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Thread: best antenna

  1. #1201
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    It's an interesting proposal. I have a window looking at west-southwest, a CoCo, a dongle and a computer all within 3 meters. Realtime visualization: http://ramonferreiro.dyndns.org/planeplotter/ I can even install VRS to show you all coverage plots. The only thing I don't have in excess is free time.
    Northwest Spain: F-LECO1, F-LEST1

  2. #1202
    Captain abcd567's Avatar
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    Quote Originally Posted by Breitling View Post
    It's an interesting proposal. I have a window looking at west-southwest, a CoCo, a dongle and a computer all within 3 meters. Realtime visualization: http://ramonferreiro.dyndns.org/planeplotter/ I can even install VRS to show you all coverage plots. The only thing I don't have in excess is free time.
    Thanks. Sure, few persons will have enough free time to do such an exercise.
    Free time & dedication are the most important requirements for such a work.

  3. #1203
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    An abstract from "ARRL Antenna Book"

    Collinear Transposed-Coax Arrays
    Collinear arrays tend to be tolerant of construction tolerances, making them easy to build and adjust for VHF and UHF applications.

    The most popular collinear array is the omnidirectional array of half-wave dipoles constructed of transposed sections of coaxial cable as shown in Figure 15.10B. The original array of this type is the Franklin array shown in Figure 15.10A.

    The phase-reversing stubs allow multiple half-wave sections to operate in phase, creating gain at right angles to the antenna. An example of this array is the popular Cushcraft Ringo Ranger series of omnidirectional VHF and UHF antennas. While the phasing stubs make the Franklin array inconvenient for vertical stacking of more than two elements, a derivative of this array uses transposed sections of coaxial cable as in Figure 15.10B.




    The phasing stub is created by the inside of each coaxial section. The outer surface of the coaxial shield forms the radiating element. The resulting antenna can be enclosed in a PVC or fiberglass tube, such as the Comet GP-series of VHF/UHF omnidirectional antennas.

    The practical limit for gain in this type of array is about 10 dBi. A choke balun or other method of decoupling such as a set of λ/4 radials is required at the feed point of the array to prevent current from being induced on the outer surface of the coaxial feed line.

    Collinear Omnidirectional Array for 70 cm
    Figure 15.11 shows the basic construction of a transposed-coax array for the 70 cm band with dimensions in millimeters for accuracy. The λ/4 whip at the end of the array is optional. The gain of this array is approximately 9 dBi (slightly less without the whip). The original design of this antenna is credited to the Radio Amateur Society of Norwich (www.rason.org). More information is available via the “Projects” page of the RASON website.



    The physical length of each λ/2 section of coax must account for the velocity factor of the cable which should be measured accurately before cutting any cable. Once the physical length of λ/2 has been determined, add 8 mm to allow for creating the 4 mm connecting surfaces on each end.
    For a VF = 0.66, the λ/2 sections should be 223 mm long plus 8 mm for a total of 231 mm. RG-58, RG-8, RG-8X or RG-213 can be used for this antenna. Do not remove the outer jacket from the cable other than at the connecting ends as this will allow the individual braid strands to loosen, reducing the shield’s effectiveness as a continuous conductor.

    Use a 169 mm segment of #16 AWG copper wire for the top whip section.

    A λ/4 coaxial sleeve balun is attached at the feed point of the antenna. (See the Transmission Line Coupling and Impedance Matching chapter.) The balun is made from copper tubing that is soldered to the shield of the feed line using strips of brass or copper shim. If 5⁄8-inch tubing is used, the length should be 160 mm. The feed line should be centered in the balun tubing by using small pieces of plastic inserted between the coax jacket and the tubing’s inner surface. Approximately λ/4 beyond the end of the balun’s closed end add an additional choke balun of three type 43 ferrite beads (choose the ID to fit the feed line coax).

    The entire antenna should be enclosed in a length of PVC or fiberglass tubing to protect it from the weather. If necessary for mechanical stability, support the antenna sections with a length of wooden dowel or plastic rod, secured with electrical tape.
    Last edited by abcd567; 2014-06-03 at 20:43.

  4. #1204
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    Another abstract from "ARRL Antenna Book"

    DETUNING SLEEVES
    The detuning sleeve shown in Figure 24.61B is essentially an air-insulated λ/4 line, but of the coaxial type, with the sleeve constituting the outer conductor and the outside of the coax line being the inner conductor.



    Because the impedance at the open end is very high, the unbalanced voltage on the coax line cannot cause much current to flow on the outside of the sleeve. Thus the sleeve acts just like a choke to isolate the remainder of the line from the antenna. (The same viewpoint can be used in explaining the action of the λ/4 arrangement shown at Figure 24.61A, but is less easy to understand in the case of baluns less than λ/4 long.)

    A sleeve of this type may be resonated by cutting a small longitudinal slot near the bottom, just large enough to take a single-turn loop which is, in turn, link-coupled to a dip meter. If the sleeve is a little long to start with, a bit at a time can be cut off the top until the stub is resonant.

