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40 Metre Delta Loop on 80 metres Severe RF on audio

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  • david_oxon
    I have a 40 metre full wave delta loop antenna. The apex of the triangle is 11 metres high and the base is 2 metres high. I feed it approx 2 metres from one
    Message 1 of 3 , Jun 6, 2010
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      I have a 40 metre full wave delta loop antenna. The apex of the triangle is 11 metres high and the base is 2 metres high. I feed it approx 2 metres from one of the base corners on the upward sloping side of the antenna with 50 ohm coax via a 4:1 balun. My rig is the Yaesu FT-950. The antenna works very well on 40, 20 & 10 metres. However, when I try to use it on 80 metres I get severe RF on my audio. I have tried the following all to no avail:
      • 6 turns of the coax about 150 mm in diameter close to the balun.

      • Using a large loading coil in the middle of the base of the antenna with several taps to try to resonate the antenna on 80 metres.

      • Using my Yaesu FC-301 10 to 160 metre selectable ATU.

      Any suggestions please?
      Best 73
      David
      G3SBM
    • M HOLDEN
      Hello David. It is highly unlikely that the 4:1 balun is seeing a suitable impedance on it s output side. Ideally it is looking for 200 ohms to match in the
      Message 2 of 3 , Jun 7, 2010
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        Hello David. It is highly unlikely that the 4:1 balun is seeing a suitable impedance on it's output side. Ideally it is looking for 200 ohms to match in the 4:1 ratio. The quick solution could be to break the loop at about 63 feet from the feedpoint  and use it with the other section as counterpoise. You would then close the break to resume ops on 40 -10M. The balun is not wanted on 80M.
        Mike G4HOL

        --- On Sun, 6/6/10, david_oxon <david_oxon@...> wrote:

        From: david_oxon <david_oxon@...>
        Subject: [loopantennas] 40 Metre Delta Loop on 80 metres Severe RF on audio
        To: loopantennas@yahoogroups.com
        Date: Sunday, 6 June, 2010, 21:29







         









        I have a 40 metre full wave delta loop antenna. The apex of the triangle is 11 metres high and the base is 2 metres high. I feed it approx 2 metres from one of the base corners on the upward sloping side of the antenna with 50 ohm coax via a 4:1 balun. My rig is the Yaesu FT-950. The antenna works very well on 40, 20 & 10 metres. However, when I try to use it on 80 metres I get severe RF on my audio. I have tried the following all to no avail:

        • 6 turns of the coax about 150 mm in diameter close to the balun.



        • Using a large loading coil in the middle of the base of the antenna with several taps to try to resonate the antenna on 80 metres.



        • Using my Yaesu FC-301 10 to 160 metre selectable ATU.



        Any suggestions please?

        Best 73

        David

        G3SBM






















        [Non-text portions of this message have been removed]
      • david_oxon
        Hi Mike, and thank you very much for your very quick response to my plea for help including your comments and suggestion. I will give it a try. Just a thought,
        Message 3 of 3 , Jun 7, 2010
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          Hi Mike, and thank you very much for your very quick response to my plea for help including your comments and suggestion. I will give it a try. Just a thought, I also have a 1:1 balun in my junk box. would this be worth trying for 80 metres albeit the antenna might not be ok on 40 metres then. I guess it would be looking for 50 ohms which is closer to the normal 100 ohms associated with a delta loop antenna--I think. Sorry my antenna theory is not brilliant.

          73

          David
          G3SBM

          --- In loopantennas@yahoogroups.com, M HOLDEN <mholden909@...> wrote:
          >
          > Hello David. It is highly unlikely that the 4:1 balun is seeing a suitable impedance on it's output side. Ideally it is looking for 200 ohms to match in the 4:1 ratio. The quick solution could be to break the loop at about 63 feet from the feedpoint  and use it with the other section as counterpoise. You would then close the break to resume ops on 40 -10M. The balun is not wanted on 80M.
          > Mike G4HOL
          >
          > --- On Sun, 6/6/10, david_oxon <david_oxon@...> wrote:
          >
          > From: david_oxon <david_oxon@...>
          > Subject: [loopantennas] 40 Metre Delta Loop on 80 metres Severe RF on audio
          > To: loopantennas@yahoogroups.com
          > Date: Sunday, 6 June, 2010, 21:29
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          > I have a 40 metre full wave delta loop antenna. The apex of the triangle is 11 metres high and the base is 2 metres high. I feed it approx 2 metres from one of the base corners on the upward sloping side of the antenna with 50 ohm coax via a 4:1 balun. My rig is the Yaesu FT-950. The antenna works very well on 40, 20 & 10 metres. However, when I try to use it on 80 metres I get severe RF on my audio. I have tried the following all to no avail:
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          > • 6 turns of the coax about 150 mm in diameter close to the balun.
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          > • Using a large loading coil in the middle of the base of the antenna with several taps to try to resonate the antenna on 80 metres.
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          > • Using my Yaesu FC-301 10 to 160 metre selectable ATU.
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          > Best 73
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          > David
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          > G3SBM
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