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Trap dipole performance vs no traps

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K3RW, Oct 17, 2017.

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  1. K3RW

    K3RW Ham Member QRZ Page

    I have a 10-40m trap dipole I built myself, using Unadilla traps. The last leg is 40m with no trap on the end--so its using a 10, 15, and 20m trap per side. Performance seems okay for what I'm using so far.

    If all else is equal, how does this system compare to a fan dipole, monoband dipoles for these bands, or a 40m doublet?
  2. G3YRO

    G3YRO Ham Member QRZ Page

    It obviously depends on the design of the Traps . . .

    On 40m it's using all 3 traps in each leg as slight loading coils, so will be slightly less efficient than a full-sized 40m dipole. (how much shorter is it?)

    On 20m it's using 2 traps . . . etc etc.

    The difference between using Traps and just having a 40m Doublet, is that with traps on each band it's a resonant half-wave dipole, with the classic waveform. A 40m Doublet will have multiple lobes on the higher bands, with big peaks and nulls in different directions. (and you'd obviously need a balanced ATU)

    Having separate dipoles on the same feeder (in a Fan arrangement) is also a compromise, as they will always interact in practice.

    I personally prefer to use a W3DZZ trap dipole, as it gives me a good match to coax on 80, 40, 20 15 & 10, but only using 1 pair of traps. (but again, it has lots of lobes on the higher bands)

    It's VERY hard to predict the relative efficiencies of these different antennas in practice, they all have pros and cons . . . that's why you will get LOTS of different answers to this question!

    Roger G3YRO
    K3RW likes this.
  3. K3RW

    K3RW Ham Member QRZ Page

    Thanks for the reply. I realize now its a tough question due to lots of variables. FWIW, the traps are Unadilla/Reyco ones, not coaxial traps.

    I'm not sure if a fan dipole has a different pattern than a standard dipole. I'm irked that WARC band traps are hard to come by (not that I like traps), but particularly that a commercial multiband WARC wire antenna is rare. Looks like I will be building one :eek:

    I didn't realize a doublet had a different pattern. Oops. I see why it does though. Thanks for mentioning that.

    Good heads up on the design you recommended. I've got a pair of 40m traps laying in a box. 80m is pretty long for this lot. Could I do 60m instead of 80, with the 40m traps and still have most of the other bands? The Reyco guide doesn't have any 60m info in it. Their 60m traps keep seeling out.
  4. G3YRO

    G3YRO Ham Member QRZ Page

    Remember that a dipole for 80m with 40m traps will be shorter than normal, as the traps provide slight loading on 80m.

    So the length up to the 40m traps will be normal (and adjusted for resonance), typically around 32' 6" . . . but you'll only need another 20ft or so beyond the traps to resonate it on 80m.

    You could make a 60m dipole with 40m traps . . . but it won't be resonant on the higher bands - that is unique to the W3DZZ design.

    Also - for the W3DZZ to work properly on the higher bands, you can't just use ANY 40m traps. They should really have the correct L to C ratio. (60pF capacitors)

    But you could try the ones you have and see. I previously used commercial traps, but last year made my own, and it's now a much better match on 20m & 10m.

    Roger G3YRO
    K3RW likes this.
  5. N2EY

    N2EY XML Subscriber QRZ Page

    Couple of notes....

    1) The pattern of a typical half-wave wire dipole in free space is the classic "donut" - a single lobe at right angles to the wire. This pattern is also true for dipoles shorter than a half-wave. For longer wires, side lobes begin to appear, and by the time you get to 3/2 wave dipole length, the free-space pattern looks like a cloverleaf. How much this matters depends on the application.

    2) The real-world pattern of a real-world dipole depends on a whole bunch of factors, such as height above ground, type of ground, feedline interaction/radiation, nearby object interaction ("nearby" meaning "within a few wavelengths"), etc. For example, a typical house is full of metal pipes, wiring, ductwork, capping, etc. - all of which interacts.

    3) All traps have loss. Whether the trap loss is significant depends on trap construction. A good indicator is the Q of the trap - the higher the Q, the lower the loss. (All tuners have loss, too - you don't get anything for nothing). The more traps, the more loss, which is why the W3DZZ design has been so popular for so long (more than 60 years!)

    4) Trap dipoles have two advantages: First, they permit the use of coax feed with reasonable SWR around the resonance points. Second, a trap dipole is slightly shorter because of the loading effect of the traps, which can be a big plus for the ham with limited space.

    5) In most real-world cases, a trap dipole will need a bit of adjustment to get the minimum-SWR points right where you want them.
    K3RW likes this.
  6. W1VT

    W1VT Ham Member QRZ Page

    I just put up a trap collinear for 12 and 15 meters. It seems to work well--EZNEC predicts that it has a dB more gain than a dipole after losses are taken out for trap and matching losses.

    Zack W1VT
    Last edited: Oct 17, 2017
    N2EY likes this.
  7. N2EY

    N2EY XML Subscriber QRZ Page

    6) A tip for wire antenna adjustment....trap and otherwise.

    When putting up wire antennas where the length(s) may require adjustment, save yourself a lot of time by investing in some "split bolt" connectors, aka "Burndys" or "bugs".


    They permit you to make a rapid solderless connection that is easily taken apart. Once you have the lengths correct, just make a Western Union splice and solder.

    73 de Jim, N2EY
    K3RW likes this.

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