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Loop Capacitance

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  • ozzy_1kw
    If the tuning is done by the small loop for best swr then for a single band hf cb i can put the exact capacitance required or is the tuning too sharp to cover
    Message 1 of 3 , Apr 1 12:11 AM
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      If the tuning is done by the small loop for best swr then for a single band hf cb i can put the exact capacitance required or is the tuning too sharp to cover the entire cb band??

      what is the purpose for the variable capacitor ,is it multiband usage ?

      TY Rob


      PS, i see on youtube the guy that sells loops.his appear to be upside down with the tuning capacitor on the bottom that he tunes with his thumb. is there any issues feeding it from the top and running the coax down the side of the loop ?

      Cheers Rob
    • richard
      On Thu, 01 Apr 2010 07:11:43 -0000 ... Rob If you would bother to use the spreadsheet program of OH7SV, you would see that on a loop diameter roughly the same
      Message 2 of 3 , Apr 1 2:55 AM
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        On Thu, 01 Apr 2010 07:11:43 -0000
        "ozzy_1kw" <ozzy_1kw@...> wrote:

        >
        >
        > If the tuning is done by the small loop for best swr then for a single band hf cb i can put the
        > exact capacitance required or is the tuning too sharp to cover the entire cb band??
        >
        > what is the purpose for the variable capacitor ,is it multiband usage ?
        >
        > TY Rob
        >
        >
        > PS, i see on youtube the guy that sells loops.his appear to be upside down with the tuning
        > capacitor on the bottom that he tunes with his thumb. is there any issues feeding it from the top
        > and running the coax down the side of the loop ?
        >
        > Cheers Rob
        >
        Rob
        If you would bother to use the spreadsheet program of OH7SV, you would see that on a loop diameter
        roughly the same as a bike wheel, the 3dB point is about 50 KHz, whats that 4 CB channels ?

        Come on you should know that you will never learn anything if you don't try.
        Use the spreadsheet that I've now sent you twice.
        All you have to do is alter the value in the left hand column, that's frequency , length of the
        loop, the conductor diameter( how thick it is) and how much power you are going to put in it.

        Results
        Inductance (uH) 1.580
        Capacitance (pF) 21.2
        XL = XC (ohms) 273.0
        Distributed C (pF) 5.4
        Tuning capacitance (pF) 15.8
        Main loop diameter (m) 0.637
        Feeding loop diameter (m) 0.127
        Radiation resistance (ohms) 0.227
        Loss resistance (ohms) 0.067
        Parallel resistance (kohms) 127
        Efficiency (%) 77.2
        Compared to ideal loop (dB) -1.12
        Loaded Q 465
        Bandwidth -3dB (kHz) 59.2
        Capcitor rms voltage (V) 2519

        Thats what you get roughly for what you are using.

        If you can't get enough decimal places in the left hand column you can change the cell properties,
        details of how to do this are in the "help" column in EXCEL and Open office.

        If you really want to become a Ham you will have to learn some basic theory
        Google is your friend.
        If you go to your local club you will probably get given a ARRL or RSGB Handbook where someone has
        bought a new one , and doesn't want the old one.
        The process of learning is not asking questions to which you have already been given the answer.
        but being able to understand what's in front of you.









        --
        Best wishes

        Richard Bown

        ###################################################################################
        Registered Linux User 36561
        OS: Mandriva 2010.0 Powerpack on an AMD Dual Athlon 64 +4400: 8 GB RAM DDR2
        Ham Call: G8JVM , QRA IO82SP, Interests Microwave 1.3 GHz,2.3 GHz & 5.7GHz & 10 GHz
        ###################################################################################
      • Chris Harris
        All my loops have fed from the top and tune at the tune at the bottom as the cap is heavy and the feed is not. The feed loop is untuned and the line runs down
        Message 3 of 3 , Apr 1 4:50 AM
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          All my loops have fed from the top and tune at the tune at the bottom as
          the cap is heavy and the feed is not. The feed loop is untuned and the
          line runs down the centre line of the support pole, keeping everything
          symmetrical as reasonably possible although I wonder how much the feed
          loop being unbalanced upsets the symmetry due to capacitive interaction.

          Any thoughts on this anyone?

          I think you will want to tune the capacitance of your 11m loop as it
          will be tuning over ~1.2% and you can expect a Q in the 100s if the
          thing is well built and if it isn't it won't work.....

          For you're frequency, a simple DIY variable capacitor should be pretty
          easy to conjure up as you only need 20pF or so. I have seen cute designs
          using water pipe or copper sheet or PCB laminate.

          C



          -----Original Message-----
          From: loopantennas@yahoogroups.com [mailto:loopantennas@yahoogroups.com]
          On Behalf Of ozzy_1kw
          Sent: Thursday, 1 April 2010 7:12 p.m.
          To: loopantennas@yahoogroups.com
          Subject: [loopantennas] Loop Capacitance






          If the tuning is done by the small loop for best swr then for a single
          band hf cb i can put the exact capacitance required or is the tuning too
          sharp to cover the entire cb band??

          what is the purpose for the variable capacitor ,is it multiband usage ?

          TY Rob

          PS, i see on youtube the guy that sells loops.his appear to be upside
          down with the tuning capacitor on the bottom that he tunes with his
          thumb. is there any issues feeding it from the top and running the coax
          down the side of the loop ?

          Cheers Rob







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