    The diameter of the coaxial detuning sleeve in Figure 24.61B should be fairly large compared with the diameter of the cable it surrounds. A diameter of two inches or so is satisfactory with half-inch cable. The sleeve should be symmetrically placed with respect to the center of the antenna so that it will be equally coupled to both sides. Otherwise a current will be induced from the antenna to the outside of the sleeve. This is particularly important at VHF and UHF.

    In both the balancing methods shown in Figure 24.61 the λ/4 section should be cut to be resonant at exactly the same frequency as the antenna itself. These sections tend to have a beneficial effect on the impedance-frequency characteristic of the system, because their reactance varies in the opposite direction to that of the antenna. For instance, if the operating frequency is slightly below resonance the antenna has capacitive reactance, but the shorted λ/4 sections or stubs have inductive reactance. Thus the reactances tend to cancel, which prevents the impedance from changing rapidly and helps maintain a low SWR on the line over a band of frequencies.

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    Last edited by abcd567; 2014-06-03 at 11:50.

  5. #1205
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    Quote Originally Posted by F-EGLF1 View Post
    If you look at the photos of the commercial one you wil see that it has a similar arangement on the outside of the tube,
    I thought that it was just to re-inforce the tube for the clamp, however it made a big difference to the result (it is 1/4-wavelength long) I have seen other plans with a similar arrangement but without the internal balun, (the coin sized plate is in fact a UK penny as it was all I had to hand at the time!).
    I will try the next one without the upper internal balun and see what effect it has.
    My build was based on the earlier photos, but with all the lengths re-calculated and the 1/4 stub tweaked on the analyser.
    There is still much learning to do as so many people and sites have their own way of building these things, my hope is to provide a tested working design that anyone can copy with basic tools and get good results.
    Using RG402 is far better than mucking about with ordinary coax as it can be pre cut to the exact length and solders easily so it should be more repeatable, in a future post I will detail the tools and methods I used on the MK2 build as I am photographing it stage by stage.
    Ben.
    I feel that the "upper coin-sized copper disk with 4-down legs" is in fact "a set of λ/4 radials", doing the same job as sleeve i.e. decoupling the common mode currents (the unbalanced current on outer surface of the coaxial shield). The sleeve alone does not fully do the decoupling, and requires additional methods such as "a set of λ/4 radials" or "a set of ferrite beads choke".
    Last edited by abcd567; 2014-06-01 at 02:59.

  6. #1206
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    Quote Originally Posted by Breitling View Post
    It's an interesting proposal. I have a window looking at west-southwest, a CoCo, a dongle and a computer all within 3 meters. Realtime visualization: http://ramonferreiro.dyndns.org/planeplotter/ I can even install VRS to show you all coverage plots. The only thing I don't have in excess is free time.
    I do have some free time. I fancy a few weeks holiday in Spain

  7. #1207
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    Quote Originally Posted by abcd567 View Post
    .
    I spent this weekend to do some maths for optimizing a CoCo.
    The results are shown at the bottom of the calculation sheet below:


    Has anyone tried and tested that design?

  8. #1208
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    Source: ECE Dept, McMaster University, Ontario, Canada. Antenna Lectures by Prof. Natalia K. Nikolova

    Balanced-to-unbalanced feed
    Sometimes, when high-frequency devices are connected, their impedances might be well matched, and still we may observe significant reflections. This is sometimes referred to as “field mismatch.”
    A typical example in antennas is the interconnection between a coaxial line of Zc = 75Ω and a half-wavelength dipole of Zin = 73Ω. The reflections are much more severe than one would predict using equation Γ2 = {(SWR-1)/(SWR+1)}2 . This is because the field and the current distributions in the coaxial line and at the input of the wire dipole are very different [see figure below]. The unequal currents on the dipole’s arms unbalance the antenna and the coaxial feed and induce currents on the outside of the coax shield which are the reason for parasitic radiation. To balance the currents, various devices are used, called baluns (balanced-to-unbalanced transformer).









    Sleeve (bazooka) balun 1:1
    The sleeve and the outer conductor of the coaxial feed form another coaxial line, which has a characteristic impedance of Z′c. This line is shorted quarter-wavelength away from the antenna input terminals. Thus, its input impedance is very large and results in: (i) suppression of the currents on the outer shield (I3), and (ii) no interference with the antenna input impedance, which is in parallel with respect to the coaxial feed. This is a narrowband balun, which does not transform the impedance (1:1 balun).



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    Last edited by abcd567; 2014-06-03 at 20:45.

  9. #1209
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    My LDF1-50 coco just about ready. It'll be installed in fiberglass case and I'll be adding a 1/4 wave choke balun at the base.

    http://ads-b.ca/LDF1-50/coco/

    top element is a 1/4 wave 1/4 wave with shorted center.

  10. #1210
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    Quote Originally Posted by 1090 MHz View Post
    My LDF1-50 coco just about ready. It'll be installed in fiberglass case and I'll be adding a 1/4 wave choke balun at the base.

    http://ads-b.ca/LDF1-50/coco/

    top element is a 1/4 wave 1/4 wave with shorted center.
    Saw the pictures. Looks great. Waiting for completion & trial run. please post the photos of balun when you add it,

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