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Re: [loopantennas] Re: Stupid antenna tricks

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  • Ray Phillips
    Why not try a skirting board loop antenna? In your largest room, run one or more turns of multi-stranded, insulated, copper wire (speaker cable should be OK)
    Message 1 of 11 , Jan 1, 2008
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      Why not try a skirting board loop antenna?

      In your largest room, run one or more turns of multi-stranded, insulated,
      copper wire (speaker cable should be OK) around the outside of the room up
      against the skirting board. Or one or more turns around the outside of
      the room under the carpet. Because a loop's capture ability is a function
      of its area, you get the maximum indoors catch area, in the warm, in an
      inconspicuous fashion. A 4 foot loop resonant at about 4 MHz will have a
      gain in the region of about -15 dB. A skirting board loop resonant at the
      same frequency can have a gain approaching 4 dB. Don't take my figures
      for gospel but there is a large difference. If any of the technophiles
      on the forum want to post the definitive comparison, I welcome them (I'd
      like to know but am too lazy to run the experiments).

      The number of turns depends on the perimeter of your room relative to your
      frequency of interest plus a number of other factors such as the method of
      construction - eg is there iron mesh in the concrete floor?, position of
      electrical wiring, plumbing etc. A single turn loop in a room 24 feet by
      20 feet in a wooden building should resonate somewhere around 8-9Mhz (This
      is a guess - anyone confirm or deny?). It is harder to get that perfect
      length than using a multiple turns on a small area loop because each turn
      is just so long but the performance benefits are worth the struggle.
      Anyway you should aim for your loop to to be slightly shorter than the
      resonant length so you can tune it down to your desired frequency with a
      variable capacitor. Just connect the variable capacitor to the ends of
      the loop wire. If your length is right you will hear an increase in
      signal as you adjust the capacitor to resonate the loop at your frequency
      of interest. If the loop resonates above the FOI, lengthen
      it----resonates below, shorten it. You can, of course, use the loop
      untuned by running just one turn and leaving the capacitor out but the
      advantages of the tuned loop will be unavailable - just put in a loop with
      the largest area. There is no way to predict the actual performance of a
      skirting board loop because of the wide variability in the construction
      and location of rooms and so some experimentation is needed.

      Inside the building you expose your antenna to more electrical
      interference. Self-adhesive aluminium foil tape under and on top of the
      loop will go a long way to provide some electrostatic rejection. This is
      because the aluminium is highly conductive to electricity but has poor
      magnetic abilities which allows the magnetic part of the radio signal to
      get to the magnetically sensitive loop. Don't make the tape continuous
      around the whole loop as it will become a shorted turn and suck all your
      signals away - leave a gap of a few inches (at the tuning capacitor is
      good).
      The tape will alter the resonant frequency of the loop - experiment.

      Now you've got to couple the loop to the radio. If your loop is tuned
      don't connect the ends of the loop directly to the radio because a tuned
      loop is a high impedance device. The input to most radios is a low
      impedance and this low impedance will load up the tuned loop and severely
      impair its performance. Wind a few turns of thin, insulated wire on a
      ferrite rod and connect the ends to a suitable plug using coax cable. Lay
      the rod across the loop.
      If your loop is untuned, connect the ends to the coax or use a balun
      between the loop and the coax. I don't know what ratio balun to recommend
      because of the variability in room sizes and FOI but a 4:1 is a good place
      to start.

      Before committing to lifting your carpet or other serious activities, just
      place the wire near as near as possible to its final position and do your
      experiments/adjustments. Once you've maximised its performance, put it
      into its final position, the performance won't change much.

      Cost: speaker cable 40c/metre; ferrite rod <$5; plug $1.50; coax
      $1.10/metre; aluminium tape - don't know - never bothered; coupler wire
      (free - pull an old transformer apart).










      > Unfortunately an outdoor resonant antenna for anything other than, oh,
      > 11 meters, is out of the question for me. I'm in an apartment and
      > while there's a large fenced in backyard where I could put up a
      > stealth antenna, it's on the wrong side of the building from my apartment.
      >
    • cank13cat
      I think my copy of Carr s Receiving Antenna Handbook has a very similar scheme. I ve been trying to avoid using a tuner, for the sake of ease in
      Message 2 of 11 , Jan 1, 2008
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        I think my copy of Carr's "Receiving Antenna Handbook" has a very
        similar scheme.

        I've been trying to avoid using a tuner, for the sake of ease in
        bandscanning, but I'm not sure that's practical. What I might end up
        doing is attaching a random wire and tuned loop to the receiver via a
        switchbox. Use the random wire just to check for signals in the clear
        and switch to the tuned/tunable loop to improve reception, reduce
        noise, etc.

        As a photographer I've accumulated way too many tripods. I'll
        probably dedicated one of 'em to a loop stand. The nearly infinitely
        adjustable angles should make it handy for reorienting a loop to
        minimize local noise.

        --- In loopantennas@yahoogroups.com, "Ray Phillips" <ray@...> wrote:
        >
        > Why not try a skirting board loop antenna?
        >
        > In your largest room, run one or more turns of multi-stranded,
        insulated,
        > copper wire (speaker cable should be OK) around the outside of the
        room up
        > against the skirting board. Or one or more turns around the outside of
        > the room under the carpet. Because a loop's capture ability is a
        function
        > of its area, you get the maximum indoors catch area, in the warm, in an
        > inconspicuous fashion. A 4 foot loop resonant at about 4 MHz will
        have a
        > gain in the region of about -15 dB. A skirting board loop resonant
        at the
        > same frequency can have a gain approaching 4 dB...
        > (snip)
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