element (copper perhaps?) in place. Based on this, what number of elements, and also a 5 1/2 element design?

    With Dielectric Insulated elements, there is a conflict in element length due to difference in inner & outer mediums (138mmxVF for dielectric inside & 138mm for air outside).

    With air as dielectric both inside & outside the element, both lengths become same (138mm) & this conflict will resolve.
    Please see attached sketch below.

    The design will remain same, number of elements according to your choice. Only the material used for making the elements is copper/aluminum/brass tube with inner conductor also of copper/aluminum/brass, held in place by insulating spacers. The length of elements will be 138 mm.

    As far as feed cable is concerned, it does not perform receiving & phasing functions & hence there is no conflict.

    Air Insulated CoCo-2.png

    Later addition:
    The electromagnetic field INSIDE the cable is produced by the current flowing in the central conductor. This field is totally confined within the cable in the space between central conductor & shield.

    The electromagnetic field OUTSIDE the cable is produced by the aircraft antenna, and is totally in air outside the cable. It does not penetrate into the space inside the shield. The outer surface of the shield is the actual receiving antenna
    Last edited by abcd567; 2014-07-21 at 16:50.

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