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Optimal Collinear Antenna

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  • Strix technica
    replied
    Originally posted by cmgam View Post
    Now the question, how to connect the antenna to dongle?
    To use a bnc conhector on antenna and then a coax cable? Connect directly the bnc conhector to the last element of the antenna and cable?
    It doesn't really matter provided the connector you use is the same impedance as the coax you used. Eg, for (75Ω) RG6, use F-connectors; for (50Ω) RG58, use SMA or BNC. Then get a suitable BNC or F-con adaptor for whatever your dongle uses, typically SMA.

    I used good quality (-2dB/10m) CT-100 RG6 and F-cons and then F-con (female) to SMA (male) adaptor at the SDR.

    I also inserted a decoupling sleeve (which I understand amounts to a 1:1 balun) between the co-lin and the feeder. I'm not sure how much difference this makes, but it is sensible in terms of RF theory because it provides high-impedance on the end of the outside conductor on the feeder end and should limit the degree to which the shield of the feeder acts as part of the antenna and therefore how much noise is coupled into your receiver.

    I did this by inserting an F-con coupler bulkhead (ie one that has a nut on one side of the coupler). The cut needs to be such that the length to the mating face of the coupler to the first 1/2w segment is equal to that of the VF-adjusted 1/4w. I made the decoupler from a metal can cut to about 95% of the free-air 1/4w (95% to compensate for end-effect).

    The open end of the can faces the feeder, which is directly attached to the coupler inside. Note that this forms the first 1/2w element in the co-linear so be sure that you cut the appropriate bit of coax or an element from the end of the co-lin to keep an even number of 1/2w elements.
    Last edited by Strix technica; 2017-05-01, 15:44.

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  • cmgam
    replied
    Hi
    Antenna is done. Now the question, how to connect the antenna to dongle?
    To use a bnc conhector on antenna and then a coax cable? Connect directly the bnc conhector to the last element of the antenna and cable?
    Any other way?
    Thanks in advance
    Regards


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  • Strix technica
    replied
    Originally posted by turbo825 View Post
    I am building a collinear antenna using RG6 coaxial cable for the elements. Due to difficulties attempting to insert the elements into each other,
    Obviously a bit late for turbo825, but for the benefit of anybody else who comes across this thread, I found that careful application of an awl inserted between the outer plastic sheathing (note: not the dielectric!) and the foil shield made it much easier to insert the pins (2cm, in my case) during construction.

    The resulting 8-element colinear is working better than anything else I've got, though doubtless it could be improved if I had suitable testing equipment.

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  • ameins
    replied
    Sorry, the cable doesn't have a specific name or type. I bought it on amazon.de because it had the velocity factor in the description. The cable is described as CCS (copper clad steel), 135 db, 75 ohm.
    Most of the time a was able to push the pieces together without punching through the outer shield. And when I did punch the core through the shield, I simply took a box cutter and carefully cut the shield open
    to push the core back in (used some more tape to close the opening later ;-) . I used a design with 4 elements I found here: forum.planefinder.net/threads/ads-b-diy-antenna.23/page-18#post-1319.

    ant.jpg

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  • turbo825
    replied
    What type of cable did you use for the second antenna? Building a collinear antenna using RG6 cable has been difficult, but I have had the most luck with the RG6 cable sold by Mediabridge on Amazon. I am not sure what brand of RG6 cable people use to build antennas on YouTube tutorials. They seem to have no difficulties.


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  • ameins
    replied
    Hello,

    I used RG6 on my first antenna also and had the same problems that you describe. Took me many hours to stick just four elements together.
    (It was a pain...)
    The resulting antenna worked fine, though.

    My second try was with a different sort of coax, steel core (copper-clad). This cable typ was a lot easier to handle and the antenna works fine.
    Second antenna was build in just over an hour .

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  • turbo825
    replied
    Originally posted by abcd567 View Post
    No, the length of pins do not affect performance.
    However it does affect the mechanical strength of joints.
    With shorter pins, you will need to apply more tape at joints to prevent joints from pulling out during handling/use.
    The method of construction you are using is called "push-pin" method as it requires pushing the pin-like center wire of coax into outer sheath of adjacent piece of coax.
    Okay, thank you for your assistance.

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  • abcd567
    replied
    No, the length of pins do not affect performance.
    However it does affect the mechanical strength of joints.
    With shorter pins, you will need to apply more tape at joints to prevent joints from pulling out during handling/use.
    The method of construction you are using is called "push-pin" method as it requires pushing the pin-like center wire of coax into outer sheath of adjacent piece of coax.

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  • turbo825
    started a topic Optimal Collinear Antenna

    Optimal Collinear Antenna

    I am building a collinear antenna using RG6 coaxial cable for the elements. Due to difficulties attempting to insert the elements into each other, I shortened the distance of copper core that extends past the edge of the elements. Will shortening this length affect the antenna's effectiveness? Thanks in advance for advice.
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