Very long dipole design/effeficacy...

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KF7DM, Oct 8, 2017.

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  1. KF7DM

    KF7DM Ham Member QRZ Page

    I live on a tree-less 20-plus acre lot in southwest Utah (700 feet x 1400 feet). What are pros/cons of putting up a doublet antenna (center-fed at 35-40 feet with homemade ladder line) that can span 600 to 650 feet (end to end)? Can this dipole be multi-band with antenna tuner? I run an ICOM 706MKIIG barefoot. If more pros than cons, where can I find some easy-to-use software or tables to guide me in best end-to-end length?
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    IMO that's way too long.

    A few reasons:

    It might be okay for 160m, where it's 1.15-1.2 WL long and will probably have four lobes -- but at 35-40' above ground, it will be pretty omnidirectional with most of the energy launched "upwards," so probably no real pattern. Good enough.

    But on 80m, where it's >2WL long it will have 6-8 lobes with nulls between. Again, at 35-40' above ground, probably not really distinctive lobes since still most of the energy will go straight up and still be a cloud warmer. So, probably no real issues.

    The problem is on the higher bands, where it's so long it can have 12-16 lobes or more and it will be high enough to have pretty distinct lobes with deep nulls in between them. On the higher bands, most of the energy will not be focused broadside to the wire axis, but more along the wire axis and this can become almost like two beam antennas aimed in opposite directions towards the ends.

    That might be okay if those are the directions you want to work; but if you want to work "everybody," it's not great.

    With that much available property, instead of a doublet I'd install a horizontal loop. That will also have lobes, and more on higher bands; but because it has four sides (or better still, a circle!) the lobes will be in many directions and the nulls won't be as obvious.

    Of course, a loop is ideally circular but a square isn't a bad compromise and only requires four supports.

    If you use ladder line and a good balanced tuner, you should be able to match whatever it is, so overall length isn't really critical -- although knowing the length will allow you to model it and see where the radiation is likely to all go.

    Also, when making "really long" wire antennas, use heavy-gauge copper (or copperweld) to minimize resistive loss. For a very long span, even #12 copper will stretch and possibly break, so #12 copperweld (CCS) or even #10 CCS is probably a good choice.
    NH7RO likes this.
  3. SM0AOM

    SM0AOM Ham Member QRZ Page

    Steve is right. What you essentially is proposing are two long-wire antennas end-to-end.

    On "lower" frequencies, there will be a somewhat predictable radiation pattern with two or three major lobes, but on the higher bands the pattern will break up in a mess of sidelobes almost in-line with the antenna wires.

    There will be a total of at least 8 or 10 lobes with deep nulls in between. The exact direction of these lobes will be very difficult to predict.

    This amount of wire is better utilised in a horisontal loop together with one or two separate inverted-V dipoles from a common support.

    NH7RO likes this.
  4. KF7DM

    KF7DM Ham Member QRZ Page

    Thanks immensely for great information! I am a 73-years (young!) ham and now with much free time I try to find out as much as I can before I put up another antenna on this new property. I have had a 1000-foot rhombic (4 x 250 feet) so the wire is quite stretched out now! I wish to use the wire and poles that support my rhombic into another more efficient and useful antenna. The loop ideas are greatly appreciated -- I will study up on them and post anything that I do not understand. KF7DM
    NH7RO likes this.
  5. KW4TI

    KW4TI Ham Member QRZ Page

    A very long dipole, which is 2+ wavelength on the band of use, is going to be a multiband antenna. It will have an impedance on the order of 400-600 ohms at frequencies for which it is 2+ wavelengths. To match this high impedance antenna, 450 ohm ladder line should work ok, or for coax something like a 9:1 unun + common mode choke is needed here, or a 9:1 Guanella balun, or you could just use an antenna tuner at the feedpoint. If you use ladder line, you may get some imbalance and ladder line radiation. It may start to be very directional with the radiation occuring off the ends, so you can bend the dipole around to make it more omnidirectional like a long wire, unless of course you want it to be directional. There will be many nulls but they probably won't be that deep due to radiation losses reducing the standing waves on the antenna wire. The best lengths would avoid having the total length of the antenna being near a multiple of a half wavelength at any band of interest. But a long dipole could be fun to play with, and probably will have good results. A large horizontal loop is also a good use of a lot of land as well. When the wire gets very long it doesn't matter much if the long wire is a loop or not when its bent around.



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