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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 QRZ Page

    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 QRZ Page

    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.

  3. NN4RH

    NN4RH Premium Subscriber QRZ Page

    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 QRZ Page

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

    G3TXQ Ham Member QRZ Page

    Debatable :)

    The ARRL Antenna book says:
  6. K4UUG

    K4UUG Ham Member QRZ Page

    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

  7. W5DXP

    W5DXP Ham Member QRZ Page

    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 QRZ Page

    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 QRZ Page

    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 QRZ Page

    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.
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