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Re: [loopantennas] 50MHz reception using a modified Wellbrook 1530 Loop

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  • Richard (Rick) Karlquist
    ... Are you saying that the amplifier input impedance is a positive reactance that tracks the positive reactance of the loop? Or is it possibly a resistive
    Message 1 of 4 , May 16 11:16 PM
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      On 5/16/2013 2:17 PM, Andrew Ikin wrote:
      >
      > input z above 14MHz and thus improve the loop/amplifier matching, to
      > give a 5dB gain increase. Hence, the amplifier would provide a
      > reasonable loop impedance track over a 1:20 ratio. Previously this had
      > been approx. 1:12 ratio. The loop and amplifier input z is about 10-15
      > Ohms at 1MHz. With the present amplifier design, the transistor Miller
      > capacitance limits the HF gain and input z. One option would be to use a
      > cascode amplifier, this can provide 1:30 impedance tracking ratio. At
      > the moment it is too early to say whether this option is worthwhile.
      > However it may improve the lower VHF gain with loop loading caps removed!!!
      >
      > 73
      >
      > Andrew Ikin

      Are you saying that the amplifier input impedance is a positive
      reactance that tracks the positive reactance of the loop? Or
      is it possibly a resistive input impedance that tracks the
      magnitude of the loop reactance? Is the amplifier connected
      directly to the loop feedpoint, or is there intervening coaxial
      line?

      73

      Rick N6RK
    • Andrew Ikin
      Rick N6RK wrote on May 17. Are you saying that the amplifier input impedance is a positive reactance that tracks the positive reactance of the loop? Or is it
      Message 2 of 4 , May 17 8:51 AM
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        Rick N6RK wrote on May 17.


        Are you saying that the amplifier input impedance is a positive
        reactance that tracks the positive reactance of the loop? Or
        is it possibly a resistive input impedance that tracks the
        magnitude of the loop reactance? Is the amplifier connected
        directly to the loop feedpoint, or is there intervening coaxial
        line?

        73

        Rick N6RK



        Rick, The amplifier input impedance is positive approx. +70 degrees over
        most design bandwidth and this tracks the loops rising positive reactance
        with frequency. The Amplifier connexion is directly to the loop.

        If one looks the A F of the ALA1530 and a very low input z amplifier of
        Wellbrook LFL1010 using basically the same amplifier. The LFL1010 has a
        nearly constant A F of 15dB from 70kHz to 30MHz, whereas the A F of the
        ALA1530 at 70kHz is 21.0dB gradually rising to 0.5dB at 25MHz
        On average the ALA1530 impedance tracking yields a 10dB increase in
        sensitivity from 500kHz to 30MHz.

        Put it another way, very low input z amps. are okay from low sensitivity
        loops for say EMC work.

        73

        Andrew


        P. S.


        A F is the Antenna Factor in dB



        Antenna Factor to receiver indication converts dBuV to dBuV/m

        Receiver indication assumes the antenna driving into a 50 Ohm system.

        For example, if the receiver indication at 300kHz was 90dBuV, the Field
        strength is 99dBuV/m. Hence, the A F would be 9dB

        The lower the A.F. the more sensitive the antenna.
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