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deformed delta loop

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  • leblanccamloop
    Hi gang, I just had a crazy idea and was wondering if anyone could comment on it. I have been looking into building a 60 foot by 22 foot delta loop for
    Message 1 of 5 , Jul 7, 2013
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      Hi gang,

      I just had a crazy idea and was wondering if anyone could comment on it. I have been looking into building a 60 foot by 22 foot delta loop for primarily LW/MW and from what I've been reading they tend to be good low noise antennas. However I don't need it to be directional and would prefer it to be more non directional. Here comes the crazy idea: Would it be possible to run half the loop in one direction and the other half about 90 degrees from that direction using the apex support point as the pivot point? What effect would this have on the antenna sensitivity, pattern and feed point impedance? The terminating resistor would therefore not be selected to give any deep nulls as this is what I don't want. Would the standard 1.2K ohms be a good starting pojnt or perhaps a bit lower in value? Any advise would be appreciated.

      Thanks
      Cam
    • Andrew Ikin
      Cam, wrote on July 7. Hi gang, I just had a crazy idea and was wondering if anyone could comment on it. I have been looking into building a 60 foot by 22 foot
      Message 2 of 5 , Jul 7, 2013
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        Cam, wrote on July 7.
         
        "Hi gang,

        I just had a crazy idea and was wondering if anyone could comment on it. I have been looking into building a 60 foot by 22 foot delta loop for primarily LW/MW and from what I've been reading they tend to be good low noise antennas. However I don't need it to be directional and would prefer it to be more non directional. Here comes the crazy idea: Would it be possible to run half the loop in one direction and the other half about 90 degrees from that direction using the apex support point as the pivot point? What effect would this have on the antenna sensitivity, pattern and feed point impedance? The terminating resistor would therefore not be selected to give any deep nulls as this is what I don't want. Would the standard 1.2K ohms be a good starting pojnt or perhaps a bit lower in value? Any advise would be appreciated."
         
         
         

        Cam, you will probably end-up with a pattern that where the null is slewed at 45 degrees. An omni-directional pattern is only possible with two orthogonal loops that are combined at 90 degrees. I would recommend that you stick to the normal Delta.

         

        As a guess your loop will have an inductance of 70uH (1.7uH/m), thus the impedance will be approx. 70 Pi Ohms at 500kHz or 220 Ohms. So a simple 4:1 z Balun will do for the 50 Ohm Rx. Make the balun input centre tapped and connected to the feeder to maintain the loop is balanced to reduce the E Field response.

         

        Another point try the keep the loop circumfrence no larger that 0.2 Lambda at the highest frequency.

         

        Kind regards

         

        Andrew Ikin
         
         
         
         
      • Don
        Hi Cam. I am a LW NDB Dxer. You will want directionality but it will somewhat broad in my experience anyway except for a few degrees around 90° to the plane
        Message 3 of 5 , Jul 7, 2013
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          Hi Cam.

          I am a LW NDB Dxer. You will want directionality but it will somewhat
          broad in my experience anyway except for a few degrees around 90° to the
          plane of the antenna. I assume your length is 60ft on the ground. Why
          not make two 30 x 22ft 90° apart? If you are going to DX MW stations I
          would think you would want some directionality.

          My ALA100L's are 30Lx12H (ft), 4ft off the ground rectangles 90° apart.
          I have 4 support masts but you could get by with 3. Use large diameter
          stranded wire like 14 or 12ga (US) or even large gauge twinlead or
          windowlead to increase the surface area of the wire (ends connected
          together). They could be lower but I have mine at 4ft to make it easier
          to cut the grass and what not.

          I'd like to use 1/2in or 3/4in copper or aluminum tubing but it would
          probably get stolen and possibly become an eyesore to some neighbors. I
          painted the masts to match the house color and used black wire.
          Practically invisible from a distance.

