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Thread: DDRR Antenna

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  1. #1

    Default DDRR Antenna

    First of all, I'm intent on discussing HALF wave DDRR antennas, not the quareter wave version which is usually examined.

    I took my insipiration for the antenna from Les Moxon's excellent "HF Antennas for All Locations", which details Italian experiments with the half lambda DDRR from several decades ago.

    I found this image which shows, on the right hand side, the antenna I mention which, usefully, doesn't appear to require a high voltage capacitor.

    9zBAl.jpg

    I have thrown together a 20m wire version of the antenna, accepting it may have significant losses, and run it on WSPR against others known to be running high-quality experimental small loops. I've used a second ring for the first experiment. The lower ring is only at 1m, strung on fenceposts. The results are not at all bad, though there is clearly room for improvement, notably the use of wide bore tubing instead of wire. RF measurements show a pattern with a strong, very low angle vertical radiation, falling away above about 20 degrees, and then with a narrow, very high angle peak above the antenna.

    I wonder if there are any ops out there who have experiemented long before I came to scratch my head about a DDRR, and what were your findings and experiences? If you want to be abusive or ask why I don't buy a SteppIR, I'm not interested, thanks all the same...

  2. #2

    Default

    I had not heard of a half wave version. But I have the book you mention so I will look it up. I am interested in knowing the differences between that and a 1/4 wave version.

  3. #3

    Default

    The DDRR is essentially a short vertical wire with a horizontal non-radiating top loading wire; an inverted-L.

    I have not experimented with the DDRR but have built several very short monopoles around 0.02 wavelength tall. Examples are a 9" 28 MHz top loaded monpole and a 12' 1.8 MHz monopole.

    NEC simulation with loss added so it and the real antenna exhibit the same VSWR bandwidth will tell you the antenna efficiency. If you prefer real-world gain measurements a reference antenna can be used. A 1/4 wavelength vertical having a few dozen 1/8 wavelength (or longer) radials will work. Repeated A-B receive measurements made within seconds and plotted enable one to compare gain within 1 dB.

  4. #4
    Join Date
    Mar 2003
    Location
    3763 Lyle Avenue, North Pole, AK 99705
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    23,061

    Default

    Quote Originally Posted by MW1CFN View Post
    First of all, I'm intent on discussing HALF wave DDRR antennas, not the quareter wave version which is usually examined.

    I took my insipiration for the antenna from Les Moxon's excellent "HF Antennas for All Locations", which details Italian experiments with the half lambda DDRR from several decades ago.

    I found this image which shows, on the right hand side, the antenna I mention which, usefully, doesn't appear to require a high voltage capacitor.

    9zBAl.jpg

    I have thrown together a 20m wire version of the antenna, accepting it may have significant losses, and run it on WSPR against others known to be running high-quality experimental small loops. I've used a second ring for the first experiment. The lower ring is only at 1m, strung on fenceposts. The results are not at all bad, though there is clearly room for improvement, notably the use of wide bore tubing instead of wire. RF measurements show a pattern with a strong, very low angle vertical radiation, falling away above about 20 degrees, and then with a narrow, very high angle peak above the antenna.

    I wonder if there are any ops out there who have experiemented long before I came to scratch my head about a DDRR, and what were your findings and experiences? If you want to be abusive or ask why I don't buy a SteppIR, I'm not interested, thanks all the same...
    Hi John:
    I have a lot of experience with the "conventional" DDRR on 160 meters. The DDRR is best considered a CURRENT mode antenna; the vast majority of the radiation results from the very short vertical member, the radiation of which is vertically polarized. As such, you need a LOT of current in this first few feet of conductor, which dictates a 1/4 wave antenna. Undoubtedly the 1/2 wave version would have DRAMATICALLY less current in the vertical section, where it matters most.

    To properly evaluate the performance of the DDRR, you need to look at just the vertical component of radiation, best measured on the ground in the "near far-field", perhaps ten wavelengths away with a DECIDEDLY vertical receive antenna. I'm not a betting man, but I would be willing to place money on the assertion that the 1/2 wave version will be DRASTICALLY worse by this criteria.

