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Re: FT857D & FT897D reducing TX power for transverter

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  • Geert Jan de Groot
    ... The 897/857 is probably not the best choice as driver for a transverter. Trying to reduce the output power via ALC, as proposed, does not work well: - Most
    Message 1 of 8 , Sep 11, 2013
      > There is usually little or no point in removing the in-line attenuator, in
      > fact doing so may make things worse rather than better.
      > Most transverters have enough gain, and a low noise figure, in receive to
      > make the loss in the attenuator insignificant, and removing it may well
      > cause the additional gain in the transverter to overload the receiver with
      > strong signals.

      The 897/857 is probably not the best choice as driver for a transverter.
      Trying to reduce the output power via ALC, as proposed, does not work well:
      - Most transceivers generate an output spike while the ALC circuitry
      kicks in
      - The output signal may not be very clean if the circuit is driven back
      this far.

      But, there may be hope. If you disconnect the coaxes between the
      main board on top and the PC board on bottom, you will find that
      you can use the green coax as input for the receiver, and the red coax
      as TX output (5-7 watts out). So, it's even TX-RX separated.
      There are a few drawbacks:
      - Neither TX or RX is protected. Forget to connect the transverter and
      you may find you just lost the driver stage (which is a bitch to replace)
      - I'm not sure whether the 8x7 uses a feedback circuit to drive output
      power. if it does, then you may need to adjust the circuits to use
      drive level, instead of PA level, for the feedback loop.

      One advantage though: Old 8x7's use a transistor in the VHF/UHF PA stage
      that has been out of production and is no longer available (not even as
      spare). It doesn't fail often, but there are radios that have a broken
      PA and cannot be repaired easily, hence available cheaply and
      and which may be used for transverters.

      (one of the changes with the 8x7D is that this PA stage is changed from
      the unobtainable transistor to an LDMOS device, requiring circuit changes;
      changing is not trivial)

      In general, I probably would use a radio with better receiver dynamic
      characteristics for VHF/UHF/SHF transverter work. Keep in mind that
      the radio is a compromise!

      Geert Jan
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