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Loop-On-Ground Antenna

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AI5DH, Aug 1, 2018.

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  1. K0OKS

    K0OKS Premium Subscriber QRZ Page

    It was not extremely obvious at first, but here is a picture of how they suggest you construct a 1:9 impedance ratio transformer to go from 450 Ohm to 50 Ohm.

    And thanks for the additional link.

    A5C5F617-8F69-42EE-98FD-4D76FB097BB0.jpeg
     
  2. AI5DH

    AI5DH Ham Member QRZ Page

    OK correct me if I am wrong, but I am pretty sure that is an Unun for an end fed wire antenna using an Auto Transformer. Pretty sure you want an Isolation Transformer running Balanced on at least the Loop side, and I run Balanced on the coax side to keep ground out of the circuit. When you reference a circuit to ground sets you up for common mode noise.

    [​IMG]

     
    N0TZU likes this.
  3. K0OKS

    K0OKS Premium Subscriber QRZ Page

    I get the difference, and yes, this is an autotransformer. But it is what they are recommending for a Beverage. Perhaps it is different since a Beverage is grounded at the opposite end.

    My current transformer looks more like a classic, isolated transformer, like your illustration.

    I was going to omit the ground (though my feedline is still currently grounded at the receiver).
     
    N0TZU likes this.
  4. AI5DH

    AI5DH Ham Member QRZ Page

    That makes it a Ground Referenced antenna which means Unbalanced requiring a Unun if you used coax back to shack. The Loop is Balanced, so you would not use a Unun. For a Balanced Loop you want a Isolation Transformer and on the primary you connect the Loop across the primary coil. On the secondary coax side, you can run either Balanced or Unbalanced if you ground one side of the secondary. Recommended to run Balanced on both sides of transformer despite using coax.

    Thinking out loud sure seems using 450 Ohm Ladder Line would make this even easier to interface to a Loop? I can't use it due to obstructions getting outside.
     
  5. K0OKS

    K0OKS Premium Subscriber QRZ Page

    You would think the 450 Ohm would be nice, but you still have to translate it to 50 Ohm at some point to get to the transceiver. I am having very good results blending the Main antenna with the LOG using the MFJ-1026.
     
  6. K0OKS

    K0OKS Premium Subscriber QRZ Page

    FWIW, I made a new transformer with a .87" mix 75 torroid. I used 44 and 11 turns. It works about the same on receive as my mix31 setup. It may be a little bit better. It did save me a more expensive 2.4" Mix 31 torroid.

    Dereck,KF5LJW, earlier you said higher impedance cable was preferred for receive. Can you explain this? I am curious from an academic (and practical) standpoint. Thanks.
     
    KC9UR likes this.
  7. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    With this long discussion and one other one that passed thru earlier, has anyone just measured the impedance of these loops laying on the ground (15' x 15')? With the impedance in hand it would be very easy to just match it. I see a lot of ideas for transformers and types of transmission lines to use. All of this would be easy to nail down knowing the Z. Maybe its been published and is out there already and I missed it. As for what transmission line type and impedance to use, at these frequencies just use what is available and adapt to what you have. If no one has actually measured the feed point of one of these, I'll do it. I did an on the ground 20 meter dipole for a discussion that KL7AJ started. I simply measure the Z looking into the transmission line with its far end shorted to determine its electrical length. Then I remove the short and attach the antenna to be tested and measure the Z again looking into the transmission line. Using a Smith Chart or the program TLW I rotate this Z to out to the feedpoint (knowing the electrical length in wavelength) and that's its Z at the feedpoint. Respectfully, Pete, WB2UAQ
     
    Last edited: Aug 14, 2018
  8. K4SAV

    K4SAV Ham Member QRZ Page

    See this:
    http://forums.qrz.com/index.php?threads/measure-the-impedance-of-a-loop.623487/#post-4759969

    You would think that measuring the impedance of a dipole on the ground would be simple, but it is not. The impedance varies a lot depending on ground moisture content as you measure it at different times. Also unexpected resonances show up and those vary also.

    Jerry, K4SAV

    edit: If you want to see some of my results for measuring a dipole on the ground it is here:
    http://lists.contesting.com/archives/html/Topband/2018-08/msg00073.html
     
    Last edited: Aug 14, 2018
    K0OKS likes this.
  9. AI5DH

    AI5DH Ham Member QRZ Page

    At what frequency and impedance would that be? A 15 x 15 loop would be resonant around 15 MHz (300ish ohms) which I have no intention of using. I want 500 KHz up to 5 or 10 MHz. It is a Broadband antenna and Impedance is all over the place from a few a hundred up to over a 1000 Ohms depending on what frequency you are talking about.
     
  10. G3YRO

    G3YRO Ham Member QRZ Page

    Exactly . . . as I stated previously, the feed impedance will vary WILDLY from band to band.

    But the bottom line is it DOESN'T MATTER if you have a bad match to your coax, or to your receiver !

    You're not transmitting . . . so even if you lost 10dB due to mismatch, so what? The noise will be reduced as much as the signals . . . it's the Signal to Noise ratio you want to be good on this antenna, right?

    Roger G3YRO
     

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