5/8 wave dipole, Why not?

Discussion in 'General Technical Questions and Answers' started by K5ROX, Apr 12, 2009.

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  1. K5ROX

    K5ROX Ham Member

    Ive read over and over that 5/8 wave is good or best, I never see a 5/8 wave dipole or inverted Vee.

    There must be a reason, i mean were not talking much longer and wire isnt that expensive so someone please enlighten me.

    Dave K5Rox
     
  2. G3TXQ

    G3TXQ Ham Member

    Dave - it's called something different when it's a dipole. Look up "Extended double Zepp" - that's a dipole with each leg close to 5/8 wave.

    Steve
     
  3. NN4RH

    NN4RH Subscriber

    The point of the 5/8 wave vertical is that it is sort of the optimum length if what you want most of your radition with a low take-off angle with the highest gain. The penalty is that the feedpoint needs an inductor to get a match.

    A 5/8 wave horizontal dipole that you asked about is just a bit longer than a 1/2 wave dipole and the patterns are virtually identical.

    I think you probably were wondering about a 1.25 wavelength dipole (i.e. a dipole with two 5/8 wave elements). Such a horizontal antenna would give you about the highest gain possible, almost 10dBi, with a wire dipole, with a two-lobe pattern. Any longer than that and the pattern breaks up into multiple lobes. The penalty is that as with the vertical, the feedpoint impedance is very high. The pattern is narrow which can be good or bad depending on what you are trying to do.

    A 40 meter dipole (66 feet) is fairly close to a 1.25 wavelength dipole on 17 meters. You could feed it with ladder line and a wide-range tuner and use it on multiple bands.


    You might want to download EZNEC demo version to get an idea of what the patterns are.
     
  4. ND6M

    ND6M Ham Member

    exactly correct, by definition a dipole is 1/2 wl @ the designed freq.
     
  5. G3TXQ

    G3TXQ Ham Member

    Debatable :)

    The ARRL Antenna book says:
    Steve
     
  6. K4UUG

    K4UUG Ham Member

    5/8 Wave Vertical
    There's nothing magical about the 5/8 proportion of a wavelength (62.5%), a whip this long has no special properties - a coil is needed to bring the antenna into resonance. It just happens to be close to the limit (0.64 of a wavelength) to which you can extend a single element (1/4 wave style) antenna before the radiation pattern breaks up in undesirable ways. By making the whip as long as possible compared to a 1/4 wave, we get a lower angle of radiation and a squashed radiation pattern (less signal goes upwards) and so a higher effective gain (3dB) than a 1/4 wave. Popular for mobile use, but they can lead to worse results in hilly areas as less signal gets up into the hills. If used on a handheld, you'll need to keep it vertical - which is why manufacturers usually provide 1/4 waves instead. 7/8 waves are slightly different, they are collinears - a 1/4 wave (2/8) combined with a 5/8 section, yielding a dB or so more than a 5/8 on it's own.



    Double Extended Zepp
    A dipole where each side is 5/8 wave instead of 1/4 wave. The center is a folded stub 0.11 of a wavelength, with a feedpoint along the stub. Often fed at a higher impedance point using an impedance transforming stub of a 1/2 wave of coax arranged as a 4:1 balun. 4dB gain over a normal 1/2 wave dipole. See Links
    http://tcarc.ca/2mdez.html
    http://www.qsl.net/wd8rif/text/zepp.txt


    :)
     
  7. W5DXP

    W5DXP Ham Member

    There's absolutely nothing prohibiting the use of a 5/8WL center-fed dipole. It would be about 600/F feet long and would perform well. It would be about 84 feet long on 40m and have a feedpoint impedance of around 200+j470 ohms according to EZNEC.
     
  8. WA9SVD

    WA9SVD Ham Member

    A matter of semantics?. A "dipole" is two. USUALLY equal (and even that is open to interpretation) length elements; regardless of length.
    Now, often the ASSUMPTION, as erroneous or inaccurate as it may be, (made in the ARRL Handbook, VEC question pool, and many other places) is that a "dipole" only applies to a center-fed antenna that is an electrical half wave at a given frequency, or an electrical quarter wave long in each of two legs.

    Now, if you specify "center-fed Half wave" dipole, there is no confusion or amkbiguity.
     
    Last edited: Apr 12, 2009
  9. WA7OET

    WA7OET Moderator

    Exactly.
    I believe the proper definition is from the IEEE which defines a dipole by the current on the wire, not that it has two sides.
     
  10. WA9SVD

    WA9SVD Ham Member

    But then a "dipole" would not haver to be ONLY a "half" wave length, center-fed antenna...
    The term actually is ambiguous, the IEEE notwithstanding. a "di-pole" merely describes two elements. Would not a 3/2 wave, center-fed antenna still be a "dipole" by the IEEE definition?

    What is ASSUMED in ARRL,or IEEE publications, or the Amateur vernacular is not always accurate or technically correct.
     
  11. NA0AA

    NA0AA Ham Member

    I suppose that I used "Dipole" as shorthand for a 'resonant dipole', and I use Doublet for any antenna that is a dipole but not necessaily to be resonant at any one specific frequency.

    But technically, I guess you guys are correct.

    IIRC, the magic of the resonant dipole is a feepoint impedence suitable for a 50 ohm radio and coax feedline, and that it gives a very consistant radiation pattern.
     
  12. KC7YPJ

    KC7YPJ Ham Member

    The formula per leg for an edz is 585/f , for an overall length 1170/f
    A 40m EDZ is around 163.6 feet long at band center.
    Throw in calculation for velocity factor and it shortens up a bit to 156'

    I have a 156' edz fed with hb 520'ish ohm balanced line as one of my main antenna.

    The thing works wonders on 30m into zl land, it works quite nicely on 40,80,
    ja and south pacific like gangbusters on 20m, and 160m is liveable for around
    the region, better if I tie the 2 sides together at the feedpoint and feed it as
    a T working against my ground mount verticals radial field.
     
  13. WA7OET

    WA7OET Moderator

    I agree, the publications are not always accurate but I put more weight in the IEEE definition. The definition that I read was about the current distribution on a half wave wire which would be maximum current in the middle and maximum voltage at the ends. That was the essential qualifications for a dipole. Wish I could remember where I read it now... thinking it was in a W1FB publication...
     
  14. G0GQK

    G0GQK Ham Member

    Are we to suppose then that one of these fantasy double size G5RV's at 204 feet long is not a dipole ?

    G0GQK
     
  15. W5DXP

    W5DXP Ham Member

    From The IEEE Dictionary: "dipole antenna - any one of a class of antennas having a radiation pattern approximating that of an elementary electric dipole." Seems that would include all dipole antennas from infinitesimal up to 1.25 wavelength.
     
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