Shared Feed Point Delta Loop Array

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by N7BB, Oct 14, 2018.

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  1. N7BB

    N7BB Premium Subscriber QRZ Page

    Has anyone ever tried feeding two identical Delta loop antennas from the same feed point?
    I've seen many plans to construct wire beams, whereas only the driven element is fed.
    But, if two delta loops are fed from the same apex with a 4:1 balun, and both sharing the same apex for the single mast support, would both antennas exhibit the same feed point impedance? Could this be as simple as the fan or bow tie dipole? Or, would I have interaction and feed point impedance problems? Again, these are identical loops. (I've seen different sized loops fed this way for multi banding). I would appreciate any feedback.
     
  2. AF7ON

    AF7ON Ham Member QRZ Page

    I've used double-diamond and double-delta loops on everything from HF to VHF - basically vertically stacked loops fed at the common point. An antenna I built and wrote about some years ago was a two-element double diamond (four one-wavelength loops in a square formation) in front of a wire mesh reflector for two meters. I'm not sure from your description if this is the configuration you're asking about.

    Mike
    AF7ON
     
  3. WA4SIX

    WA4SIX Ham Member QRZ Page

    What about doing it as an array, using the feedline between them to phase them?

    Ed
     
  4. KK5JY

    KK5JY Ham Member QRZ Page

    The arrangement you are describing has been used for feeding pairs of dipoles and inverted V antennas. So I wouldn't expect the loops to be any different, assuming they are for different bands.

    Just for fun, I modeled a couple of apex-fed vertical loops in EZ-NEC, at right angles to each other:

    upload_2018-10-14_20-41-24.png

    The SWR sweep did produce two dips near the expected frequencies at 200Ω (the high-side of a coax-fed 4:1 transformer). That said, they do have a bit of influence on each other in the model. The feed point impedance changes slightly on both bands, when compared to individual loops alone, as does the resonant frequency. That just means that you'll probably want to tune the loops together, with both connected as you adjust each one. My guess is that on any given band, the other loop acts as an unwanted/unintentional hairpin, which causes the Z value of the on-band loop to drop a bit.

    But at least EZ-NEC doesn't see any major problems with what you describe. You should try it and then return here and share your results. :)
     
  5. N7BB

    N7BB Premium Subscriber QRZ Page

    Thank you so much for modeling that! I do want to try this; it seems as if there will be a bit of pruning to be done - more so than with a single loop.
     
  6. N7BB

    N7BB Premium Subscriber QRZ Page

    This is a GREAT idea; I suspect one feed line would have to be a bit longer than the other? It seems I need to read up on phasing again. I've forgotten the basics.
     
    KA2RRK likes this.
  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    Didn't the OP want the two loops to be identical?

    What is the goal? If directivity, then would you not have to switch feeds, or switch the phase of the delay line if feeding both?
     
  8. KK5JY

    KK5JY Ham Member QRZ Page

    I can't see why, given that the apexes were shared. A shared apex loop pair only makes sense for putting two loops for different bands within the same space, sharing a support. If identical loops are installed that way, they aren't really a pair of loops anymore -- they form a sort of "cage" loop.
     
  9. KA2RRK

    KA2RRK Platinum Subscriber Platinum Subscriber QRZ Page

    Bi-Quad array:


    Deltas should work also.

    KA2RRK
     
  10. WA4SIX

    WA4SIX Ham Member QRZ Page

    There are many different phasing systems.
    What is the actual goal? Is it directivity for 1 band, or are you using 2 different bands?

    Ed
     

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