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Re: [loopantennas] DF loop to locate local 20m broadband interference

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  • Andy
    I am certainly no expert, but I ll throw in my beliefs and opinions. With loops, there are small loops (much less than a wavelength), and large loops (similar
    Message 1 of 8 , Oct 1, 2010
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      I am certainly no expert, but I'll throw in my beliefs and opinions.

      With loops, there are small loops (much less than a wavelength), and
      large loops (similar to a wavelength). The two behave totally
      differently. The small loop has nulls perpendicular to the loop,
      whereas a full wavelength loop's null is exactly the opposite.
      Between the two, you probably get a mishmash of both patterns, with no
      good null. So keep the dimensions small.

      I *think* the dimension in question is the total wire in the loop, so
      it would seem that a single turn loop might have better nulls than a
      multi turn loop with the same outer dimensions. For a loop to be
      considered small, the current in the wire should be everywhere the
      same. Once the wire length starts to approach a quarter wavelength,
      that no longer holds true.

      Bigger size means more signal gathering capability (more gain). If
      you get enough signal with a smaller loop, there may be no reason to
      make it bigger.

      Electrical balance is supposed to be an important characteristic for
      having good nulls. If you unbalance the loop it might screw up the
      pattern.

      So-called "shielded" loops supposedly help maintain that balance.

      Tuning the loop with a capacitor increases the amount of signal it
      receives (its gain), but I think wouldn't affect its pattern. If you
      get enough signal without tuning it, maybe it's good enough.

      Regards,
      Andy
    • tenorman1952
      ... Build a loop like this: http://www.mtmscientific.com/swloop.html using a 365 pf tuning cap and some wire (I used insulated stranded 12 ga hookup wire). Use
      Message 2 of 8 , Oct 1, 2010
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        > I would like to have some ideas or advice on what sort of loop antenna would
        > be best for this purpose. I already built a pretty crude square shaped loop
        > on two wooden crosspieces, roughly 4 feet across. I was not overly impressed
        > by its directionality, but then again, I did not play with it very long. I
        > only tried rotating it inside my house near where the transceiver is set up.
        > Maybe this will be fine for my purposes.
        >
        >

        Build a loop like this:

        http://www.mtmscientific.com/swloop.html

        using a 365 pf tuning cap and some wire (I used insulated stranded 12 ga hookup wire).

        Use Bruce Carter's Calculator:

        http://www.angelfire.com/mb/amandx/loop.html

        1 turn of wire on a 44" x 44" frame, and 365 pf tuning capacitor should do well.

        74
        Paul - AE5JU
      • vbifyz
        I tried this at my QTH (typical suburban subdivision) with both tuned and untuned loops. At 14MHz, a 1m diameter loop is already quite big and the null is not
        Message 3 of 8 , Oct 1, 2010
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          I tried this at my QTH (typical suburban subdivision) with both tuned and untuned loops.
          At 14MHz, a 1m diameter loop is already quite big and the null is not deep at all, but it is still there. Tuned loop gives a better null. I used 10-360pF air variable capacitor for tuning and a 15cm diameter coupling loop. The untuned loop was a 1.5m piece of wire connected to the 50 Ohm input and the ground lug of my FT-817.

          In my case, I wasn't able to isolate the source. The QRM is radiated from all wiring everywhere in the neighborhood. The only way to reduce it is go at least 50m from all houses. When you move around the powerlines, the direction changes randomly, there is no "point source". It also changes with frequency, i.e. the 14MHz null and 7MHz null may be 90 degrees apart.

          I may have found a maximum around 3 or 4 particular houses, but that was the extent of my "direction finding".

