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Re: [loopantennas] Re: Tranmitting Loop for 6m & 10m

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  • W2XJ
    That would work and it would fit the capacitor you have but you could go larger. Your numbers are off in a couple of places. The conversion from English to
    Message 1 of 21 , Jul 31, 2007
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      That would work and it would fit the capacitor you have but you could go
      larger. Your numbers are off in a couple of places. The conversion from
      English to metric are wrong it is not a division by 4 but 3.3. Also, you
      can not assume the velocity to be .66. It think that is way too much for
      this exercise.




      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 for coupling the RF feed.
      > Is there any usage of Balun and tuner should be considered?
      >
      >>From my calculation, with the velocity factor of 0.66, for 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.
      >
      >
    • 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 2 of 21 , Jul 31, 2007
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        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 3 of 21 , Aug 1, 2007
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          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
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        • 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 4 of 21 , Aug 1, 2007
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            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 5 of 21 , Aug 1, 2007
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              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 6 of 21 , Aug 1, 2007
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                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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