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Re: [loopantennas] Re: How long horizontal wire do i need for a 1/2wave AM Di-Pole.. where do i get or how do i make a Balun for AM

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  • Jim Dunstan
    ... Hi, The short answer to your original question if there was a directional antenna i could make to pick up a weak station ... is no. I am assuming you
    Message 1 of 20 , Nov 11, 2006
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      At 11:23 PM 10/11/2006 +0000, you wrote:


      >my origional question was if there was a directional antenna i could
      >make to pick up a weak station,10,000 watts day 5K nights.. 77 miles
      >away with a lot of stations flaming over it... i was linked to a
      >article about a directional di-pole.. this is obviously not what i was
      >looking for.. thank you for you help.. and i was asking because i do
      >not have the knowledge to do it myself.
      >
      >thanks for you help..


      Hi,

      The short answer to your original question "if there was a directional
      antenna i could make to pick up a weak station ..." is no. I am assuming
      you are referring to an antenna that has 'gain'. A directional (gain)
      antenna implies using creative techniques to deform the radiation pattern
      such that reception is greater in one particular direction at the expense
      of radiation in other directions. The usual techniques is to use multiple
      elements fed and located such that the phase of the signals add and
      subtract in some way. A dipole is a single element .... fed in the centre
      .... thus giving it its name. A dipole does have a radiation pattern which
      is uneven in all directions ..... and I suppose it could be called
      'directional'. However, the pattern is very broad in all directions except
      for two reductions 180 degree from each other .... called nulls. These
      nulls are not very sharp or are they very deep.

      Directional or gain antennas in the MW band are possible but they are the
      exclusive domain of 'commercial' operations that have the resources to
      construct huge antenna farms.

      However, radio listeners can create antennas that help pick up stations in
      the MW band that have "a lot of stations flaming over it ..."..... but a
      directional dipole is not one of them.

      Large wire antennas within the realm of possibility for the DX listener
      are: the "EWE", the Beverage, the K9AY loop, the 'Flag or Pennent', es I
      am sure a couple more. They are all well represented on the internet and
      you can find plenty of articles on all of them. For the most part they are
      what are called 'terminated' or 'travelling wave' antennas.

      Another strategy is to use small magnetic loop antennas. The
      characteristic of a well constructed magnetic loop is that it has very
      sharp well defined 'nulls' which if carefully oriented can separate signals
      on the same frequency enough to copy one from the other. A well
      constructed loop allows orientation in both angle and azimuth ... which
      adds to the separation of signals. Again there are hundreds of great
      construction articles on the internet.

      A third method a DX listener can use to separate the "flaming" interference
      is to use a phasing device (MFJ sells such a device) that allows two
      antennas to operate simultaneously. The trick is to use the phase of the
      "flaming" station signals to cancel each other out .... leaving the desired
      signal in the clear. This technique can be used with the specialized
      'terminated' or 'magnetic loop' antennas mentioned above.

      None of these techniques provide any gain or directional radiation patterns
      .... in fact the strength of the signals coming to the radio is in fact
      reduced considerably .... but modern receivers usually have sufficient gain
      or small RF amplifiers are used to compensate for some of the loss. In the
      case you gave the strength of the signal from the station you want to
      receive is not the issue .... reducing general noise and interference from
      other stations on or near the frequency is.

      There is lots of good reading on the internet .... to help you get started
      in sorting out the problem.

      Jim



      >
    • Greg W:-)
      I agree with that 100% :-D heheh gW:-)
      Message 2 of 20 , Nov 11, 2006
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        I agree with that 100% :-D heheh

        gW:-)

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


        --- In loopantennas@yahoogroups.com, John Popelish <jpopelish@...> wrote:

        >
        > I would expect excellent reception of both AM band radio
        > transmissions and lightning.
        >
      • Greg W:-)
        Ahh ,,but the built in safety fuse will trip during a lighnting or severe static event . Unfortunately that fuse is the radio :-) gregW:-)
        Message 3 of 20 , Nov 11, 2006
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          Ahh ,,but the built in safety fuse will trip during a lighnting or
          severe static event .

