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Re: Tranmitting Loop for 6m & 10m

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  • Len Warner
    ... You do not say what the winding pitch is: my bet is that your loop is close wound, in which case the loop s self- resonance will be at around 2Mhz. (It
    Message 1 of 21 , Jul 31, 2007
      At 7:40 am ((PDT)) Mon Jul 30, 2007, in Digest 881, Richards wrote:

      >[snip] For my experiment, I tuned weak and moderate signals
      >in the 31 m short wave band (around 9.4 MHz).
      >
      >What seemed to work as well as anything, was a mere 16 seat of wire
      >looped twice around a 2 foot by 2 foot square PVC pipe frame I built for
      >another purpose. Using coax cable feed line, it did not seem to matter
      >whether I connected just one end of the wire loop, or both ends - one
      >end to the coax cable center conductor, and the other to the shield.
      >I don't know why it did not matter how I connected it, but it
      >seemed to work the same either way.

      You do not say what the winding pitch is: my bet is that
      your loop is close wound, in which case the loop's self-
      resonance will be at around 2Mhz. (It depends somewhat
      on wire diameter, which you also didn't care to tell us.)

      You would benefit from RJELOOP3.EXE by Reg Edwards.

      This is designed for square receiving loops and will tell you
      the inductance, approximate self-resonance frequency,
      capacitor for resonance (if used) and other interesting
      parameters.

      By using RJELOOP3 you will discover that you need to
      increase the winding pitch to 2 wire diameters or more
      in order to raise the self-resonance above your frequency
      of interest.

      If you are planning to tune the loop I would suggest a bigger
      spacing, since self-capacitance is worse for Q than a good-
      quality external C and will also limit the tuning range attainable.

      >[snip]
      >I wonder whether tuning capacitor would help adjust the wire to various
      >frequencies above

      Not practical: the loop is already capacitive above its
      parallel resonance.

      >or below its resonant frequency?

      Yes, but first you need to raise its self-resonance above
      your frequency of interest.

      >I wonder whether I could locate the tuning capacitor in
      >the shack, rather than out at the loop in the backyard?

      Yes, if you don't mind adding the resistance and capacitance
      of your feeder to the loop resonant circuit (which you should).

      >I am, based on the discussion here, presuming that my
      >16 foot long wire in a

      How does

      >figure-8 double loop

      relate to

      >(i.e. going twice around my 2 feet per side pvc frame)

      ?

      Or are you trying to tell us at this late stage in the discussion
      that you wound it with figure-8 zip cord?

      >is approximately 1/10 wavelength for the 31 m (9.5 MHz)
      >broadcast band.

      300m/9.5Mhz = 104ft approx.; 16ft = 0.154 wavelength;
      a >50% error over 1/10 wavelength but, since you have
      coiled it, you have brought the low-current ends back to
      overlap the maximum-current centre, so the spacial
      distribution of current will be more uniform than a single
      turn of this wire length would have been and I'm guessing
      that means it will give you good "small loop" performance.

      >I also presume that I would have to select a different
      >length of wire, perhaps, or a different band in the spectrum. Or is
      >this mistaken, would any short loop work? Would any short length work,
      >but with a tuner?

      Any loop of wire below its self-resonant frequency behaves
      just as you should expect a loop of wire to behave: it has an
      area which determines how it interacts with the spatial RF field
      and an inductance and capacitance which determines how
      you can couple it to the rest of your circuit.

      When the length is greater the spatial arrangement and
      possible self-resonant modes take over, and you will need
      either experiment or an antenna-modelling package to
      discover what its impedance and radiation pattern are,
      which is one reason for sticking with published designs
      so you can benefit from someone else's experience.



      Regards, LenW
    • Len Warner
      ... Only if it is to be a small receiving loop: it is marginal as a small transmitting loop; and there is no restriction on (approx) multiple
      Message 2 of 21 , Aug 1, 2007
        At 1:21 pm ((PDT)) Tue Jul 31, 2007, in Digest 882, W2XJ wrote:

        >I agree that the loop is too large. The circumference needs to be less
        >than 1/4 wave.

        Only if it is to be a "small" receiving loop: it is marginal as
        a "small" transmitting loop; and there is no restriction on
        (approx) multiple half-wavelengths being used in a large loop.

        For example, see "The Loop Skywire" and the following
        "7-MHz Loop" in ARRL Antenna Book chapter 5, both of
        which have multiple modes above their fundamental (and
        the Skywire as a top-hat loaded vertical too).

        >With 47 cm diameter, you are right at 1/4 wave length
        >before you consider velocity and loading.

        Both MLOOP31 and RJELOOP1 say it should tune 50MHz
        and work with good efficiency with a 25mm conductor...

        >If you go to a diameter
        >slightly less than 40 CM, it should tune on 6 M and be about 60 %
        >efficient on 10 M if you use around 1 inch (25mm) tubing. Also, your
        >capacitor is to large. You need about 26 pf for 10 m and half that on 6.

