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

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  • Andrew Ikin
    Phil, All ALA1530 loops built before 2010 would have a similar VHF responce as mentioned in your post. The reason for the inclusion of the VHF filter was due
    Message 1 of 4 , May 16 2:17 PM
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      Phil,

       

      All ALA1530 loops built before 2010 would have a similar VHF responce as mentioned in your post.

       

      The reason for the inclusion of the VHF filter was due to some North American users being concerned that the loop had too much gain in the FM band. The only satisfactory way I could reduce the VHF gain without degrading the upper HF performance was to move the loop/amplifier resonance from approx. 40MHz down to 14MHz by loading the loop with some capacitance. This also had the added benefit of increasing the amplifier 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
    • 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 2 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 3 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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