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958Re: Transmitting loop

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  • dldorrance
    Aug 1, 2005
      --- In loopantennas@yahoogroups.com, "Danny" <dannibou@y...> wrote:
      > --- In loopantennas@yahoogroups.com, <vze3v8dt@v...> wrote:
      > > Not enough info given to solve that problem. Tell us more about
      > your configuration. Important info is physical dimensions of the
      > loop, diameter of the loop conductor, frequency. The two things
      that
      > would likely limit your ability to transmit would be the trimmer
      > capacitor that you have needs to be rated for the fairly high
      voltage
      > even at low power and the conductor size of the main loop needs to
      be
      > very low loss (large diameter) for Tx in order to achieve reasonable
      > efficiency.
      >
      > Hi
      > It's a 4 feet loop wired with # 14 insulated wire 7 turns spaced 1/2
      > inch. Without capacitor, it resonate at 1.8mhz. My transmitter has
      > only 100mw and I have a 1 watt booster. My aim is to transmit about
      > 200 feet away, no more. May be I'll not need to use the booster. I
      > have a variable capacitor which I salvaged from an old 3 watt CB. I
      > just don't want to destroy my transmitter output transistor.
      > Thanks
      > Danny

      Ok this setup might work. Be advised that you have big time losses in
      this loop, possibly on the order of 20 dB. For receiving this would
      only be noticeable on the margin, where you would never hear very weak
      signals. Since you wouldn't hear them you would have no idea that
      your antenna might not be very efficient.

      There is one other consideraton, however, which could destroy your 1
      watt amplifier. Keep in mind that receivers are much more forgiving
      than transmitters regarding mismatches . Chances are overwhelming
      that the SWR at the transmitter would be very high. This reflected
      power will heat up your final. When a transistor gets too hot it will
      be destroyed. THis can happen in a millisecond.

      I have read on this list advise to retune receive-only loops every 500
      kHz. Well for a transmitter you must tune possibly every 10 kHz to
      maintain an adequate match for the transmitter to prevent fireworks.

      Do you have an antenna analyzer so that you can pre-tune your loop to
      an SWR of maybe 2:1 or less?

      Dave WA6YSO
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