          Unfortunately that fuse is the radio :-)

          gregW:-)

          http://www.swdxer.co.nr/

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


          --- In loopantennas@yahoogroups.com, QRP_1@... wrote:
          >
          >
          > I would expect excellent reception of both AM band radio
          > transmissions and lightning. [snip]
          >
          > Be careful on a cold windy day too, static electricity can build up
          > enough on that wire to knock you on your butt and can damage the radio.
          > If you use it, connect a 250 mH choke from the wire to ground to bleed
          > off any static charges.
          >
          > Rick McKee - KC8AON - Southern Ohio
          > http://www.angelfire.com/electronic2/qrp
          > http://www.angelfire.com/electronic2/qrp/forsale.html
          > Monthly QRP Field Day Group: http://groups.yahoo.com/group/mqfd
          > With God all things are possible <><
          >
        • QRP_1@juno.com
          Ahh ,,but the built in safety fuse will trip during a lighnting or severe static event . Unfortunately that fuse is the radio :-) [snip] Fuses won t protect
          Message 4 of 20 , Nov 11, 2006
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            Ahh ,,but the built in safety fuse will trip during a lighnting or
            severe static event .

            Unfortunately that fuse is the radio :-) [snip]

            Fuses won't protect you if you are in contact with the lead in wire ! I
            grabbed a lead in wire once and got a nasty shock ! It was a very cold
            windy day, and after it happened I went outside to make sure the power
            line hadn't fallen on the antenna. All it was, was a static charge on
            the antenna wire. But it hurt as bad as a shock from the 117 volt AC
            power would or maybe even worse ! I later switched to insulated antenna
            wire to help keep the static off the antenna. I'm a ham now for the last
            11 years, and I still use insulated wire for my antennas !

            Rick McKee - KC8AON - Southern Ohio
            http://www.angelfire.com/electronic2/qrp
            http://www.angelfire.com/electronic2/qrp/forsale.html
            Monthly QRP Field Day Group: http://groups.yahoo.com/group/mqfd
            With God all things are possible <><
          • Laurence Taylor
            ... If you do exactly that, you ll short your signals to earth! I take it you meant the Ae and E terminals. I should think you d get excellent reception - and
            Message 5 of 20 , Nov 15, 2006
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              In message <ej2ung+qc2v@...> "Danny" writes:

              > What do you think about the following. I know a place where I can go
              > anytime. This place is a mile away from any power line. If I put a 10
              > feet pipe in the ground and a 250 feet of vertical wire attached to a
              > helium balloon and I connect both wires to the antenna input of a
              > receiver, what could be the predicted result of a such arrangment?

              If you do exactly that, you'll short your signals to earth!
              I take it you meant the Ae and E terminals.

              I should think you'd get excellent reception - and
              transmission, if you ue it for that. Better still with an
              ATU.

              The problem, of course, is that baloons tend to blow about
              in the wind, so you'd have to anchor it, but it's certainly
              a good way of getting a decent aerial out!

              It's not an original idea, though; Radio Marti in Florida
              and Radio Laser in Europe both experimented with baloon-
              mounted aerials.

              rgds
              LAurence

              ... >°<<<=< (Thread Herring)
              ~~~ Tag-O-Matic V.13F
            • Greg W:-)
              ... SNIP ... And marconi did it 3 thousand years ago too ,,well , maybe not exactly 3 thousand years ,,but a long time ago anyway. :-P gregW:-)
              Message 6 of 20 , Nov 15, 2006
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                --- In loopantennas@yahoogroups.com, Laurence Taylor <lozt@...> wrote:
                >
                SNIP
                > It's not an original idea, though; Radio Marti in Florida
                > and Radio Laser in Europe both experimented with baloon-
                > mounted aerials.
                >
                > rgds
                > LAurence


                And marconi did it 3 thousand years ago too ,,well , maybe not exactly
                3 thousand years ,,but a long time ago anyway. :-P

                gregW:-)

                http://www.swdxer.co.nr/

                =======================================================================
              • dariusfranke
                I think also some of the winners of the crystal set DX contests do much the same technique, so it must be pretty effective - more wire up higher , as they
                Message 7 of 20 , Nov 15, 2006
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                  I think also some of the winners of the crystal set DX contests do
                  much the same technique, so it must be pretty effective - 'more wire
                  up higher' , as they say...

