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FM loop antenna for nulling

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  • Tony
    I posted this on the other group, but did not know if I was sounding too crazy here. I wanted to know if there was any way to design a loop antenna for the FM
    Message 1 of 5 , Aug 29, 2004
      I posted this on the other group, but did not know if I was sounding
      too crazy here.

      I wanted to know if there was any way to design a loop antenna for
      the FM broadcast band (88-108 MHz) which would have the same nulling
      ability/technique as the Radio Shack AM loop or the Terk loop on the
      AM broadcast band. Another example would be comparable to the Palomar
      loops, or something I could construct myself. I want something that
      has the ability to null out the stronger stations in favor of the
      more distant ones. I know this is a popular/common aspect with LW &
      AM broadcast antennas, but what about 88-108 MHz?
      I am aware of an array which would consist of at least 2 Yagis, but
      do not have the space to implement/employ such. Again, the key is
      nulling out the strong stations.

      Does such a concept exist? I would like to null out some of the semi-
      local (roughly 40 miles away) FM stations to allow reception of other
      stations.

      Please let me know, & thanks in advance.

      Tony
    • Steve Baker
      Tony, You could probably use the same construction method and calculations used for MW and LW loops but the problem you might have is that the Q would be too
      Message 2 of 5 , Aug 29, 2004
        Tony,

        You could probably use the same construction method
        and calculations used for MW and LW loops but the
        problem you might have is that the Q would be too
        high. AM signals found on MW are about 10KhZ wide
        while FM signals are 200KHz wide. From what I hear a
        good loop has a Q that gives you a 25KHz bandwidth (or
        better). You would loose much of the FM signal and
        since it is frequency modulated it may be
        indisguishable. You could purposely decrease the Q to
        widen the received bandwidth by adding resistance
        across the loop; but this would end up decreasing the
        nulling effect that the loop has and defeat the whole
        purpose of the loop in the first place.

        I just started reading up on loops 6 months ago so I
        could be wrong. It may be that the bandwidth is
        actually a percentage of the received frequency in
        which case the loop would work. I'm sure the other
        guys in the group will correct me if I'm wrong.

        Steve




        --- Tony <Plutonyum@...> wrote:

        > I posted this on the other group, but did not know
        > if I was sounding
        > too crazy here.
        >
        > I wanted to know if there was any way to design a
        > loop antenna for
        > the FM broadcast band (88-108 MHz) which would have
        > the same nulling
        > ability/technique as the Radio Shack AM loop or the
        > Terk loop on the
        > AM broadcast band. Another example would be
        > comparable to the Palomar
        > loops, or something I could construct myself. I want
        > something that
        > has the ability to null out the stronger stations in
        > favor of the
        > more distant ones. I know this is a popular/common
        > aspect with LW &
        > AM broadcast antennas, but what about 88-108 MHz?
        > I am aware of an array which would consist of at
        > least 2 Yagis, but
        > do not have the space to implement/employ such.
        > Again, the key is
        > nulling out the strong stations.
        >
        > Does such a concept exist? I would like to null out
        > some of the semi-
        > local (roughly 40 miles away) FM stations to allow
        > reception of other
        > stations.
        >
        > Please let me know, & thanks in advance.
        >
        > Tony
        >
        >
        >
        >




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      • c. beijersbergen
        These kind of antennas for the 3meter band (88-108 MHz) are usually not called loops but square dipoles. If you make a metal square with sides of 50 cm., slit
        Message 3 of 5 , Aug 30, 2004
          These kind of antennas for the 3meter band (88-108 MHz) are usually not
          called loops but square dipoles. If you make a metal square with sides of 50
          cm., slit the bottom in in the middle and connect your feed wires to the
          receiver to these two points you automatically have a tuned, directional
          sensitive antenna, with the right feed impedance. You might want a 1:1 balun
          to connect your 50-75 Ohm coax to it. No capacitor is needed. The bandwidth
          will be such that you can receive the complete band with it, but at the
          sides you will have some attenuation. By changing the length of the sides
          you can make it sensitive for a specific section of the band.
          A circular version can also be made, provided the length of the conductor is
          200 cm's.

          On remark about Q and bandwidth. The Q is a quality factor, independent of
          the frequency. It gives the relation of bandwidth and center frequency. So
          a Q factor that give a bandwidth of 10kHz at 1MHz will give 1 MHz at 100 MHz
          center frequency. If you ever succeed in achieving this , which I doubt, you
          will not have any problem with the modulation being filtered out.

          regards,

          Cor Beijersbergen van Henegouwen
        • R H F
          TONY, Instead of a Loop Antenna, try using a basic (Ready Made) 300 Ohm TV Twin Lead FM Folded Dipole Antenna. (Cost about $3) The FM Folded Dipole Antenna
          Message 4 of 5 , Aug 30, 2004
            TONY,

            Instead of a Loop Antenna, try using a basic (Ready Made)
            300 Ohm TV Twin Lead "FM" Folded Dipole Antenna.
            (Cost about $3)

            The FM Folded Dipole Antenna can be layed out Horizontally
            or 'formed' around a Hula Hoop.

