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Thread: best antenna

  1. #2391
    Captain abcd567's Avatar
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    Quote 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 at 02:55.

  2. #2392
    Purser Rooster's Avatar
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    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.

  3. #2393
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    Quote 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 at 21:21.

  4. #2394
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    The Successful "Franklin Spider" by forum member caius.

    Wire used:
    From electric cable, 2.5 sq mm (1.8mm dia).

    Quote Originally Posted by caius
    I'm still using a franklin spider, as I found it gave better results than the normal one. I'm averaging 6900 aircraft and over 800000 positions per day from an antenna in the loft, so I'm quite happy with it.

    My experimenting is on hold at the moment as I'm traveling for a while. When I get back I'll be putting it outside.
    Last edited by abcd567; 2015-07-14 at 14:55.

  5. #2395
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    This is my AirNav antenna.Is it a spider or is it a simple collinear with ground plane and does anyone one know the retailer for this one.Apparently it's a 3 in 1 antenna(119.1 + 1090 + 1575 Mhz).

    ex-NZAL1

  6. #2396
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    119mHz I can assume is voice comms, but what happens on 1575mHz ?

  7. #2397
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    Quote Originally Posted by HermanZA View Post
    119mHz I can assume is voice comms, but what happens on 1575mHz ?
    GPS L1 1575.42 MHz.
    ex-NZAL1

  8. #2398
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    Franklin Spider Failure Reason - Impedance Mismatch


    The Spider has an impedance close to ideal 75+j0 ohms. The SWR is about 1.3

    The Franklin Spider has an impedance 176+j0.14, far from ideal 75+j0 ohms. The SWR is about 2.35

    As a result Franklin Spider gave inferior performance than Spider.

    I now attempted a CRUDE impedance match (many approximations in calculations and factors & use of capacitor 10% lower than required) and still got a dramatic improvement. Please see data graphs below:


    .


    And this is how I achieved it:

    Step 1 of 3: Starting point is antenna impedance by simulation


    Step 2 of 3: Calculation of length of transmission line piece & value of series capacitor by Smith Chart.
    Length of Transmission Line between Antenna & Capacitor (including length of Connectors) = 210 mm RG6, VF = 0.84 (assumed)
    Capacitor = 2.2 pF.
    I did not have a 2.2 pF capacitor, but had many 1 pF capacitors, so I put 2 capacitors of 1 pF in parallel, which gave me 2 pF, 10% less than required 2.2 pF. I will now purchase a 2 - 5 pF trimmer (adjustable capacitor) and replaced ceramic ones by trimmer and try to fine tune the response to optimum.


    Step 3 of 3: Built & put on trial run


    Later Addition: Later tweaking showed that using a 3 pF Capacitor (instead of 2 pF Capacitor) and 240 mm Transmission Line (instead of 210 mm transmission line) improved performance, and Franklin Spider surpassed Spider.

    Last edited by abcd567; 2015-07-20 at 05:20.

  9. #2399
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    I have noted something wrong in the graph of collected data I posted above with impedance matching. When I changed ¼ wavelength whip with Franklin whip (i.e. converted Spider to Franklin Spider), all the graphs became nearly zero. I have previously swaped the Spider & Franklin whips several times, and each time the graphs dropped to about 40%, but not almost zero.

    To clear the matter, tomorrow I will repeat the experiment about the same time and post graphs.

    Inspite of above abnormal behaviour, the jump back of Franklin Spider graph to level of Spider after addition of impedance matching, clearly prove the effectiveness & success of impedance matching method I have used.

  10. #2400
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    In the above trial run of matching network for Franklin Spider, I had to make some assumptions & aproximations. As a result, the matching is aproximate, not optimum. I am now planning to optimize matching network by trial & error.

    (a) I will replace fixed ceramic capacitor by adjustable (2pF to 5 pF) trimmer capacitor.

    (b) I will make & try two more pieces of transmission line, one 5mm less (i.e.205 mm total length including connectors at both ends) other 5mm more (i.e. 215mm total length including connectors at both ends).

    I hope I will be able to get still better results than the crude matching I have done now.
    Last edited by abcd567; 2015-07-18 at 13:26.

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