    Eric
    "The more you know, the less you don't know."

  5. #5

    Default

    Thanks for all the constructive, thought-provoking ideas, and without dismissing anyone, especially Eric for his insight into the current at play. It's clearly worth me getting hold of a decent capacitor and trying out the quarter wave version. Ah, more garden space given over to antennas...

  6. #6
    Join Date
    Feb 2004
    Location
    Bakersfield, CA
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    Default

    I'm certain you have already come upon most of the sites that detail the DDRR but for those that haven't here is some good information; http://www.orionmicro.com/ant/ddrr/ddrr1.htm.
    It's a two part article full of information. Hope this helps a bit...
    Have fun
    73
    Gary

  7. #7

    Default

    To: MW1CFN

    Is the intent to create current strictly in the loop, and if so, what part does connecting it to ground play?

    If the intent is to create currents in the loop via E-fields to ground, then the 'loop' looks more like a wide conductor worked against ground ... is that the intent?

    Also, the diagram is not clear, but your words seem to state the majority of your loop is horizontal - is that correct?



    de Jim WB5WPA

  8. #8

    Default

    Quote Originally Posted by MW1CFN View Post
    Thanks for all the constructive, thought-provoking ideas, and without dismissing anyone, especially Eric for his insight into the current at play. It's clearly worth me getting hold of a decent capacitor and trying out the quarter wave version. Ah, more garden space given over to antennas...
    PS. A word to the wise - I use about a 3 foot length of 1 5/8" Heliax on a QW vertical loop for a 160m at the 100W power level w/no arcing. If you're hard pressed a 100 pF air variable with reasonable spacing (a little more than BCB variable spacing) will allow 100 W w/o arcing on such a loop, but the change in Q is noticeable going from the mechanical var cap to the Heliax Cap.

    de Jim WB5WPA

  9. #9
    Join Date
    Mar 2003
    Location
    3763 Lyle Avenue, North Pole, AK 99705
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    Default

    Quote Originally Posted by WB5WPA View Post
    To: MW1CFN

    Is the intent to create current strictly in the loop, and if so, what part does connecting it to ground play?

    If the intent is to create currents in the loop via E-fields to ground, then the 'loop' looks more like a wide conductor worked against ground ... is that the intent?

    Also, the diagram is not clear, but your words seem to state the majority of your loop is horizontal - is that correct?



    de Jim WB5WPA
    Hi Jim:
    Yes the loop is horizontal, and being so close to the ground, the radiation resistance is extremely low (for any type of DDRR). Although Robert Dome did an extensive analysis of the DDRR, describing it in some exotic waveguide analogies, in reality, it's nothing but a VERY SHORT vertical, and subject to all the restrictions and requirements thereof....such as using FAT conductors to reduce losses.

    Eric
    "The more you know, the less you don't know."

  10. #10
    Join Date
    Mar 2003
    Location
    3763 Lyle Avenue, North Pole, AK 99705
    Posts
    23,061

    Default

    Quote Originally Posted by WB5WPA View Post
    To: MW1CFN

    Is the intent to create current strictly in the loop, and if so, what part does connecting it to ground play?

    If the intent is to create currents in the loop via E-fields to ground, then the 'loop' looks more like a wide conductor worked against ground ... is that the intent?

    Also, the diagram is not clear, but your words seem to state the majority of your loop is horizontal - is that correct?



    de Jim WB5WPA
    Hi Jim:
    Yes the loop is horizontal, and being so close to the ground, the radiation resistance is extremely low (for any type of DDRR). Although Robert Dome did an extensive analysis of the DDRR, describing it in some exotic waveguide analogies, in reality, it's nothing but a VERY SHORT vertical, and subject to all the restrictions and requirements thereof....such as using FAT conductors to reduce losses.

    Eric
    "The more you know, the less you don't know."

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