            Then the FM Folded Dipole can be rotated for the best Null
            of the unwanted FM Station, or for the best Peak Signal of
            the wanted FM Station.

            READ: FM Reception and FM Relex Antenna
            http://groups.yahoo.com/group/Shortwave-SWL-Antenna/message/1109

            READ: The Search... For FM Radio Reception Excellence !
            http://groups.yahoo.com/group/Shortwave-SWL-Antenna/message/409
            http://groups.yahoo.com/group/Shortwave-SWL-Antenna/message/392

            iane ~ RHF
            .
            .
            = = = In loopantennas@yahoogroups.com,
            = = = "Tony" <Plutonyum@v...> wrote:
            > I posted this on the other group, but did not know if I was
            sounding
            > too crazy here.
            >
            > I wanted to know if there was any way to design a loop antenna for
            > the FM broadcast band (88-108 MHz) which would have the same
            nulling
            > ability/technique as the Radio Shack AM loop or the Terk loop on
            the
            > AM broadcast band. Another example would be comparable to the
            Palomar
            > loops, or something I could construct myself. I want something
            that
            > has the ability to null out the stronger stations in favor of the
            > more distant ones. I know this is a popular/common aspect with LW
            &
            > AM broadcast antennas, but what about 88-108 MHz?
            > I am aware of an array which would consist of at least 2 Yagis,
            but
            > do not have the space to implement/employ such. Again, the key is
            > nulling out the strong stations.
            >
            > Does such a concept exist? I would like to null out some of the
            semi-
            > local (roughly 40 miles away) FM stations to allow reception of
            other
            > stations.
            >
            > Please let me know, & thanks in advance.
            >
            > Tony
            .
          • Stumm, John C
            Tony, I took a folded dipole, (one using coax) opened it up and put it in a hula hoop. It works well for me, especially on radios that accept 75 ohm inputs. An
            Message 5 of 5 , Aug 30, 2004
              Tony, I took a folded dipole, (one using coax) opened it up and put it
              in a hula hoop. It works well for me, especially on radios that accept
              75 ohm inputs. An impedance matcher allows good reception with 300 ohm
              inputs as well. Here's the text of my message from this other group. The
              tilt feature I incorporated helps too.
              John
              http://groups.yahoo.com/group/Shortwave-SWL-Antenna/message/1055
              Here's an idea that I had that works well for me for FM. It is made of
              coax cable so the impedance will be low to begin with, (whatever the
              impedance of the cable is) RG-58a/u works for me. Anything will work as
              long as it is not too big, (you'll see why) If You have a hula-hoop
              handy, pull out the plug that splices it together. Install a half inch
              PVC "T" on one end. Take 18-20 feet of the coax, (depending on how much
              lead cable you need, about 9 feet will be in the loop). Bare back the
              lead end of the cable so just a 1/4 inch of bare center conductor is
              showing. Thread this end through the hula-hoop and tee until it just
              protrudes out the other end. At this point, carefully bare a small area
              of the coax shielding where it first entered the hula-hoop. Solder the
              bared center conductor end back to the bared spot on the shield.
              Incorporating the tee in this joint so that the lead in cable comes out
              the center of the tee is the tricky part. Leave yourself enough cable to
              work with when soldering and, if the coax is small gage, it will easily
              go back in the hoop when the tee is made up. A little tape will make the
              tee fitting snugger, the idea being that this antenna will TILT as will
              as be directional. I've found this to be an important feature.
              Flattening this arrangement down, (no hula-hoop) works well too. It, in
              effect, becomes a coaxial folded dipole, (75ohm) If this is used with a
              radio that takes a 300 ohm balanced input, just add a matcher, I
              guarantee this will be worth the effort!

              -----Original Message-----
              From: Tony [mailto:Plutonyum@...]
              Sent: Sunday, August 29, 2004 1:24 PM
              To: loopantennas@yahoogroups.com
              Subject: [loopantennas] FM loop antenna for nulling

              I posted this on the other group, but did not know if I was sounding
              too crazy here.

              I wanted to know if there was any way to design a loop antenna for
              the FM broadcast band (88-108 MHz) which would have the same nulling
              ability/technique as the Radio Shack AM loop or the Terk loop on the
              AM broadcast band. Another example would be comparable to the Palomar
              loops, or something I could construct myself. I want something that
              has the ability to null out the stronger stations in favor of the
              more distant ones. I know this is a popular/common aspect with LW &
              AM broadcast antennas, but what about 88-108 MHz?
              I am aware of an array which would consist of at least 2 Yagis, but
              do not have the space to implement/employ such. Again, the key is
              nulling out the strong stations.

              Does such a concept exist? I would like to null out some of the semi-
              local (roughly 40 miles away) FM stations to allow reception of other
              stations.

              Please let me know, & thanks in advance.

              Tony







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