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    Hot to match HF loop antenna?

    HF (8.2MHz) loop antenna is resonant at 6.3 MHz (lower than required) without any capacitors across terminals. How to re-match it to make it resonant at 8.2MHz?
    Antenna is driven by a MOSFET driven by on-off digital pulses - like a switch. Antenna is connected to Drain of MOSFET and there is current-limiting resistor in MOSFET Sink - that's all the circuit.

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    Re: Hot to match HF loop antenna?

    Are you talking about resonance of the whole circuit (e.g. with MOSFET Cds) or unloaded antenna?

    Did you consider a parallel connected inductor (with sufficient high SRF) to shift the resonance upwards?


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    Re: Hot to match HF loop antenna?

    Or - depending on exact output configuration, add capacitance in series with the loop. Please show a schematic, without one we can't predict the DC and AC signal paths and how the resonance is achieved.

    Brian.
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    Re: Hot to match HF loop antenna?

    Of course FvM, Inductor in parallel! How can I forget? Thank you FvM!

    - - - Updated - - -

    Please find attached schematic.
    I see now how to tune it higher - add parallel inductor. What impedance to match it to? It is not 50 Ohm, FET will present dfferent impedance in 'on' and 'off' states.?



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    Re: Hot to match HF loop antenna?

    Those values make theoretical resonance at 7.15MHz, ignoring the capacitance of the FET.
    You can make it resonate at higher frequency by reducing the value of C1. Somewhere around 110pF would be more appropriate.

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    Re: Hot to match HF loop antenna?

    Brian,
    I guess I did not make myself clear. The L and C values for the antenna are values derived from measurements of existing antenna. I can not reduce value of C1.
    Andrey



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    Re: Hot to match HF loop antenna?

    What's the role of L2?



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    Re: Hot to match HF loop antenna?

    Quote Originally Posted by FvM View Post
    What's the role of L2?
    It is part of the circuit. I believe it is there to compensate capacitance of mosfet but can not say for sure. I tried shorting it, changed it's value. It did not result in improvement.

    - - - Updated - - -

    Also note, mosfet on the schematic is not correct part number. I do not have correct part in this library.



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    Re: Hot to match HF loop antenna?

    If the objective is to maximise the current in the inductor (the radiating part) at 8.2MHz, from the values given either reduce the value of R1 to about 240R or, counter intuitively increase the value of L2, somewhere around 6.8uH should do it.
    The circuit is a little like a resonant power converter circuit. What happens when the 'receiver' is close to the antenna, it may be that in the intended applcation that the circuit is off resonance until power is coupled out of the circuit.



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    Re: Hot to match HF loop antenna?

    Without knowing the actual MOSFET type and e.g. its output capacitance, it's impossible to estimate the circuit behavior. L2 can be part of a matching network, but it needs a shunt capacitor at the MOSFET output to achieve an up transformation. There's also a risk of MOSFET breakdown when feeding an inductive load.



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    Re: Hot to match HF loop antenna?

    Actually the resonance of the antenna (3.3uH in parallel with 150pF) is 7.1MHz, and not 6.3MHz.
    Adding a parallel 10uH inductor to the terminals of the antenna it will bring the resonance to 8.2MHz.
    In this situation, don't expect the antenna will radiate the same as if is resonating itself on 8.2MHz, without adding any external components.


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    Re: Hot to match HF loop antenna?

    vfone, this is correct - antenna by itself resonates at 7.1MHz, when transmitter connected - at 6.3MHz.
    Why after matching with parallel inductor antenna will not radiate the same - because of losses in matching inductor?



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    Re: Hot to match HF loop antenna?

    Best way to use such a circuit would be to tune it such that you get zero voltage switching at the FET. If the self resonant frequency of the coil is already too low, then adding a parallel inductance would be the proper way to tune it..



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    Re: Hot to match HF loop antenna?

    This is valid for any antenna type. A resonant antenna (on a desired frequency) works better (or much better) than a non-resonant antenna with a matching network used to bring the input impedance to other value.
    Loop antennas anyway have poor efficiency, and on top of this, if they are not resonating on the desired frequency make things even worse.



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