        ... with about 8pF or 9pF respectively, though we know
        this value must be marginal for Dexter's variable capacitor.

        (RJELOOP1 says only 4.4pF with a 1mm conductor.)

        For 10MHz, RJELOOP1 predicts 29.3pF.

        > With a capacitor 10 times too large, tuning at the bottom of the range
        >could be difficult. What kind of match are you using - gamma match or
        >loop coupling? How much power do you plan to use?

        There are many specific details Dexter hasn't given us,
        (though I see there are some later in this Digest)
        probably because he doesn't understand them himself yet
        or have enough information to reach a decision.

        I suggest it is time for Dexter to do his own legwork and at least
        play with RJELOOP1 (or MLOOP31) until he gets a feel for the
        numbers and preferably also read chapter 5 of ARRL Antenna Book
        to get a handle on the theory and some specific designs.

        And would contributors please _not_ quote most of the history
        of this thread into their messages - especially top-posted into
        reverse or higgledy-piggledy order. I can't easily follow much
        of the quote in your message and I see I wrote most of it!

        Quotes should give _context_ - and they need to be come before
        the reply to do that - not _history_ - we have an Archive for that.


        Regards, LenW
        --
        If you are sending a reply to a message... be sure you summarize the
        original at the top of the message, or include just enough text of
        the original to give a context. This will make sure readers under-
        stand when they start to read your response.... Giving context helps
        everyone. But do not include the entire original! (rfc1855)
      • Len Warner
        ... Faraday: Michael Faraday, an English natural philosopher http://en.wikipedia.org/wiki/Michael_Faraday ... A balun provides a BAlanced to UNbalanced (or an
        Message 3 of 21 , Aug 1, 2007
          At 9:05 pm ((PDT)) Tue Jul 31, 2007, in Digest 882, de_9w2dtr wrote:

          >--- In loopantennas@yahoogroups.com, W2XJ <w2xj@...> wrote:
          > >
          > >
          > > I agree that the loop is too large. The circumference needs to be less
          > > than 1/4 wave. With 47 cm diameter, you are right at 1/4 wave length
          > > before you consider velocity and loading. If you go to a diameter
          > > slightly less than 40 CM, it should tune on 6 M and be about 60 %
          > > efficient on 10 M if you use around 1 inch (25mm) tubing. Also, your
          > > capacitor is to large. You need about 26 pf for 10 m and half that
          >on 6.
          > > With a capacitor 10 times too large, tuning at the bottom of the
          >range
          > > could be difficult. What kind of match are you using - gamma match or
          > > loop coupling? How much power do you plan to use?
          > >
          >
          >I am using a Farady Loop

          Faraday: Michael Faraday, an English natural philosopher
          http://en.wikipedia.org/wiki/Michael_Faraday

          >for coupling the RF feed.
          >Is there any usage of Balun and tuner should be considered?

          A balun provides a BAlanced to UNbalanced (or an
          unbalanced to balanced) transformation.

          You only need one if your antenna coupling is balanced
          and your feedre is unbalanced; or your antenna coupling
          is unbalanced and your feeder is balanced.

          > From my calculation, with the velocity factor of 0.66,

          Where did you get that from? The velocity factor is one
          in free space - are you operating in some sticky liquid?

          >or 54Mhz
          >frequency. The loop diameter for 1/4w will be about 29cm. Is this
          >correct? I use this formula, 300/54=5.55, 5.55/4=1.388,
          >1.388*0.66=0.916, 0.916/pi=29.1cm.
          >
          >I am using a 12mm microwave coax cable for the loop.

          If you are calculating the loop dimensions using the
          velocity factor of your coax, remember that the loop is
          radiating from the outside of the conductor and this
          has nothing to do with what is happening inside the
          coax (otherwise, how would coax feeders work?)


          Regards, LenW
          --
          From Yahoo! Groups Help: ... trim all the irrelevant quoted text
          out of your message (as a courtesy to the other members of
          the group to make the digest easier to read).
        • Richards
          Brief Off topic comment -- Feels lit it some days... I liked your sticky liquid question, ... but mostly because I feel that is what I am operating in most
          Message 4 of 21 , Aug 1, 2007
            Brief Off topic comment -- Feels lit it some days...

            I liked your "sticky liquid" question, ... but mostly
            because I feel that is what I am operating in most of
            the time... slogging along... slowly... inexorably...

            Life is highly viscous.

            Happy trails. /// Richards ///
            ==============================================================

            Len Warner wrote:
            - are you operating in some sticky liquid?
            >
            ==============================================================
          • de_9w2dtr
            I live in an apartment 13th floor above the ground. I plan to install the loop antenna on the balcony. Although I can install a vertical antenna, I think it is
            Message 5 of 21 , Aug 1, 2007
              I live in an apartment 13th floor above the ground. I plan to install
              the loop antenna on the balcony. Although I can install a vertical
              antenna, I think it is wiser to go loop due to it's size.
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