          Don KPC6NDB
          Upland, CA
          34.10 -117.63 1250ft
          R-75 w/250Hz 2x R-71A w/250Hz R-70
          Perseus SDR-IQ FUNcube Dongle Pro+
          FRG-100B DX-1000 SR-AF & LPF
          DSP599zx MFJ-784B HD-1418 AF-1
          MSB-1 PA0RDT Mini-Whip x2 @ 20ft
          RYO Active Antenna @ 20ft
          LFL1010 ALA100L x2

          On 07-Jul-13 12:27, leblanccamloop wrote:
          > Hi gang,
          >
          > I just had a crazy idea and was wondering if anyone could comment on it.
          I have been looking into building a 60 foot by 22 foot delta loop for
          primarily LW/MW and from what I've been reading they tend to be good
          low noise antennas. However I don't need it to be directional and would
          prefer it to be more non directional. Here comes the crazy idea: Would
          it be possible to run half the loop in one direction and the other half
          about 90 degrees from that direction using the apex support point as the
          pivot point? What effect would this have on the antenna sensitivity,
          pattern and feed point impedance? The terminating resistor would
          therefore
          not be selected to give any deep nulls as this is what I don't want.
          Would the standard 1.2K ohms be a good starting pojnt or perhaps a bit
          lower in value? Any advise would be appreciated.
          >
          > Thanks
          > Cam
          >
          >
          >
        • Camille LeBlanc
          Thanks Don and Andrew for your replies.  You have both given me food for thought.  I have recalculated my planning and I now think I will build a regular
          Message 4 of 5 , Jul 8, 2013
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            Thanks Don and Andrew for your replies.  You have both given me food for thought.  I have recalculated my planning and I now think I will build a regular form delta 41 feet long by 18 feet high.  This should reduce my old circumference from 134 feet (41m) to around 95.5 feet (29.1m).  It would be built along the back fence and be feed from the ENE end therefore nulling the backside New England states. (I'm in SE New Brunswick, Canada)  This should give me a better shot at TAs. 
             
            Andrew,  I was intregued by the way you calculated the impedance.  What formula did you use.  I would like to recalculate my new design at various frequencies.   I was also planning on rolling my own impedance transformer on a 100mm ferrite rod but from what I've read it is recommended to isolate both windings and not tap a single winding. (perhaps I misunderstood what you were telling me).  I presume the smaller circumference would raise the feedpoint impedance from the 220 ohms you quoted me.
             
            Thanks again gentlemen
            Cam
          • Andrew Ikin
            Carmile, LeBlanc wrote on the 9th July, Andrew, I was intregued by the way you calculated the impedance. What formula did you use. I would like to
            Message 5 of 5 , Jul 9, 2013
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              Carmile, LeBlanc wrote on the 9th July,
               
              "Andrew,  I was intregued by the way you calculated the impedance.  What formula did you use.  I would like to recalculate my new design at various frequencies.   I was also planning on rolling my own impedance transformer on a 100mm ferrite rod but from what I've read it is recommended to isolate both windings and not tap a single winding. (perhaps I misunderstood what you were telling me).  I presume the smaller circumference would raise the feedpoint impedance from the 220 ohms you quoted me."
               

              Carmile, generally speaking, 1m of thin wire has a measured inductance of 1.7uH. The Impedance is 2Pi F x L where 2Pi is 6.28, F MHz and L uH. Hence at 1MHz a 1uH inductor has a reactance of 6.28 Ohms.  I would recommend that the transformer should be constructed using 10 turns turns trifilar  36 SWG (0.2mm) enamelled copper wound an a Fair-Rite twin hole core 2873000302. The antenna side is 2 x 10 turns in series, so you will have a centre tap. The receiver side is the other winding. Note; type 73 ferrite is conductive so you will need use 3mm nylon insulation sleeving in the core. Alternatively you could use Fair-Rite 28430000302 core, this has a much lower AL value ( 2uH per turn based on 10 turns ) but it is non conductive.  If you do end up using a Ferrite rod, then use isolated windings close to each other. Also the ferrite rod needs to housed in screened enclosure. It must be large enough so that the rod is several diameters from the metal work.

               

              A smaller loop circumfrence will reduce the impedance.

               

              Don mentioned the Wellbrook ALA100, this has been replaced by the ALA100LN. It has 8 Fets configured using Gate Source feedback, thus dynamically reducing the Fet Channel resistance and noise figure to a fraction of a dB with low  IMD and high gain.

               

               

               

              Kind regards

               

              Andrew

               
               
               
               
               
               
               
               
               
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