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  • abcd567
    replied
    Originally posted by Rooster View Post
    ..........
    Yes you could use the suck it and see approach, but with more than one coil its like juggling sand ! LOL
    I would love to potter with these myself - but without the correct test equipment, its a game of luck.
    Try your luck with "Franklin Spider".

    The Spider is a simple & proven antenna. It has a ¼ wavelength (69mm) whip, with 4, 6 or 8 radials, also ¼ wavelength, bent down 45 degrees to bring its impedance close to 75 ohms.

    Couple of months ago, I conceived a varient of Spider, and named it "Franklin Spider". In this varient, a ½ wavelength vertical + a ¼ wave stub is added on top of the ¼ wavelength whip. I ran simulations, and it showed better gain and slightly higher swr than the standard spider.

    However when me and 3 other planefinder forum members made prototype and put to trial run, only 1 (caius) out of 4 got better results than Spider, while other 3 (me, jepolch & Xforce30164) got inferior result than the Spider.

    If you want to use "suck it and see" approach, try this "Franklin Spider" varient of plain "Spider". It is simple & easy to make. For comparison, you can make either both types of Spiders, or can make only one Spider with interchangeable whips (one whip ¼ wavelength, and other whip ½wavelength vertical+¼ wavelength stub+¼ wavelength vertical)

    My built:
    Wire dia = 1 mm (core wire of RG6 coax).

    From top to bottom:
    (1) 138 mm vertical section (½ wavelength)
    (2) Stub, ¼ wavelength (wire lengths in stub = 66 mm + 6 mm + 66 mm = 138 mm = ½ wavelength).
    (3) 69 mm vertical section (¼ wavelength).
    (4) Four radials, 69 mm each (¼ wavelength each), slanting down 45 degrees from horizontal.


    Please see below photos of "Franko Spider" built by forum member xforce30164 based on my design.

    http://forum.planefinder.net/threads...na.23/page-131

    Simulation below done by me show "Franko Spider" to be superior to "Spider"

    .

    Simulation of Franko Spider



    Simulation of Spider

    Last edited by abcd567; 2015-07-13, 21:21.

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  • Rooster
    replied
    I think its really a bad idea to infer that the coil has to be an electrical 1/4 wave.... in this instance the coils are to alter PHASE, you MUST take into account the impedance, capacitance and "Q" of the coil in question...

    Yes you could use the suck it and see approach, but with more than one coil its like juggling sand ! LOL
    I would love to potter with these myself - but without the correct test equipment, its a game of luck.

    Leave a comment:


  • abcd567
    replied
    Originally posted by Rooster View Post
    This bit is confusing me... why are you subtracting the wire diameter ? Surely the specs dictate the former diameter ?

    Have you built these two to test your theory ? My analyser only goes upto VHF :-( or Id be at it.
    A member of Plane-finder forum built this antenna according to the drawing using wire from electrical cable size 2.5 sq. mm, and 1-1/2 turn coil wound on 10 mm dia former, as shown in drawing. He did not take into consideration the note that wire length in coil = 65.5 mm. He reported that the antenna gave very poor result.

    Another member of Flightaware forum also made the same antenna. He also used 2.5 sq mm wire, but he played it well by first cutting 66 mm length of wire, then wound it over 11.5 mm drill bit (instead of 10 mm mentioned in the drawing), to get nearly 1-1/2 turns. He then soldered the 3 vertical limbs to the two coils. Since his wire in coil was pre-cut to 66 mm (1/4 wavelength), the coil provided good phasing. He reported that he got good performance from this antenna.


    If the coil is wound on 10 mm dia former:
    The 2.5 sq mm wire has a dia of 1.8 mm.
    The coil's dia (from center of wire-to-center of wire) = dia of former + dia of wire = 10 mm + 1.8 mm = 11.8 mm.
    The length of wire in 1-1/2 turns is therefore = 1.5 x pi x 11.8 = 55.6 mm, whereas the required length of wire in drawing is mentioned 65.5 mm.
    Since the length of wire in coil is 10 mm less than required 1/4 wavelength, the phasing provided by the coil is poor.

    If coil is wound on 11.5 mm dia former:
    The dia of coil (from center of wire to center of wire) = dia of former + dia of wire = 11.5 mm + 1.8 mm = 13.3 mm
    With wire pre-cut to 66mm, the number of turns obtained by winding this wire on 11.5 mm drill bit = 66 mm / pi x dia of coil = 66 / 3.14 x 13.3 = 1.6 turns, almost 1-1/2 turns. Anyway, the exact dia of coil & number of turns are not critical if the total length of wire in the coil is 66 mm ( i.e. 1/4 wavelength).


    The coil made of 66mm long wire, wound on 11.5 mm drill bit, by Flightaware member

    Last edited by abcd567; 2015-07-13, 02:55.

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  • Rooster
    replied
    If a 2 mm dia wire is used:
    The 14 mm dia coil should be wound over a rod/drill bit of dia = 14 mm - 2mm = 12 mm
    The 10 mm dia coil should be wound over a rod/drill bit of dia = 10 mm - 2mm = 8 mm
    This bit is confusing me... why are you subtracting the wire diameter ? Surely the specs dictate the former diameter ?

    Have you built these two to test your theory ? My analyser only goes upto VHF :-( or Id be at it.

    Leave a comment:


  • abcd567
    replied
    WIRE COLLINEAR
    I have noted some contradiction in coil dimensions in this drawing of wire collinear by G-7RGQ.

    adsb-ant-drawing.gif

    It says the coil consists of 1½ turns of 10 mm dia. The length of wire in coil therefore works out to π x d x 1.5 = 3.14 x 10 x 1.5 = 47mm. Adding 4 mm wire to provide gap between turns, the total wire length becomes 51 mm. However the drawing mentions "total wire length in coil = 65.5 mm (¼ wavelength)".

    SIMULATIONS
    I have run 2 simulations of wire collinear by G-7RGQ:
    Simulation 1: Using 10 mm dia, 1½ turns coil, as given in drawing.
    Simulation 2: Using 14 mm dia, 1½ turns coil, calculated by wire length in coil = 66 mm, i.e. ¼ λ.

    The simulation results show a marked improvement in Radiation Pattern as well as in Horizontal Gain when 14 mm dia coil (i.e. wire length = 66 mm) is used. SWR in both cases are more or less same and substantially high (i.e. SWR > 6)

    10 mm dia coil
    simulation coiled whip 10mm dia coil.PNG.png

    14 mm dia coil
    simulation coiled whip 14mm dia coil.PNG.png

    NOTE:
    Diameter of the coils in simulations is measured from center of the wire.
    Hence the inner dia of coil (i.e. the dia of rod or drill bit on which the coil is wound) = dia of coil - dia of wire.

    If a 2 mm dia wire is used:
    The 14 mm dia coil should be wound over a rod/drill bit of dia = 14 mm - 2mm = 12 mm
    The 10 mm dia coil should be wound over a rod/drill bit of dia = 10 mm - 2mm = 8 mm

    AN EASIER WAY to make the coil accurately is to measure & cut 66 mm length of wire and then wind it over a rod or drill bit. In this case exact dia & number of turns is not important as the wire length is pre-cut to 66 mm (i.e. ¼ λ). A 1/2" (12.5 mm) or 3/8" (9.5mm) drill bit can be used to wind the coil. The coils and vertical sections can then be joined by soldering/brazing/welding

    Leave a comment:

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