Parallel Dipoles in the Attic

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by WD9DWT, Jun 1, 2021.

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  1. WD9DWT

    WD9DWT Ham Member QRZ Page

    Thank you so much for the explanation and the procedure. But what if I can't find a frequency on the 20 meter antenna that has a purely zero reactance? The frequency with the least reactance is about 10 ohms with an impedance of about 4 ohms ohms. Would that be resonance? Do I need to find a balun to transform that impedance to the feedline impedance?
     
  2. WD9DWT

    WD9DWT Ham Member QRZ Page

    You raise a very good point. I should terminate the antenna I am not analyzing.
     
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  3. WD9DWT

    WD9DWT Ham Member QRZ Page

    That would be a great alternative! Thanks for the suggestion. But I guess I'm kind of a purist. I'm just not convinced about the idea of using an antenna tuner at the radio-end of the feedline because it makes the feedline part of the antenna system which affects efficiency and causes near field effects.
     
    AK5B likes this.
  4. N3DT

    N3DT Ham Member QRZ Page

    Disconnect the 20M antenna, set it aside. Get the 40M one working again. Now just space your 20M wire away from the 40M wire by about 1.5-2". Don't connect the 20M wire to the 40M wire, just a 20M half wave wire spaced from the 40M antenna by 1.5-2". Now you will have both bands on the one piece of coax. It's called 'coupled resonator' in the ARRL 2nd Ed. of the Antenna handbook chapter 7. There is very little if any interaction between the 2 antennas unlike the fan. I have 2 multiband 'CR' antennas. One is 80M with coupled 17 and 20M and another 40M antenna with coupled 30, 12 and 10 with the whiskers on the 40M dipole to bring the 15M band down where it belongs. There is some interaction on the 12/10M wires.
     
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  5. WD9DWT

    WD9DWT Ham Member QRZ Page

    What a fantastic idea! I will try that. (In my Antenna Book 23rd ED. it's in chapter 10 section 6 - a great read!)
     
    Last edited: Jun 2, 2021
  6. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    Double post. Please see below.
     
    Last edited: Jun 2, 2021
  7. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    OP should definitely try this. However:

    So there may not be sufficient coupling between the hamstick element and the 1/4 wave 20m element on that side. OP will just have to try it and see.

    Please, OP if you do try it, report back here and tell us how it works. I don't know if anyone has ever tried to couple a full-size wire with a hamstick before and shared the result.
     
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  8. WD9DWT

    WD9DWT Ham Member QRZ Page

    To clarify, the frequency of least reactance yields an impedance of 4 +j10 on the 20m dipole.
     
  9. WB3BEL

    WB3BEL Ham Member QRZ Page

    If you are not measuring the impedance right at the antenna, or correcting for the transmission line impedance transformation, then you are looking at the wrong thing.

    If you add a different length of transmission line, you will find that the frequency of resonance of the system has changed.

    Fixation on resonance (near zero reactance) is a mistake for most simple antenna systems.

    What you really want to look at is the frequency range where the SWR is lowest. When your SWR is good enough that the system losses are minimal and your transceiver can match it, then you really are probably close enough. The reactance can provide clues on how to adjust an antenna, but you can't measure it in a random way.

    I would not recommend using a hamstick-in-attic antenna. This is a recipe for poor performance. These antennas are mediocre when used on mobile stations operating above 14 MHz. They are less than mediocre when used on mobile stations below 14 MHz.

    Bend the antenna wire to fit in the attic. Try to keep the wires near the feedpoint as straight as possible for the longest distance possible before bending them.
    Keep it away from metal objects. Put a good choke at the antenna feedpoint.
     
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  10. WD9DWT

    WD9DWT Ham Member QRZ Page

    I am measuring it at the antenna - no balun, no transmission line.
     

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