          Good luck, 73
          Mike
        • legonedijk
          Message 4 of 8 , Oct 2, 2010
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            --- In loopantennas@yahoogroups.com, Eamon Egan <eamon.egan@...> wrote:
            >
            > I often experience pretty bad local broadband interference on the 20m band
            > and I want to be able to track down the source. It seems powerline related
            > as it sounds like it's modulated at 60 or 120Hz. It may be on other bands
            > as well, but I am mostly active on 20m at this point. When it happens, it is
            > often reaches up to between S5 and S7 or sometimes S9 on my 20m dipole.
            >
            >
            >
            > It could conceivably be from somewhere inside my house but I am pretty sure
            > it is from somewhere nearby outside. I plan to move up and down the street
            > with my transceiver on battery power and connected to a loop antenna, to see
            > if I can null out the interference source and thereby locate it. Before I
            > move around on battery power, I will be 100% sure it isn't in my house by
            > momentarily cutting off the AC.
            >
            >
            >
            > I would like to have some ideas or advice on what sort of loop antenna would
            > be best for this purpose. I already built a pretty crude square shaped loop
            > on two wooden crosspieces, roughly 4 feet across. I was not overly impressed
            > by its directionality, but then again, I did not play with it very long. I
            > only tried rotating it inside my house near where the transceiver is set up.
            > Maybe this will be fine for my purposes.
            >
            >
            >
            > I don' t need a super sharp null, but I do need to be able to see it well
            > enough to walk in the right direction and stop in front of the right
            > neighbor's house or utility pole :) I know that a simple loop will give two
            > nulls at 180 degrees from each other, and I can deal with this ambiguity.
            >
            >
            >
            > So here are a few more specific questions:
            >
            > * How many turns, at what diameter, would be best? Is one turn better
            > or worse than several? Are there any relevant design formulas that yield
            > this kind of guidance?
            > * What construction details or tolerances are most critical, for the
            > loop to display a useful null for this type of direction finding?
            > * Do you think I should bother trying to make it shielded (i.e. use
            > coax cable for electrostatic shielding)?
            > * Should I tune the loop with a variable capacitor? Will this help me
            > in what I am trying to accomplish?
            > * How important is balance in achieving a useful null? Should I
            > connect the loop to my transceiver with (unbalanced) coax, or use the 300
            > ohm balanced line terminal on my tuner? Or should I make a separate balun
            > using a ferrite toroid?
            >
            >
            >
            > Any ideas or advice would be appreciated. I mostly want to know whether I
            > should just start with something very simple like what I've already built,
            > or whether I need to put something fancier together for this application.
            >
            >
            >
            > Eamon Egan, VE2EGN
            >
            >
            >
            > [Non-text portions of this message have been removed]
            >
          • Eamon Egan
            Thanks to all who replied so far. Andy: I was aware of the distinction between large and small but thanks for mentioning that it is the total length. that
            Message 5 of 8 , Oct 2, 2010
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              Thanks to all who replied so far.



              Andy: I was aware of the distinction between "large" and "small" but thanks
              for mentioning that it is the total length. that seems to make sense. I will
              keep the diameter small enough, stick with a single turn, and possibly
              simulate the antenna to see at what size the pattern starts getting
              complicated.



              Paul: thanks for the recommendation on the site for the kit, and the
              calculator. These look helpful.



              Mike: thanks for mentioning how difficult it can be to pinpoint. I will not
              place my expectations too high. And of course even if I do think I know
              where it's coming from, knocking on the front door and explaining that they
              need to stop interfering will not necessarily be easy. You seem to be saying
              that your untuned loop connected to your receiver (1.5m) was longer than
              your tuned loop (you call it a 15cm diameter coupling loop). Isn't the loop
              connected to the radio usually called the coupling loop, and isn't it
              usually smaller than the tuned loop?



              I have two different opinions on whether the tuning capacitor affects the
              pattern or just the signal strength. I may simulate this to form my own
              opinion, or since I have a serviceable tuning capacitor I may just have a go
              at tuning the loop anyway, just to see how it goes.



