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LTSpice AC simulation

Discussion in 'General Electronics Discussion' started by Richard D, May 18, 2017.

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  1. Richard D

    Richard D

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    May 18, 2017
    I have an active antenna project which I would like to simulate using an AC source whose impedance changes across the frequency the amplifier operates over. I can see no way to do this, and would appreciate some help.
    The first attached file shows the impedance graphically to give an idea of the range of Z, but I can produce a csv file of the data if that's helpful.
    The second file shows the part of the circuit with the V2 AC source I would like to run with the Z data.
    active hf ant Z.jpg active hf antenna cct.jpg
    Any ideas would be most useful.
     
  2. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Hello Richard
    I don't know a way of doing this in LT-Spice but others on here may have an answer. Could you not just put an RLC network on the output of the AC source to mimic what you want? Just a thought.
    Thanks
    Adam
     
  3. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Just did a sim on a 2pF cap and it's reactance is 2.7 K @ 29 MHz and 19.8 K @ 4 MHz, quite close to what you want. Not sure if this helps?
    Thanks
    Adam
     
  4. Richard D

    Richard D

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    May 18, 2017
    Hi Adam, Thanks for your reply. Yes, a network after the source would do the job. The capacitance is very close to 2.5pF across the complete frequency range. The difficulty is the changing resistance ('real' value of the impedance) across the frequency range. This is the bit of the network I cannot figure out how to place in my circuit for simulation.
    I think this changing resistance value is due to skin effect of the conductors I used to simulate the antenna. It consists of a 1m dipole (0.5m each leg) of a 2mm radius stainless steel rod (used in a harsh environment so can't use copper or aluminium due to corrosion effects). The resistance is 0.29972 ohms at 3MHz and 3.092 at 30MHz, almost linear 10:1 ratio in both ohms and frequency over this range. What component acts like that?

    Richard.
     
  5. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Hi Richard. How are you measuring the resistance of the AC source you are using in real life? I am not sure that it's skin effect. Can you draw a block diagram showing the system blocks including the antenna and where you are taking the measurements. Some pictures would be good also.

    Thanks
    Adam
     
  6. Richard D

    Richard D

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    May 18, 2017
    Hi Adam, I modelled the short dipole in the 4nec2 program, and the result was the impedance graph as in the initial post. I have not actually constructed one for real life measuring.
    Today I changed the values of R5 and R10 to match the modelled values at both 3 and 30MHz (each resistor half the 0.3 and 3 ohms respectively), and when I re-ran the LTSpice simulation I could not see any difference in the overall gain of the circuit, so it seems that it will not be affected by the R component of the source impedance anyway.
    So thanks for all your input.
    Regards, Richard.
     
  7. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Ok no probs. Cheers
    Adam
     
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