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Re: Wideband Active Loop Amplifier

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  • vbifyz
    I tried a similar design of my own. Linearity wasn t good, maybe because I only had about 5ma current through each transistor. I could hear products from
    Message 1 of 36 , Jul 20, 2010
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      I tried a similar design of my own. Linearity wasn't good, maybe because I only had about 5ma current through each transistor. I could hear products from nearby AM band stations up to 7-10MHz. It was a quick prototype without proper shielding, so it also suffered from instability because of the feedback from the output transformer (on a toroid, not a binocular core) back to the loop.
      Nulling wasn't good because of the common mode electrical component pickup.
      I guess when properly done, the design can work acceptably.

      Mike

      --- In loopantennas@yahoogroups.com, Chris Trask <christrask@...> wrote:
      >
      > I would also disconnect the two emitters, have a separate biasing resistor from each emitter to ground, and then a suitable capacitor between the two emitters. Then the two capacitors from the collectors to the output transformer primary winding can be eliminated. The 1:1 UnUn output transformer could be replaced with a 1:1:1 BalUn, the centre tap of the primary connected to the supply line. This would increase the P1dB a bit plus raise the gain by an additional 6dB.
      >
      > Chris
      >
    • Daniele
      Hello folks, I m new to this group so please let me briefly introduce myself. I m Daniele Tincani from Livorno, Italy. I m a SWL (with special interest for LF,
      Message 36 of 36 , Jun 12, 2011
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        Hello folks,

        I'm new to this group so please let me briefly introduce myself. I'm Daniele Tincani from Livorno, Italy. I'm a SWL (with special interest for LF, MF and low HF) and I like to homebrew simple projects like the one discussed in the attached messages.
        I would like to know if someone tried in practice to apply modifications suggested by Chris to the amplfier circuit proposed by John 9V1VV. In particular I would be interested in how to build the input BalBal and the output 1:1:1 BalUn and how connect the latter into the circuit.
        Best regards
        Daniele

        --- In loopantennas@yahoogroups.com, Chris Trask <christrask@...> wrote:
        >
        > >
        > >I would appreciate any comments you may have on this design;
        > >
        > >http://xephemera.blogspot.com/2009/08/16-30-mhz-broadband-low-noise-loop.html
        > >
        > >It is not an original - the circuit has been around for years.
        > >
        > >In my noisy urban and tropical environment I can detect no difference
        > >when switching to my ALA 100L-1 amplifier.
        > >
        > >Some Finnish hams reported that the design was pretty good in quiet
        > >environments compared with the Wellbrook.
        > >
        > >I now use both amps on separate loops orientated 90 degs and switch
        > >between the two.
        > >
        >
        > I've seen a number of amplifiers similar to that. The first one that comes to mind is that of the British fellow who has an extensive online presentation of a large loop antenna. Can't recall his name or the URL at the moment, but I'm certain that there are list members who will readily recognize who I'm refering to.
        >
        > It would be interesting to compare the radiation patterns that result between high input impedance and low input impedance amplifiers used for wideband active loop antenna applications. I suspect that the former approach would deteriorate the null.
        >
        > One item I find missing from this design is an input BalBal transformer. Such a transformer would serve a number of uses, in particular providing a degree of immunity from common-mode interference such as lightning strikes. It would also provide a ground reference for the antenna proper, and in addition would allow the two transistors to have an identical base voltage. The limiting diodes would be placed across the primary side of the transformer.
        >
        > I would also disconnect the two emitters, have a separate biasing resistor from each emitter to ground, and then a suitable capacitor between the two emitters. Then the two capacitors from the collectors to the output transformer primary winding can be eliminated. The 1:1 UnUn output transformer could be replaced with a 1:1:1 BalUn, the centre tap of the primary connected to the supply line. This would increase the P1dB a bit plus raise the gain by an additional 6dB.
        >
        > Chris
        >
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