              One more thing I'd like to ask for comments on: does anyone know why it is
              better to use a separate coupling loop, as opposed to connecting the tuned
              loop directly to the radio? I know coupling loops are used in transmitting
              loops also, but I am not sure I understand why. What would happen if you
              tried to connect the tuned loop directly to the radio?



              Eamon VE2EGN



              [Non-text portions of this message have been removed]
            • Eamon Egan
              I have found an answer to my own question about coupling loops, at http://www.radiobanter.com/showthread.php?t=74935 Apparently the impedance ratio equals the
              Message 6 of 8 , Oct 2, 2010
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                I have found an answer to my own question about coupling loops, at
                http://www.radiobanter.com/showthread.php?t=74935



                Apparently the impedance ratio equals the square of the diameter ratio. So
                it is a matter of impedance matching.



                - Eamon



                [Non-text portions of this message have been removed]
              • dldorrance
                Easiest way to track down RFI is to walk your neighborhood with a battery operated portable broadcast band AM radio. It has a ferrite loop built into it. Dave
                Message 7 of 8 , Oct 3, 2010
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                  Easiest way to track down RFI is to walk your neighborhood with a battery operated portable broadcast band AM radio. It has a ferrite loop built into it.

                  Dave WA6YSO

                  --- In loopantennas@yahoogroups.com, Eamon Egan <eamon.egan@...> wrote:
                  >
                  > I often experience pretty bad local broadband interference on the 20m band
                  > and I want to be able to track down the source. It seems powerline related
                  > as it sounds like it's modulated at 60 or 120Hz. It may be on other bands
                  > as well, but I am mostly active on 20m at this point. When it happens, it is
                  > often reaches up to between S5 and S7 or sometimes S9 on my 20m dipole.
                  >
                  >
                  >
                  > It could conceivably be from somewhere inside my house but I am pretty sure
                  > it is from somewhere nearby outside. I plan to move up and down the street
                  > with my transceiver on battery power and connected to a loop antenna, to see
                  > if I can null out the interference source and thereby locate it. Before I
                  > move around on battery power, I will be 100% sure it isn't in my house by
                  > momentarily cutting off the AC.
                  >
                  >
                  >
                  > I would like to have some ideas or advice on what sort of loop antenna would
                  > be best for this purpose. I already built a pretty crude square shaped loop
                  > on two wooden crosspieces, roughly 4 feet across. I was not overly impressed
                  > by its directionality, but then again, I did not play with it very long. I
                  > only tried rotating it inside my house near where the transceiver is set up.
                  > Maybe this will be fine for my purposes.
                  >
                  >
                  >
                  > I don' t need a super sharp null, but I do need to be able to see it well
                  > enough to walk in the right direction and stop in front of the right
                  > neighbor's house or utility pole :) I know that a simple loop will give two
                  > nulls at 180 degrees from each other, and I can deal with this ambiguity.
                  >
                  >
                  >
                  > So here are a few more specific questions:
                  >
                  > * How many turns, at what diameter, would be best? Is one turn better
                  > or worse than several? Are there any relevant design formulas that yield
                  > this kind of guidance?
                  > * What construction details or tolerances are most critical, for the
                  > loop to display a useful null for this type of direction finding?
                  > * Do you think I should bother trying to make it shielded (i.e. use
                  > coax cable for electrostatic shielding)?
                  > * Should I tune the loop with a variable capacitor? Will this help me
                  > in what I am trying to accomplish?
                  > * How important is balance in achieving a useful null? Should I
                  > connect the loop to my transceiver with (unbalanced) coax, or use the 300
                  > ohm balanced line terminal on my tuner? Or should I make a separate balun
                  > using a ferrite toroid?
                  >
                  >
                  >
                  > Any ideas or advice would be appreciated. I mostly want to know whether I
                  > should just start with something very simple like what I've already built,
                  > or whether I need to put something fancier together for this application.
                  >
                  >
                  >
                  > Eamon Egan, VE2EGN
                  >
                  >
                  >
                  > [Non-text portions of this message have been removed]
                  >
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