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Re: [loopantennas] Questions for Chris Trask

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  • Chris Trask
    Had to think about this a bit. ... stage. ... Yes, you can probably get away with that. I made T3 and T6 identical so as to reduce that number of different
    Message 1 of 2 , Jun 29, 2011
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      Had to think about this a bit.

      >
      > After trying several options (gain, bandwidth, stability - pick any two
      :) ),
      > I've decided to build the amplifier described in your article
      > WidebandLoopAntennaAmplifierRevA.pdf.
      >
      > The problem is I don't have the binocular cores made of 61 material, only
      > made of 43 material. I understand this will kill the performance of T3,
      > because it is loaded with the very low input impedance of the second
      stage.
      >
      > Question 1: Can 43 material still be used for T6? Its loading impedance is
      > 50 Ohm.
      >

      Yes, you can probably get away with that. I made T3 and T6 identical so
      as to reduce that number of different parts. 43 material performs poorly in
      low-impedance circuitry so I use either 61 or 73.

      >
      > Question 2: If I omit T3 altogether and couple the stages directly (with
      > the necessary bias circuit modifications), how much gain would I lose?
      > My naive opinion is that in the end, voltage gain of the whole amplifier
      > is the ratio of 50 Ohm to the input impedance. I don't see how interstage
      > and output transformers can increase it. T6 is still nice to have for
      > balanced to unbalanced connection and for splitting/combining the power
      > and the signal.
      >

      Transformer T3 gives you a current gain factor of 2 (6dB of power gain).
      It also provides second-order IMD immunity from even-order products from the
      first amplifier stage. So, you'll lose 6dB of gain plus the OIP2 will
      suffer.

      Chris Trask
      N7ZWY / WDX3HLB
      Senior Member IEEE
      http://www.home.earthlink.net/~christrask/
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