                  Dave



                  --- In loopantennas@yahoogroups.com, "Greg W:-)" <onegammyleg@...> wrote:
                  >
                  > --- In loopantennas@yahoogroups.com, Laurence Taylor <lozt@> wrote:
                  > >
                  > SNIP
                  > > It's not an original idea, though; Radio Marti in Florida
                  > > and Radio Laser in Europe both experimented with baloon-
                  > > mounted aerials.
                  > >
                  > > rgds
                  > > LAurence
                  >
                  >
                  > And marconi did it 3 thousand years ago too ,,well , maybe not exactly
                  > 3 thousand years ,,but a long time ago anyway. :-P
                  >
                  > gregW:-)
                  >
                  > http://www.swdxer.co.nr/
                  >
                  > =======================================================================
                  >
                • Danny
                  ... would do ... into the ... but not an ... vertical ... but ... But given a ... has a ... match to 50 ... definitely ... you could ... the ... is for the ...
                  Message 8 of 20 , Nov 18, 2006
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                    --- In loopantennas@yahoogroups.com, Ken Javor <ken.javor@...> wrote:
                    >
                    > It certainly sounds impractical, but here is what that antenna
                    would do
                    > theoretically. First, I would not rely on a ten foot pipe driven
                    into the
                    > ground for a ground plane connection. That is a lightning ground,
                    but not an
                    > rf ground. You want 250' radials emanating from the point where the
                    vertical
                    > comes to ground. And I think you want a radial every 22.5 degrees,
                    but
                    > there are probably others here who have a better feel for that.
                    But given a
                    > decent ground plane, at 1 MHz you have a quarter-wave monopole. It
                    has a
                    > source impedance of about 36 Ohms, so it is a reasonably close
                    match to 50
                    > Ohms. If you had to pick between 50 and 75 Ohm coax, you would
                    definitely
                    > use 50 Ohm. However if you had a 2:1 unun or step-up transformer,
                    you could
                    > use 75 Ohm coax.
                    >
                    > The effective height of a half-wave dipole into a matched load is
                    the
                    > wavelength divided by 6.28 (2*pi). I think it is the same for a
                    > quarter-wave monopole, but the matching impedance is half what it
                    is for the
                    > dipole. The effective height is the transfer function between the
                    rf field
                    > intensity impinging on the antenna and the rf potential the antenna
                    delivers
                    > into a matched load. Thus:
                    >
                    > Vrcvr = E * he
                    >
                    > where
                    >
                    > Vrcvr = is rf potential delivered into matched load, Volts or
                    microvolts
                    > E = rf field intensity, Volts per meter or microvolts per meter, and
                    > he = lambda/(2*pi) = effective height, meters
                    >
                    > In this case, he = 48 meters. If your receiver has a sensitivity
                    of one
                    > microvolt, that means you can receive a signal strength of 1/48
                    > microvolt/meter, which is very good indeed.
                    >
                    > However, in order to do this your receiver would have to have very
                    good
                    > front end filtering to eliminate responses to stronger signals at
                    nearby
                    > frequencies, and of course you would have to be somewhere where no
                    other
                    > signal at the same frequency will overpower the signal of
                    interest. There
                    > is some lower level of "galactic" noise below which there is no
                    point in
                    > trying to improve sensitivity, because the background noise will
                    swamp
                    > everything out. I think that is where we are at with this design.
                    This is
                    > why a tuned loop would be ever so much more practical.
                    >
                    > In contrast to that, I have a very sensitive VLF (14 - 30 kHz)
                    receiver used
                    > on a Polaris class submarine dating from the 1960s. It has a
                    sensitivity of
                    > -153 dBm or 46 dB below one microvolt (1/200 microvolt). That
                    sensitivity
                    > is unusable on land - it was designed for an underwater antenna.
                    Because
                    > sea water is conductive and attenuates rf, the antenna operates in
                    an rf
                    > pristine environment where you can use all the sensitivity you can
                    get.


                    Thanks to everyone for your reply. Greatly appreciated.
                    Danny
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