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50MHz reception using a modified Wellbrook 1530 Loop

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  • Philip
    Hello all, I have been a very keen VHF DX chaser for over 20 years, until a house move in 2009 stopped me having beam antennas. When I ordered my Wellbrook
    Message 1 of 4 , May 16, 2013
      Hello all,

      I have been a very keen VHF DX chaser for over 20 years, until a house move in
      2009 stopped me having beam antennas.

      When I ordered my Wellbrook ALA-1530 magnetic loop antenna I had enquired first
      about the 1530+ model which appeared to cover up to 100 MHz.

      I found the 1530+ was no longer in production, but that it was possible for me
      to order a 1530 and choose not to have the VHF attenuators fitted, with a slight
      loss in HF performance on 18 & 21 MHz.

      I was very pleased with my Wellbrook 1530 loop on HF over the past few months
      and now in May 2013 I have been able to compare it with my other antennas on the
      50 MHz band. The results have been quite surprising!

      My modified Wellbrook 1530 loop had consistantly outperformed both my 50 MHz
      vertical antenna and my 280' horizontal dipole when listening to 50 MHz signals
      via Sporadic-E. Signals are very much stronger on my loop from all over Europe
      and sometimes I cannot hear them at all on my other antennas which I have used
      for 50 MHz reception in recent years.

      I am going to experiment with the loop for reception of Band 1 TV carriers via
      Meteor Scatter next.

      My loop is at ground level too and no longer on a small mast.

      I just wanted to share my results with you in case it is of interest.

      73 de Philip G0ISW

      http://www.qsl.net/g0isw
    • 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 2 of 4 , May 16, 2013

        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 3 of 4 , May 16, 2013
          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 4 of 4 , May 17, 2013
            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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