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Re: 80 meter folded dipole VS loop

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  • airchecklover
    Good explanation! I ve always wondered .... -Mark
    Message 1 of 9 , May 11, 2010
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      Good explanation! I've always wondered ....

      -Mark

      ====================

      --- In loopantennas@yahoogroups.com, Andy <ai.egrps@...> wrote:
      >
      > > Thanks for the reply, from looking at images and diagrams of folded
      > > dipoles they looked like a squashed loop, but after reading some info at
      > > Wikipedia I see they don't make a connection forming a loop (they just
      > > come close) = solved that mystery.
      >
      > Actually, most folded dipoles are wired exactly like a loop. If you start
      > with a loop and squash it flat, you would end up with a folded dipole
      > exactly. But this act of squashing it, makes it behave totally differently.
      > The currents redistribute themselves and the pattern changes a lot.
      >
      > I haven't seen the Wikipedia info, but if it shows the folded dipole with
      > the ends NOT connected together, that is one way you CAN do it, but most
      > often the ends are shorted.
      >
      > There is this notion of capture area for an antenna (the more the better),
      > and it would seem like opening up a folded dipole into an open loop would
      > increase capture area and make the antenna more effective. In other words,
      > maximizing the use of your yard's space, in area not just linear space,
      > MIGHT be a good thing. But I think the difference is actually small.
      >
      > Loops and (folded) dipoles have rather different directional
      > characteristics, as well as bandwidth, and usefulness on multiple bands, and
      > those should also influence your decision.
      >
      > For example, a (folded) dipole's pattern looks like a donut, but a full size
      > loop's pattern is more like back-to-back flashlights. Thus the loop might
      > give you less "wasted" power going in directions you don't care about (like
      > straight up or down if you only want to do DX).
      >
      > Both loops and folded dipoles (with both ends shorted so that it forms a
      > loop) do have the advantage that static voltages won't build up across the
      > antenna terminals, which makes them quieter in the wind or rain.
      >
      > Andy
      >
    • tenorman1952
      ... Yes, but there is a big difference.... In a loop the difference in the wave causes the current to flow in one direction on one side of the loop, and the
      Message 2 of 9 , May 11, 2010
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        --- In loopantennas@yahoogroups.com, Andy <ai.egrps@...> wrote:
        >
        > > Thanks for the reply, from looking at images and diagrams of folded
        > > dipoles they looked like a squashed loop, but after reading some info at
        > > Wikipedia I see they don't make a connection forming a loop (they just
        > > come close) = solved that mystery.
        >
        > Actually, most folded dipoles are wired exactly like a loop. If you start
        > with a loop and squash it flat, you would end up with a folded dipole
        > exactly.


        Yes, but there is a big difference....

        In a loop the difference in the wave causes the current to flow in one direction on one side of the loop, and the opposite direction on the other side of the loop, because the two sides of the loop are being intersected, and magnetic wave cut on a different part of the wave.

        But with a "folded dipole" both sides of the "loop" formed are too close together, and the signals are going in the same direction. The other half of the "folded dipole, that is, the half not connected to the feedline, is wasted. It has the same currents and voltages as its parallel portion all along the wire. So no current flows from the feedline half to the other half and vice versa. The voltages out on the ends are the same, and have zero current flow.

        I have made folded loops and then cut them at the ends, and it makes no difference. The other piece of wire is wasted.
      • Andy
        ... Indeed there is. What I described above is just the physical topology, that the wires of the folded dipole do trace out a flattened loop. But as an
        Message 3 of 9 , May 11, 2010
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          > > Actually, most folded dipoles are wired exactly like a loop. If you
          > > start
          > > with a loop and squash it flat, you would end up with a folded dipole
          > > exactly.
          >
          >
          > Yes, but there is a big difference....

          Indeed there is. What I described above is just the physical topology, that
          the wires of the folded dipole do trace out a flattened loop. But as an
          antenna, the folded dipole behaves nothing like a loop and everything like a
          dipole ... with the two exceptions that (1) its feedpoint impedance is
          quadrupled, and (2) it has DC continuity.

          Like you say, the current directions are those of a dipole.

          > The other half of the "folded dipole, that is, the half not connected to
          > the feedline, is wasted.
          ...
          > I have made folded loops and then cut them at the ends, and it makes no
          > difference. The other piece of wire is wasted.

          I'm not sure exactly what you're saying, if it's how I'm reading it ... but
          it isn't wasted. Yes, you can cut the bridges between the two wires and
          THAT won't make much difference, but it's not the same thing as removing the
          other (now disconnected) wire. That other wire has a significant effect; it
          is quite tightly coupled to the fed dipole wires. It causes the terminal
          voltage across the other wires to double, and the current to be halved, for
          the same power. So the feedpoint impedance quadruples. Big change.

          Does it improve antenna gain, or reception? No. But it is most useful to
          move the impedance upwards. For a low horizontal HF antenna, that is an
          important matching technique. Also for FM radios, to transform the 72 ohm
          dipole impedance up to the radio's 300 ohm input impedance.

          And as a side effect, it widens the antenna's bandwidth a bit, just like a
          cage dipole does, because the antenna is effectively fatter.

          It's worth remembering that the folded dipoles we're talking about shouldn't
          be confused with T2FD's, which employ a terminating resistor(s) to achieve
          very wide SWR bandwidths while sacrificing efficiency. It's a different
          animal.

          Andy
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