receive antennas only

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KB9MZ, Mar 8, 2011.

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  1. W8JI

    W8JI Ham Member QRZ Page

    Right.

    That's a single band deal.

    Most loops are single band. There are ways to broadband them, but commonly they are tuned at the loop. They are not for transmitting unless specially designed for transmitting.

    73 Tom
     
  2. WV6U

    WV6U Premium Subscriber QRZ Page

    I am looking to use it only as a receive antenna. So even with RX, it is intended to be only a single bander?
     
  3. W0BTU

    W0BTU Ham Member QRZ Page

    I've never seen a multi-band loop design. Mine was for 75/80 meters.

    I first used the loop next to my operating position, so I could turn it manually. I soon noticed that every signal seemed to null in the same direction. Turns out, it was too close to my 70' tower (~15').

    I eventually moved it to the garage, farther away from both the tower and my ham neighbor across the street.

    At the time, I operated 100 watts on 80 CW while he ran a KW on 75 phone. And since the ends of our 80 meter dipoles were attached to the same power pole, I had real difficulty hearing the stations at the other end of the QSO while he was on the air using my inverted-V. But listening on the loop instead of the dipole, I didn't have any problems at all hearing the other stations I was working, only 100 to 200 kHz away from my neighbor.

    Seeing N3OX's design, I remember that my coupling loop was WAY too close to the main loop.

    If I ever used loops again, it would be with some kind of configuration that provided a unidirectional pattern. Either an additional loop or a sense antenna could do that.
     
  4. WV6U

    WV6U Premium Subscriber QRZ Page

  5. W5DXP

    W5DXP Ham Member QRZ Page

    You guys need to describe what kind of "loop" you are talking about in context because those blanket statements are confusing the readers. Many hams use ladder-line fed full-wave horizontal loops as multi-band antennas for harmonically related bands.
     
  6. WV6U

    WV6U Premium Subscriber QRZ Page

    I am talking about small loops like ones described in the links I posted.
     
  7. W0BTU

    W0BTU Ham Member QRZ Page

    I understand that if someone started reading at a certain point, they could be confused. We are talking about receive-only antennas (which is the subject of this thread. :)

    In this context, one- or two-turn loops are basically inductors, which are tuned to resonance with a variable capacitor. If the loop size was right, and that capacitor had enough range, it could be used on more than one band.

    I've heard of untuned loops, but I know nothing about them. They may not even be practical.
     
  8. WV6U

    WV6U Premium Subscriber QRZ Page

    I follow you up to this point. Let's say we build a loop with 3/8" copper tubing with a diameter of 2' and our we want a range of 7 to 30Mhz. Using the loop calculator here:
    http://bit.ly/ki4ffD

    At 7Mhz, we need ~320pF and at 30Mhz we need ~17pF. Certainly a range that can be covered by available trimmers. Next, we need to match impedance to our ~50ohms coax feed line. And, my question is how do we match impedance across the range?
     
  9. W0BTU

    W0BTU Ham Member QRZ Page

    What I did was add a second, smaller loop, inside the main one. This loop was connected to the receiver in series with a variable capacitor.

    Keep in mind that impedance matching is not near as important for receiving antennas as it is for transmitting. This is why random length end-fed wires pick up HF signals as well as they do.

    In any case, I think N3OX and W8JI cover matching methods on the links above. Dan uses a broadband transformer. I would probably try one using a BN-73-202 core with fewer turns.
     
    Last edited: May 7, 2011
  10. WV6U

    WV6U Premium Subscriber QRZ Page

    This is something I have a hard time wrapping my head around. From theory we need know that power transfer will not be efficient if impedance is mismatched so the RX antenna might sound quieter but that's because signal is being lost in mismatch. From practice, we know that even on a multi-band TX antenna, a station comes across better when you kick in the tuner and bring the SWR to lowest possible. To my mind, the only difference between a good RX antenna and a TX antenna would be that RX antenna components won't be able to handle TX power and radiation resistance would be low. Other than that, everything else should be the same.
     
    Last edited: May 7, 2011
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