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Mulitiband Loop with traps?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K2CRG, Feb 18, 2012.

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  1. K2CRG

    K2CRG Ham Member QRZ Page

    I have always had good success with full-wave loops for NVIS and I appreciate the low noise characteristics of loops. And since I do more listening than transmitting, they are a natural fit....

    A few weeks ago, I was inspired by an article written by WB4DFW for a multi-band, delta loop using home-brew traps. I am now looking to do something similar and build a horizontal, multi-band, 80m long, loop with traps at 60 and 40 meters. My inspiration knows bounds, however, :D and I am looking to use Unadilla traps in building this antenna. I'm wondering if these traps will work for a loop antenna and will my multi-band retain nice vertical take-off characteristics.

    I know I can't be the first to try this, but my google-fu is coming up short. I have modeled my past antennas in 4NEC2, but I'm not sure how to model the traps for the current project.

    My previous modeling has shown - as has others - that the 80m loop will become a great Dx antenna at 40m with a little tuning, but that's not what I'm after. I want to keep natural resonance, or close to it, in the hopes that I'll keep that great vertical, omni-directional, cloud-burning lobe at 60 and 40 meters by putting those traps in!
     
  2. N0AZZ

    N0AZZ Ham Member QRZ Page

    When i start looking at the low bands for antennas I first go to Low Band DXing then W8JI's website and study the ARRL Antenna Book for what answers I can find. If I still come up short (not often) start doing Internet searches with several topic lines. There is a site that has nothing but traps building and modeling that I have came across sometime but can't remember where.

    Have used several different loops I do like them a good antenna but all were full size and most for 160m and used for all bands.
     
  3. G3TXQ

    G3TXQ Ham Member QRZ Page

    I'm not really sure what you are wanting to do; but if it is to maintain the same current distribution around a loop that you would get on its fundamental frequency, you are not going to achieve it with a couple of conventional traps.

    You need distributed capacitive loading around the loop to make the wires look "shorter" at the higher frequency; for example if you inserted twelve 47pF capacitors equally-spaced around an 80m full-wave loop, you would get a current distribution on 40m that is close to what it would be on 80m without the capacitors. So, what you would need is a set of distributed loads that add little impedance at the lower frequency, and that add capacitive reactance at the higher frequency. Not trivial!

    73,
    Steve G3TXQ
     
  4. K2CRG

    K2CRG Ham Member QRZ Page

    Thanks for the input. I have decided to build an 80m loop and then a multi-band (60/40) dipole for my purposes.

    73,

    Jeff
     
  5. N7WR

    N7WR Subscriber QRZ Page

    I'm not sure why you think traps are necessary. I have a one wavelength 80 meter horizontal loop, square, only up 30 feet. SWR on half of the 75/80 meter band is less than 1.5:1 though it is obviously a NVIS antenna on that band. SWR on every ham band 40-6 meters is less than 1.5:1 across each band and it works great. Modeled it on EZNEC and some of the gain figures are pretty impressive
     
  6. W8JI

    W8JI Ham Member QRZ Page

    Where is that article at?

    I can't think of any way to build a trapped multiple band delta loop.
     
  7. G3TXQ

    G3TXQ Ham Member QRZ Page

    If you've modelled it in EZNEC you'll see that it has an "overhead null" when operated on 40m. Look at the OP's final sentence and you'll see that he had a legitimate concern

    73,
    Steve G3TXQ
     
  8. K2CRG

    K2CRG Ham Member QRZ Page

    Right...I too have a full length 80m loop already set set-up. I can operate 40m no problem...it's just that I'm talking to Italy, not Rochester.:)


    @W8JI: I'm not sure when the original article was written, but if I'm not mistaken it originally appeared in QST. I came across it in my copy of ARRL's "More Wire Antenna Classics". The "Classics" series is a great addition to their Antenna Handbook, IMO.
     
  9. G3TXQ

    G3TXQ Ham Member QRZ Page

    Tom,

    It's worth reading, just for laughs on a Sunday evening!

    The author has a vertical delta loop - apex up; it's approximately a full-wave on 75m. He feeds it in the centre of the bottom horizontal leg, which is 70ft long.

    Then he places a pair of 40m traps 30ft either side of the feedpoint along that bottom leg, and a pair of 30m traps 23ft either side of the feedpoint. Bingo - the "loop" is now resonant on 75m, 40m and 30m!

    To be fair, the loop is up pretty high - the horizontal base leg is at 40ft - so the trap dipole he is actually using on 40m and 30m is at reasonable height.

    To the OP: you could certainly do this with your horizontal loop and retain the NVIS characteristics, but don't kid yourself you are using a loop, except on the lowest frequency band!

    73,
    Steve G3TXQ
     
  10. WA6TXU

    WA6TXU Ham Member QRZ Page

    I have an 80 mtr horizontal loop up at about 40 ft (actually it is a wavelength and a half on 80 mtrs). It is 400 feet long. Just happens that my tree location dictated the extra 130 feet of wire. I get great DX on 40 and pretty good local coverage on 40 also. Best of both worlds. I don't know if the extra 130 ft (almost 1/2 wavelength on 80) is helping the local coverage on 40 or hurting it. Just know that the horizontal loop has been great for me for both local coverage or DX. Dave. WA6TXU.
     
  11. G3TXQ

    G3TXQ Ham Member QRZ Page

    It removes the null on 40m.

    On my EZNEC loop model that extra length improves the 40m overhead signal by 17dB!!

    73,
    Steve G3TXQ
     
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