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DDRR Antenna

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by MW1CFN, Aug 1, 2013.

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  1. MW1CFN

    MW1CFN Ham Member

    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. G0VKT

    G0VKT Ham Member

    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. WX7G

    WX7G Ham Member

    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. KL7AJ

    KL7AJ Ham Member

    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
     
  5. MW1CFN

    MW1CFN Ham Member

    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. KO6WB

    KO6WB Premium Subscriber

    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. WB5WPA

    WB5WPA Ham Member

    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. WB5WPA

    WB5WPA Ham Member

    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. KL7AJ

    KL7AJ Ham Member

    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
     
  10. KL7AJ

    KL7AJ Ham Member

    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
     
  11. KL7AJ

    KL7AJ Ham Member

    I just felt compelled to read Dome's analysis again...after about 30 years....and noticed another point of great importance. The CURRENTS flowing through the tuning capacitor lead are in the OPPOSITE direction of the other vertical member, and as such REDUCE the radiated field in proportion to their currents. The conclusion is this....if you can POSSIBLY avoid the use of the tuning capacitor, do so. This can be done by making the loop a little bigger than it needs to be and then pruning it to your desired frequency. If you need to make a frequency agile antenna, it might be better to work on some kind of "trombone" section to change the length of the loop conveniently.
    If you're interested in making a true contribution to the radio art, you might do some actual field tests and find out how much improvement you can really make by avoiding the tuning capacitor! I'm sure some folks would be interested in learning this (like me!)

    Eric
     
  12. K8JD

    K8JD Ham Member

    What is it ?

    I recall going through this in an older post/thread.
    NO it is not a short vertical with a non radiating top loop.
    It is a skeleton, horizontal half slot antenna.
    It is a vertically polarized antenna. it's radiation is roughly equivalent to a quarterwave vertical element.
    Instead of an E-field design it is an H-field antenna.
    There are commercial VHF and UHF versions widely used in applications where low clearance is a problem.
    Look up SLOT ANTENNAS
     
  13. WX7G

    WX7G Ham Member

    The half wavelength version is not really a half wavelength of wire. It is a short vertical wire with a horizontal length of wire configured as a circle. As such it is a short vertical wire with two horizontal 1/4 wave length wires.

    One way to think of the DDRR is by using image theory. Picture it as being in free space with a short vertical wire driven in the middle connected to two parallel horizontal wires. The magnetic field from the vertical section is unopposed while the magnetic fields of the two horizontal wires cancel almost completely. Therefore the vertical section radiates while the horizontal section does not.
     
  14. KL7AJ

    KL7AJ Ham Member

    Good analogy.
    By the way, I didn't mean to suggest above that the transmission line analogy is WRONG....just that it's unnecessary...and unnecessarily complicated. It can be thought of as a "leaky slot" as well....the important factor is that the horizontal component is nearly non-existent with its normal proportions. :)

    Eric
     
  15. KL7AJ

    KL7AJ Ham Member

    Another thing to experiment with:

    It's conventional wisdom that a DDRR needs to be made FAT to reduce IR losses to a tolerable level, as is the case with any primarily H-mode antenna. However, I SUSPECT, it may not be necessary to make the WHOLE radiator fat...just the parts with the maximum current, that is, the vertical section, and maybe the first few feet of horizontal ring.

    Someone would do well to build a "fat" DDRR and a skinny one with just a fat vertical segment and see if there's a measurable difference in field strength.

    Eric
     
  16. WX7G

    WX7G Ham Member

    Why build and test a DDRR when a NEC simulation will perfectly results?

    NEC has been proven over and over again against physical antennas. The limitations of NEC are known and the DDRR does not approach the limits of NEC.
     
  17. WX7G

    WX7G Ham Member

    In fact this antenna can be analyzed using paper and pencil; it's a trivial exercise. Given the vertical and horizontal dimensions and the wire diameter and material type the radiation efficiency can be calculated.
     
  18. KL7AJ

    KL7AJ Ham Member

    Nec will give EXCELLENT results but not PERFECT results. One thing NEC2 does NOT do very well is model resistive losses of elements as a function of their diameter.

    NEC 4 is much better at this kind of thing, but not readily available to many hams.

    Eric
     
  19. WA7KKP

    WA7KKP Ham Member

    I was curious about these many years ago, and remember an article of a DDRR on 80 and 40 meters. The ham built it from exhaust pipe, made into a hexagon. A vacuum variable cap for tuning was required (very high RF voltages). Bandwidth was quite narrow, requiring a remote-controlled motor to operate the vac variable. He also used chickenwire as a groundscreen/counterpoise, which is NECESSARY for efficient operation.

    And I remember seeing a mag-mount DDRR for 2 meters . . . cute little thing about 6 inches in diameter. Made for a very low profile antenna. I thought about making one for 6 and/or 10 meters . . . .

    Gary WA7KKP
     
  20. KL7AJ

    KL7AJ Ham Member

    Yep...the exhaust pipe construction was standard for the DDRR.
     
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