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Multi turn transmitting loop questions...

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  • David
    Hi, I m about to build a transmitting loop, ~ 8M circumference, for 80-40-30 metres. I have some 1 heliax, and a supply of 3/4 copper pipe to work with. I
    Message 1 of 6 , Aug 1, 2008
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      Hi, I'm about to build a transmitting loop, ~ 8M circumference, for
      80-40-30 metres.

      I have some 1" heliax, and a supply of 3/4" copper pipe to work with.

      I have several Jennings UCS300-15S caps, and a few HV doorknob caps.

      I also have several ~12VDC low RPM (geared) motors.

      I have done the calculations a few different ways, used the
      calculators and spreadsheets that others have provided, modeled in nec
      and I think I have the right arrangement of parts and materials.

      I have come up with a bunch of pieces of scrap lexan from the bin at
      the local plastic supply house, so I have made some pillow
      clamps,housings and mounting stuff from it.

      I believe there is enough material to make several antennas, so the
      thought occurred to me; why don't I try a two turn loop, of
      approximately the same circumference (8m x2) and see if I can get it
      to resonate on 160 & 80M? (the space I have to put it in is limited
      to this)

      I am assuming that the driven loop will be twice the size of that in
      it's one turn sibling. I am guessing that the final ratio still needs
      to be 1/5 for the impedance match. If my logic is faulty, please let
      me know what the right idea is.....


      My question is, and there seems to be a lot of conflicting opinions
      out there, is will it be worth the trouble? Has anyone done this and
      made measurements that support the multi turn design or is it just a
      fool's errand?

      Thanks,

      David
    • m0dvo
      David, In my view, experimenting with antennas is one of the few areas left to us where we can have fun. I was looking for something to use for portable
      Message 2 of 6 , Aug 2, 2008
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        David,

        In my view, experimenting with antennas is one of the few areas left to
        us where we can have fun.

        I was looking for something to use for portable working, and have
        plagiarised SGC's "Stealth" product. Mine is a four-turn square loop
        about 5' on a side tuned by an SGC237 autotuner close to one corner.
        Results so far are encouraging, if not spectacular, but the simple
        construction has not wasted time or materials it it all comes to nought.

        Modelling and measurements are all very rigorous, but will it work in
        practice? Build it and see. AND have fun.

        73s, Mike M0DVO

        --- In loopantennas@yahoogroups.com, "David" <va3ae@...> wrote:
        >
        > My question is, and there seems to be a lot of conflicting opinions
        > out there, is will it be worth the trouble? Has anyone done this and
        > made measurements that support the multi turn design or is it just a
        > fool's errand?
        >
        > Thanks,

        > David
        >
      • dldorrance
        ... This assumption will not work. Th radiation resistance of a multi-turn loop rises exponentially with increasing number of turns. Hence it is possible to
        Message 3 of 6 , Aug 2, 2008
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          > I am guessing that the final ratio still needs
          > to be 1/5 for the impedance match. If my logic is faulty, please let
          > me know what the right idea is.....

          This assumption will not work.

          Th radiation resistance of a multi-turn loop rises exponentially with
          increasing number of turns. Hence it is possible to make a multi-turn
          loop for a single band which has near 50 ohm input impedance. The
          math is in Kraus: Antennas. McGraw Hill, 1950, chapter 6, section 6-8.

          You need to measure the input Z with an antenna analyzer, as the one I
          built had a reasonable radiation resistance (?25 ohms) but had
          capacitative reactance, requiring a coil in parallel with several
          loop turns. For such a loop your capacitors will not work.

          Dave WA6YSO
        • Jim Dunstan
          ... Hi David, Some interesting questions for which I do not have answers ... just opinions. The advantage and the disadvantage of the small (1/10 wavelength)
          Message 4 of 6 , Aug 2, 2008
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            At 12:43 AM 8/2/2008 +0000, you wrote:

            >Hi, I'm about to build a transmitting loop, ~ 8M circumference, for
            >80-40-30 metres.
            >
            >
            >I believe there is enough material to make several antennas, so the
            >thought occurred to me; why don't I try a two turn loop, of
            >approximately the same circumference (8m x2) and see if I can get it
            >to resonate on 160 & 80M? (the space I have to put it in is limited
            >to this)
            >
            >I am assuming that the driven loop will be twice the size of that in
            >it's one turn sibling. I am guessing that the final ratio still needs
            >to be 1/5 for the impedance match. If my logic is faulty, please let
            >me know what the right idea is.....
            >
            >My question is, and there seems to be a lot of conflicting opinions
            >out there, is will it be worth the trouble? Has anyone done this and
            >made measurements that support the multi turn design or is it just a
            >fool's errand?
            >
            >Thanks,
            >
            >David

            Hi David,

            Some interesting questions for which I do not have answers ... just
            opinions. The advantage and the disadvantage of the small (1/10
            wavelength) single turn loop is it's Hi-Q. When resonant and properly
            coupled it provides amazing performance in restrictive space. At the same
            time the disadvantage is its narrow band-pass and the demands put upon
            components due to resultant hi RF voltages. The common feed method for the
            small tuned loop is as you point out a small driven loop approximately 1/5
            the size of the tuned loop which seems to provide the proper coupling.

