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Thread: best antenna

  1. #1771
    Captain abcd567's Avatar
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    Yesterday's quest for making an antenna tuner for 1090 MHz, and search for smallest sized Variable & fixed Capacitors & Inductors resulted in purchase of items in the photo below. All for $10

    Smallest VARIABLE Capacitor (Trimmer) found is 2.5 - 7.5 pF (top left of photo). Smallest inductor found is 0.1 uH (bottom left of photo written .1uH). Fixed ceramic capacitor smallest 1.8pF. All are much higher than required 0.3-1.5pF & 0.025 uH.


  2. #1772
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    and the other challenge...
    at these frequencies (1090 MHz) the lead length and other physical characteristics of variable cap will likely introduce
    inductance components ... thus not sure how true to the labelled capacitance they would be at 1090 ...

  3. #1773
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    Quote Originally Posted by trigger View Post
    Nice try ab cd. I await your next brain wave.
    I have not given up yet. Another brain wave. I can achieve lower capacitance by putting several small capacitances in series.

    If two capacitors C1 & C2 are put in series, the resultant Capacitance C is given by formula:

    1/C = 1/C1 + 1/C2 = (C1 + C2 )/C1 x C2
    or
    C = C1 x C2 / (C1 + C2)

    if I put a fixed capacitors of 2pF and a trimmer of 2.5pF minimum value, I get
    Minimum Value of trimmer = 2 x 2.5 / (2 + 2.5) = 1.11 pF



    If three capacitors C1, C2 & C3 are put in series, the resultant Capacitance C is given by formula:

    1/C = 1/C1 + 1/C2 + 1/C3 = (C1 x C2 + C2 x C3 + C3 x C1)/C1 x C2 x C3
    or
    C = C1 x C2 xC3 / (C1 x C2 + C2 x C3 + C3 x C1)

    if I put 2 fixed capacitors of 2pF and a trimmer of 2.5pF minimum value, I get
    Minimum Value of trimmer = 2 x 2 x 2.5 / (2x2 + 2 x 2.5 + 2.5 x 2) = 0.72 pF

    If I add 3 capacitors of 2 pF inseries with a 2.5 minimum value trimmer..... well it will be sure lesser ...between 0.5 & 0.6 ... don't want to calculate further, already have a headache
    Last edited by abcd567; 2014-10-19 at 18:29.

  4. #1774
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    16.6nH coil
    air cored, 4mm diameter (internal), 2 turns, spaced about 2mm overall length
    (inductance can be tuned by squezzing or expanding the coil length)

  5. #1775
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    sorry, read your chart wrong
    17.9nH
    air cored, 4.5mm diamter (coil internal dia). 2 turns, length 2.45mm ( note its quite sensitive to the length)

  6. #1776
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    Quote Originally Posted by gregy View Post
    sorry, read your chart wrong
    17.9nH
    air cored, 4.5mm diamter (coil internal dia). 2 turns, length 2.45mm ( note its quite sensitive to the length)
    Thanks.

    Yes, you are right. The coil inductance is very sensitive to length & dia of coil. That is why I wanted to avoid using it for DIY situations, as the average DIY enthusiast's skill level is not very high.

    I have already worked out coil dimensions couple of days ago, using a website calculator. Thanks for help anyway.
    Coil wound over 1/4 inch (6.4 mm) dia drill bit, 2 turns, coil length 9mm gives an inductance of 17.84 nH.

    Website address: http://wcalc.sourceforge.net/cgi-bin/air_coil.cgi



  7. #1777
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    Quote Originally Posted by gregy View Post
    16.6nH coil
    air cored, 4mm diameter (internal), 2 turns, spaced about 2mm overall length
    (inductance can be tuned by squezzing or expanding the coil length)
    Yes, you are right. The coil inductance changes by changing length. Expanding reduces inductance, squeezing increases it.

    This phenomenon was used in the old fashioned non-digital AM/FM Radios to adjust the front end antenna & oscillator circuits.

    The FM dial's frequency adjustment was achieved during final stage of manufacture by expanding/compressing the coils.


  8. #1778
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    Quote Originally Posted by gregy View Post
    and the other challenge...
    at these frequencies (1090 MHz) the lead length and other physical characteristics of variable cap will likely introduce
    inductance components ... thus not sure how true to the labelled capacitance they would be at 1090 ...
    You are absolutely right.

    As a starting point, we have to ignore lead indutances & stray capacitances, and assume the labelled value only, then adjust the trimmers & coil to get best results, using software "rtl1090" 's air data indicator lights at bottom left to judge the output. For those lucky ones having access to a VNA, adjustment can be much easier & accurate.

  9. #1779
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    I have little hope that antenna impedance matching network (T, Pi, L, Series, or Parallel compensation) with adjustable controls, will be successful in DIY situations due to extremely low values of Capacitance & inductance required for matching network.

    Variable Capacitors for very low values are not available. Variable inductor can be made by winding an air core coil, but due to very few turns, accurate adjustments will be very difficult by expanding/squeezing the coil.

    Impedance matching network may succeed with those lucky ones who have access to a VNA (Vector Network Analyzer).
    Last edited by abcd567; 2014-10-20 at 21:05.

  10. #1780
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    There are GHz Active Inductors & Active Capacitors using CMOS and MESFET negative resistance. Their values can be varied by applied voltage/current by a potentiometer (like volume control knobs in radios & audio players). Maybe we can use these for impedance matching of CoCo.

    At this stage, this is just an idea, needs detailed study to find out feasibility & practicability.

    (a) 1 GHz CMOS Band-pass Filter Design Using an Active Inductor and Capacitor

    (b) Active Inductor and Capacitor for DCS receiver band (1.71GHz – 1.785GHz) using multi-MESFET Negative Resistance Circuit

    (c) A New Active Inductor and Its Application to Wide Tuning Range LC Oscillator

    (d) CMOS Active Inductors and Transformers: Principle, Implementation, and Applications - by Fei Yuan
    Last edited by abcd567; 2014-10-21 at 17:25.

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