            I have tried multi-turn wire loops with a similar circumference, 2 and 3
            turn and found the performance dramatically different. The Q is much lower
            ... Perhaps if I had constructed the multi-turn loop with copper pipe
            instead of #12 wire. The only way I could achieve sufficient coupling was
            to place a link coupled balanced antenna tuner right at the loop and used a
            noise bridge to tune the coupler. It worked well on 40m but was not very
            dramatic. 80m was mediocre.

            As the length of the loop gets longer ... eg 2 turns at 8m ... it appears
            that inductive coupling becomes less effective and direct link coupling
            more effective.

            Jim
          • David
            Hi and thanks to all for the replies. I agree with everyone so far (and our local loop guru VE2CV) that the single turn very mall loop is the optimal choice. I
            Message 5 of 6 , Aug 2, 2008
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              Hi and thanks to all for the replies.

              I agree with everyone so far (and our local loop guru VE2CV) that the
              single turn very mall loop is the optimal choice.

              I have built one already, which is a 1M diameter loop, for comparison
              against the MFJ1688 I picked up on eBay a couple of years ago. As you
              might expect, the home brew copper loop, even though it is a smaller
              diameter, beats the aluminum.

              There are really just a few design considerations didn't mention and
              those are the classical ones: size, mass and cost. (and of course these
              are multiplied by the Wife Approval Factor)

              I have an area of about ~5 metres to work with. I'd like them to be
              small enough that I can rotate the two loops (swing 'em round 90 would
              be more accurate)

              It would be nice if the snow and ice won't rip them apart (we had 14
              feet of snow in the yard last winter). So it needs to be strong and not
              have a larger span than is prudent for the pipe diameter.

              I went to the local plumbing jobber supplier yesterday, inquiring about
              the cost of various diameter copper.
              66 ft spools of type K were:
              3/4" = ~$300
              1 " = ~$750
              1 1/2" = $1300
              2" only in 12 ft sections at ~$200 each.
              Larger copper pipe by special order...... and special price!

              Type L is a bit cheaper, but as I recall it has a higher resistance,
              which is counterproductive for us....
              I have on hand 16M of straight Type K 3/4", which I have already cut for
              an octagonal loop with 1 M segments. So, I will assemble that one
              shortly.

              As I said, I have acquired plenty of spare parts for this, and I'd
              really like to see how the continuous copper will add to the efficiency.
              On paper it is undeniable so I am eager to test the on air results.

              Since the copper is so expensive, I should opt for the coil of 3/4",
              based on my remaining budget.
              Unfortunately, I don't' think the 3/4" will be strong enough for the 160
              M loop, which has the diameter of ~5M. This is also too large for the
              space I have, so it is moot, really.

              This was why I thought the multi turn might be practical for me. I read
              an interesting account of a multi turn experiment on Antennex by Anselmo
              Stiffan, IZ3BGJ.
              His results were:
              "Its performances when operated at 1.5 meters above ground against a
              1/2-wave dipole at 12 meters above ground were approximately:
              * -22dB on 160m band
              * -18dB on 80m band
              * -9dB on 40m band
              * -3dB on 20m band
              The bandwidth at SWR=2:1, which is a good indicator of the antenna Q
              factor and hence its efficiency was:
              * 1.6 kHz on 160m
              * 2.5 kHz on 80m
              * 6.4 kHz on 40m
              * 23 kHz on 20m
              "
              He seemed to get it to work, so that is why I went fishing for other
              opinions.

              I haven't convinced myself yet that it is worth several hundred extra
              dollars to see if a two turn loop for 160 will work.

              Whatever I end up with, I'll post the results.

              VY 73 de va3ae


              > Hi, I'm about to build a transmitting loop, ~ 8M circumference, for
              ............................
              > I also have several ~12VDC low RPM (geared) motors.
              snip...................


              [Non-text portions of this message have been removed]
            • dldorrance
              ... My comments above were for a loop for 20 meter. It was about 4 feet on a side. I recall it had 4 turns which were spaced maybe 2 feet apart from each
              Message 6 of 6 , Aug 2, 2008
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                --- In loopantennas@yahoogroups.com, "David" <va3ae@...> wrote:

                My comments above were for a loop for 20 meter. It was about 4 feet
                on a side. I recall it had 4 turns which were spaced maybe 2 feet
                apart from each other but were essentially made with a single
                continuous length of 12 gauge wire. The dimensions above were chosen
                for a reasonable direct match to a standard tranceiver (50 ohms).
                This is not a small loop. As stated above, its radiation resistance
                was on the order of 50 ohms (not milliohms and not 25 ohms as stated
                above) with capacitative, not inductive reactance as would be seen in
                a small loop, so it requiring a coil, not a capacitor for cancelling
                reactance. The formulas are in the Kraus reference. It can be
                designed for any band using the formulas.

                Dave WA6YSO
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